DESIGN MANUAL for ROADS and BRIDGES
PAVEMENT DESIGN MANUALS
Proposed Design for New and Reconstructed Bituminous,
Gravel and Concrete Roads
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
Table of Contents
1
2
3
4
5
General .......................................................................................................................... 1
1.1
Introduction ............................................................................................................. 1
1.2
Units of Measurement ............................................................................................. 3
1.3
Definitions and Abbreviations .................................................................................. 4
1.3.1
Pavement ........................................................................................................ 4
1.3.2
Pavement layers .............................................................................................. 5
1.3.3
General Terms ................................................................................................. 5
1.3.4
Bituminous Materials........................................................................................ 6
1.3.5
Traffic .............................................................................................................. 7
1.3.6
Abbreviations ................................................................................................... 8
1.3.7
Comparison of BS and ASTM Sieve Sizes ..................................................... 11
Traffic........................................................................................................................... 12
2.1
General ................................................................................................................. 12
2.2
Present Kenya legislation...................................................................................... 12
2.3
Evaluation of Traffic for Design Purposes ............................................................. 14
2.3.1
Traffic Counts ................................................................................................ 14
2.3.2
Axle Load Surveys ......................................................................................... 15
2.3.3
Evaluation of Axle Loads ............................................................................... 16
2.3.4
Estimating the Cumulative Number of Standard Axles ................................... 17
2.3.5
Length of Design Period ................................................................................ 18
2.4
Traffic Classification .............................................................................................. 18
Natural Environment .................................................................................................... 20
3.1
Climate ................................................................................................................. 20
3.2
Geology ................................................................................................................ 23
3.3
Demography ......................................................................................................... 25
Earthworks ................................................................................................................... 27
4.1
Cuttings ................................................................................................................ 27
4.1.1
Type, volume and position of the materials to be excavated .......................... 27
4.1.2
Level and flow of water table and springs ...................................................... 28
4.1.3
Stability of the slopes ..................................................................................... 28
4.1.4
Drainage and protection against erosion ........................................................ 28
4.2
Embankments ....................................................................................................... 29
4.2.1
Foundation Conditions ................................................................................... 29
4.2.2
Acceptable fill material ................................................................................... 30
4.2.3
Slope stability................................................................................................. 30
4.2.4
Placing and compaction of fill......................................................................... 31
Drainage and Erosion Control ...................................................................................... 33
5.1
Drainage of Surface Water.................................................................................... 33
5.1.1
Side Ditches .................................................................................................. 33
5.1.2
Cut Off Ditches .............................................................................................. 33
5.1.3
Discharge Channels....................................................................................... 33
5.1.4
Collection of Water in Embankments ............................................................. 33
5.1.5
Embankment Toe Ditches .............................................................................. 34
5.2
Drainage of Ground Water .................................................................................... 34
5.2.1
Drainage Remedies ....................................................................................... 34
5.3
Erosion Control ..................................................................................................... 35
5.3.1
Protection of Slopes ....................................................................................... 36
5.3.2
Protection of Ditches and Channels ............................................................... 37
The Republic of Kenya – Ministry of Roads 1—3
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DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
6
Subgrade ..................................................................................................................... 38
6.1
Subgrade Classes................................................................................................. 38
6.2
Classification of Kenyan Subgrades ...................................................................... 39
6.3
Determining the Subgrade Strength ...................................................................... 40
6.3.1
Recommended Subgrade CBR Test Procedure............................................. 40
6.3.2
Subgrade Compaction Requirements ............................................................ 40
6.3.3
Estimating the Subgrade Moisture Content .................................................... 41
6.3.4
Determining the Subgrade Design Strength ................................................... 41
6.4
Subgrade Requirements for Pavement Design ..................................................... 42
6.4.1
Materials Suitable for Pavement Support ....................................................... 42
6.4.2
Improved Subgrade ....................................................................................... 42
6.4.3
Lime Treated Subgrade ................................................................................. 43
7 Pavement Materials .................................................................................................. 44
7.1 Subbases ............................................................................................................ 44
7.1.1
Natural Materials ......................................................................................... 44
7.1.2
Graded Crushed Stone .................................................................................. 45
7.1.3
Stabilised Natural Materials ........................................................................... 46
7.2
Bases.................................................................................................................... 46
7.2.1
Natural gravel ................................................................................................ 46
7.2.2 Graded Crushed Stone ............................................................................. 47
7.2.3
Stabilized materials ........................................................................................ 48
7.2.4
Lean Concrete ............................................................................................... 51
7.2.5
Sand Bitumen Mixes ...................................................................................... 51
7.2.6
Dense Bitumen Macadam .............................................................................. 53
7.2.7
Dense Emulsion Macadam ............................................................................ 55
7.3
Surfacings ............................................................................................................. 56
7.3.1
Prime Coat..................................................................................................... 56
7.3.2
Tack Coat ...................................................................................................... 56
7.3.3
Surface Dressing ........................................................................................... 56
7.3.4
Slurry Seals and Cape Seals ......................................................................... 62
7.3.5
Otta seal .......................................................................................................... 63
7.3.6
Sand Seal ...................................................................................................... 64
7.3.7
Fog Spray ...................................................................................................... 64
7.3.8
Thin Surfacings .............................................................................................. 64
7.3.9
Asphalt Concrete ........................................................................................... 66
7.3.10 Gap-graded Asphalt ....................................................................................... 74
7.3.11 Sand Asphalt ................................................................................................. 75
7.4
Other Materials ..................................................................................................... 75
7.4.1
Reclaimed Asphalt Pavement (RAP) ............................................................. 75
7.4.2
Modified Bitumens ......................................................................................... 77
7.4.3
Cold Bituminous Mixes .................................................................................. 77
7.4.4
Block Paving .................................................................................................. 78
7.4.5
Geosynthetic materials .................................................................................. 79
7.4.6
Hand-Packed Stone ....................................................................................... 80
7.4.7
Rumble Devices ............................................................................................. 82
7.4.8
Speed Humps ................................................................................................ 85
8 Structural Design Method ............................................................................................. 87
8.1
Design Principles .................................................................................................. 87
8.1.1
Thicknesses and Materials Characteristics .................................................... 87
8.1.2
Design Period ................................................................................................ 87
8.1.3
Stage Construction ........................................................................................ 87
8.1.4
Safety Factor ................................................................................................. 88
8.1.5
Minimising Base and Surfacing Thicknesses ................................................. 88
The Republic of Kenya – Ministry of Roads 1—4
Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
8.2
Practical and experimental considerations ............................................................ 88
8.2.1
Use of Flexible Pavements ............................................................................ 88
8.2.2
Influence of Subgrade .................................................................................... 88
8.2.3
The Behaviour of Pavement Materials ........................................................... 90
8.3
Calculation of stress, strain, deflection and layer thickness ................................... 91
8.3.1
Calculation of stress, strain and deflection ..................................................... 91
8.3.2
Determination of layer thicknesses ................................................................ 92
8.4
Construction Principles ......................................................................................... 93
8.4.1
Minimum layer thickness ................................................................................ 93
8.4.2
Minimum significant thickness increments ..................................................... 93
8.4.3
Compliance with the specifications ................................................................ 93
9 Standard Pavement Structures .................................................................................... 95
10
Pavement Shoulders, Drainage and Cross Sections .............................................. 110
10.1
Shoulders ........................................................................................................ 110
10.1.1 Bearing Capacity of the Shoulders ............................................................... 110
10.1.2 Surfacing of Shoulders................................................................................. 111
10.1.3 Prevention of cracks in the shoulders .......................................................... 111
10.2
Drainage.......................................................................................................... 112
10.2.1 Drainage on the Road Surface and Shoulders ............................................. 112
10.2.2 Drainage of the Pavement Layers ................................................................ 112
10.2.3 Granular bases ............................................................................................ 112
10.2.4 Cemented or Bituminous bases ................................................................... 112
10.2.5 Drainage of the Subgrade ............................................................................ 113
10.3
Cross Sections ................................................................................................ 113
10.3.1 Edge Restraint ............................................................................................. 113
10.3.2 Recommended Cross-Sections.................................................................... 113
11
Problem Soils ......................................................................................................... 116
11.1
Low Strength Soils .......................................................................................... 116
11.2
Expansive Soils ............................................................................................... 116
11.2.1 Definition...................................................................................................... 116
11.2.2 Distribution ................................................................................................... 117
11.2.3 Identification ................................................................................................ 117
11.2.4 Remediation ................................................................................................ 118
11.3
Saline Soils ..................................................................................................... 120
11.4
Organic Soils ................................................................................................... 121
12
Gravel Roads ......................................................................................................... 122
12.1
Introduction ..................................................................................................... 122
12.2
Design Elements of Gravel Roads ................................................................... 122
12.3
Design of Gravel Roads .................................................................................. 123
12.4
Material Specifications..................................................................................... 124
12.4.1 Gravel wearing course materials (GW) ........................................................ 124
12.4.2 Subgrade materials (S2, S3) ........................................................................ 126
12.5
Deterioration and Maintenance........................................................................ 126
12.5.1 Gravel Loss and Recharge .......................................................................... 126
12.5.2 Maintenance ................................................................................................ 127
13
Concrete Roads ..................................................................................................... 129
13.1
Introduction ..................................................................................................... 129
13.2
Concrete Pavement Characteristics & Types .................................................. 130
13.2.1 Characteristics ............................................................................................. 130
13.3
Types .............................................................................................................. 131
13.4
Pavement Components and Functions ............................................................ 131
13.4.1 Subgrade and Subbase ............................................................................... 131
13.4.2 Concrete Slab .............................................................................................. 132
The Republic of Kenya – Ministry of Roads 1—5
Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
13.5
Factors influencing the design process and selection of pavement type .......... 141
13.6
Stress Development and Design Criteria ......................................................... 142
13.6.1 Horizontal Tensile ........................................................................................ 142
13.6.2 Horizontal Compressive ............................................................................... 142
13.6.3 Vertical......................................................................................................... 143
13.7
Concrete Pavement Design ............................................................................. 143
13.7.1 Traffic .......................................................................................................... 143
13.7.2 Failure Criteria ............................................................................................. 143
13.7.3 Thickness design ......................................................................................... 144
13.8
Construction issues ......................................................................................... 149
13.8.1 Labour Intensive works ................................................................................ 149
13.8.2 Medium mechanisation works ...................................................................... 149
13.8.3 High mechanisation works ........................................................................... 149
13.8.4 Roller-compacted concrete pavements ........................................................ 149
13.8.5 Surface finish ............................................................................................... 150
13.9
Maintenance and repair ................................................................................... 150
13.10 References ...................................................................................................... 151
Appendix : Construction details for Mbagathi Way, Nairobi............................................ 152
14
Materials Sampling and Testing ............................................................................. 159
14.1
Introduction ..................................................................................................... 159
14.2
Mass of Samples Required ............................................................................. 159
14.2.1 Soil and Gravel ............................................................................................ 159
14.2.2 Stone ........................................................................................................... 159
14.2.3 Feasibility Study........................................................................................... 160
14.2.4 Preliminary Design ....................................................................................... 160
14.3
Final Design .................................................................................................... 163
14.3.1 Earthworks and Subgrade............................................................................ 163
14.3.2 Soil and Gravel Borrow Pits ......................................................................... 166
15
Standard Methods of Testing.................................................................................. 169
15.1
Soils ................................................................................................................ 169
15.2
Aggregates ...................................................................................................... 170
15.2.1 Determination of Average Least Dimension ................................................. 171
15.3
Cement or Lime Stabilised Materials ............................................................... 171
15.4
Cement and Lime Testing ............................................................................... 172
15.5
Bituminous Binders ......................................................................................... 172
15.5.1 Sampling procedures ................................................................................... 172
15.5.2 Testing procedures ...................................................................................... 172
15.6
Bituminous Mixtures ........................................................................................ 173
15.6.1 Sampling procedures ................................................................................... 173
15.6.2 Testing procedures ...................................................................................... 173
15.6.3 CEN Tests ................................................................................................... 174
16
Footpaths ............................................................................................................... 176
The Republic of Kenya – Ministry of Roads 1—6
Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
1 General
1.1 Introduction
This Manual relates to the construction of new bituminous, gravel and concrete roads in
Kenya. It updates the Kenya Road Design Manual, Part III, Materials & Pavement Design for
New Roads (RDM III), published in August 1987.
A Seminar attended by 45 stakeholders in 1997 reviewed RDM III in the light of their own
knowledge and experience. Their conclusions, together with subsequent developments,
particularly in pavement design and asphalt mix design, are included in this revision.
The updated Manual has attempted to ‘harmonise’ with the Standards of neighboring
countries, such as Tanzania, Uganda, South Africa and Ethiopia, whose Standards have all
been updated in the last 10 years. Since the Manuals of these countries show considerable
differences in style and content, the approach adopted has been to harmonize their
principles and procedures but retain the format and style of the old Kenyan Manual in order
to retain familiarity. For the first time a section has been included on concrete roads.
When the Kenyan Manuals were first prepared the modern computer age was in its infancy.
Now it is possible to obtain information for pavement materials and design from the World
Wide Web and therefore the updating of any manual is a dynamic and on-going process.
Notwithstanding the recommendations contained in this Manual it is the engineer’s
responsibility to propose modifications he considers will result in a superior and costeffective design. The adoption of this Manual does not guarantee a serviceable and
economic road design. This can only be achieved by balancing the various controls, criteria
and elements involved.
This Manual is part of a set, listed in Table 1.1, which have now been updated in 2009 by
Egis-BCEOM, listed in Table 1.2, courtesy of a grant from the European Union. EgisBCEOM gratefully acknowledges the contribution from other works of reference in the east
and south African region together with the collaboration of various Ministry of Works staff
and other local stakeholders.
Table 1.1: Current Kenya Road Design Manuals
Road and Bridge Design
Part I
Geometric Design of Rural Roads
Part II
Geometric Design of Urban Roads (draft)
Part III
Materials & Pavement Design for New Roads
Part IV i
Bridge Design (draft)
Part IV ii
Hydraulic Design of Drainage Structures
Part V
Pavement Rehabilitation and Overlay Design
Traffic Control Devices
Part I
Road Marking
Part II
Traffic Signs
Part III
Traffic Signals (never produced)
Standard Drainage Structures
The Republic of Kenya – Ministry of Roads
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Previous Date of
Publication
1979
2001
1987
1982
1983
1988
1972
1975
Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
Part I
Small Span Concrete Bridges
Part II
Concrete Box Culverts
Standard Specification for Road & Bridge Construction
Road Maintenance
1987
1987
1986
Road Maintenance Manual (JICA)
2004
‘Roads 2000’ Manuals for rural roads
2009
Minor Roads Programme: Technical & Maintenance
Manuals
Table 1.2: Proposed New Kenya Road Design Manuals
Standard Specifications
1000
General
2000
Drainage
3000
Earthworks and Pavement Layers
of Natural or Crushed Gravel
4000
Bituminous Layers and Seals
5000
6000
7000
Design Manuals
Part 1
Geometric Design
Part 2
Drainage Design
Part 3
Design for New Bituminous,
Concrete and Gravel Roads
Part 4
Overlay and Asphalt Pavement
Rehabilitation
Ancillary Roadworks
Part 5
A) Traffic Signs and Road
Marking
B) Road Furniture, Lighting,
Traffic Control Devices
and Signals
C) Traffic Surveys
Structures
Part 6
a) Bridge
and
Culvert
Design
b) Catalogue of Typical
Bridges, Culverts and
Miscellaneous Structures
Tolerances, Testing and Quality Part 7
Environmental Guidelines
Control
The Republic of Kenya – Ministry of Roads
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Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
1.2 Units of Measurement
The standard units of measurement used are based on the International System (SI) units
together with some not strictly part of SI but applicable to road design. Multiples and submultiples of SI units are formed either by the use of indices or prefixes. Definitions of
applicable prefixes are given in Table 1.2. The basic units and the derived and
supplementary units which will normally be required for road design are listed in Table 1.3.
Table 1.3: Definitions of Prefixes
Prefix
mega
kilo
hecto
deca
deci
centi
milli
micro
Symbol
M
k
h
da
d
c
m
µ
Multiplication factor
106
103
102
10
10-1
10-2
10-3
10-6
Table 1.4: Basic Units, Multiples & Sub-Multiples
Quantity
Unit
Length
Mass
Time
Area
Volume
(solids)
Volume
(liquid)
Density
Metre
Kilogram
Second
square metre
cubic metre
Symbol Multiples and
Sub-multiples
m
km, mm
kg
Mg, g, mg
s
day (d), hour (h), minute (m)
m2
km2, hectare (=10,000m2), mm2
m3
cm3, mm3
Litre
l
ml, 1ml=10-3l=1cm3
kilogram per
cubic metre
Newton
kg/m3
Pascal
N/m2
1Mg/m3=1kg/l
= 1g/ml
MN, kN (1N = 1kgm/s2
1kgf = 9.81N)
2
kN/m (kPa), N/mm2 (MPa)
Force
Pressure
& Stress
Velocity
(speed)
Angle
N
metre per second m/s
degree or grade
Temperature degree Celsius
0
Km/h (1km/h =1/3.6 m/s)
Minute (‘), second (“),
(3600 circle)
(400g circle)
0
C
The Republic of Kenya – Ministry of Roads
3
Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
1.3 Definitions and Abbreviations
1.3.1 Pavement
Figure 1.1 Road Pavement Terminology
Fig 1-1 shows the terms used in describing the principal pavement and cross section
components.
The Republic of Kenya – Ministry of Roads
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Draft Document – October 2009
DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement
1.3.2 Pavement layers
Formation is the surface of the ground, in its final shape, upon which the pavement structure,
consisting of subbase, base and surfacing is constructed.
Subgrade consists of all the material below the subbase, including in-situ material, fill and
improved subgrade. (In AASHTO subgrade has the same meaning as roadbed: another
name for the subgrade is Foundation). For design purposes the following subgrade classes
are recognized:
Subgrade Class
S1
S2
S3
S4
S5
S6
CBR Range
2
3 to 4
5 to 7
8 to 14
15 to 29
30 or more
Fill is approved imported material used below formation level to construct embankments or
replace unsuitable natural material. Most types of soil and broken rock can be used but
highly plastic soil, expansive soil and organic soil should be avoided.
Improved (or selected) subgrade is a layer of selected fill material, the top of which is at
formation level, placed where the natural in-situ or fill material is unsuitable for the direct
support of the pavement. Its purpose is to increase the strength and stiffness of the insitu
material and thus reduce the pavement thickness.
Subbase consists of a medium quality granular layer resting on the subgrade and supporting
the base course.
Base (or road base) consists of a pavement layer lying between the surfacing and the subbase, which can be constructed from asphalt, granular or stabilised material.
Binder Course consists of the lower bituminous layer of the pavement, usually asphalt
concrete. It is not always present; the wearing course may rest directly on the base course.
Surfacing is the uppermost pavement layer which provides the riding surface for vehicles. It
will normally consist of one of the following: surface dressing, sand asphalt or asphalt
concrete. If constructed of asphalt it will include a surfacing and an optional binder course.
Wearing Course consists of the uppermost bituminous layer of the pavement, usually
asphalt concrete. The top surface of this layer should provide a smooth surface but with
adequate texture to provide adequate friction for safe vehicle braking and turning.
1.3.3 General Terms
Borrow Area is a site from which natural material, other than solid stone, is removed for
construction of the works. (The term borrow pit is also used.)
Quarry is an open surface working from which stone is removed by drilling and blasting, for
construction of the works.
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2009
Part 3 - Materials and Pavement
Stabilized Materials are naturally occurring gravels and clayey sands, or crushed stone, to
which either cement or lime, or both, have been added, in order to improve their engineering
properties.
Lean Concrete is a high quality, well graded aggregate and Portland cement mixture, mixed
in a stationary plant and laid by a paver. It is used as a high quality base.
Rock fill is rock material of such particle size that the material can only be placed in layers of
compacted thickness exceeding 300mm. Boulders with volumes greater than 0.2m³ are not
normally used.
Graded Crushed Stone consists of quarried stone which has been crushed to a range of
sizes, conforming to a high quality specification for grading, cleanliness, strength, shape and
soundness. Normally graded crushed stone is used for roadbase or as the aggregate in
bituminous bound material.
.
Gravel Wearing Course consists of a surfacing applied to a road formation where no
bituminous surfacing is to be placed. The gravel can include one or a combination of the
following materials: lateritic gravel, quartzitic gravel, calcareous gravel, some forms of partly
decomposed rock, soft stone, coral rag, clayey sands and crushed rock.
1.3.4 Bituminous Materials
Bituminous Binders are petroleum-derived adhesives used to stick chippings onto a road
surface, as in surface dressings, or to bind together a layer of surfacing or base material.
There are three principal types used in road work:




Straight-Run (or Penetration) Bitumen is bitumen whose viscosity or composition has
not been adjusted by blending with solvents or any other substance.
Cut-Back Bitumen is bitumen whose viscosity has been reduced by the addition of
volatile diluent, such as kerosene or diesel.
Short Residue Bitumen is the primary product of the refinery before the air-blowing
process, and is bitumen of variable viscosity whose penetration can be measured,
and which approximates to a slow-curing cut-back bitumen.
Bitumen Emulsion is bitumen in finely-divided droplets dispersed in water by means
of an emulsifying agent to form a stable mixture.
Surface Dressing is a method of providing a running surface to a pavement and consists of
applications of bituminous binder and single sized stone chippings. The usual form of this
method on a new road is a double surface dressing with the second layer of chips being half
the nominal size of the first. Single, triple and other types of surface dressings are also used.
Instead of chippings sand may be used (=sand seal). Two layers of chippings may also be
applied to one coat of bitumen (=’racked-in’ surface dressing).
Emulsion Slurry Seal is a surfacing material, used by itself in one or two layers, or on top of
a single surface dressing. It consists of fine aggregate, mineral filler and bitumen emulsion.
Cape Seal is a surfacing where a slurry seal is applied on top of a surface dressing to
produce a surface less harsh than a surface dressing and which is flexible and durable,
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Fog Spray is a light application of bitumen emulsion or cut-back, applied to the top of a
surface dressing, in order to improve the waterproofing quality of the surfacing and to assist
in holding the chippings.
Otta Seal is similar to a surface dressing except that a graded aggregate is used instead of
single sized chippings. It may be applied as a single or double layer.
Asphalt Concrete is a bitumen-bound premix. It consists of a mixture of coarse aggregate,
fine aggregate and filler, bound with straight-run bitumen. The proportions and grading of the
constituents may be varied by using the Marshall test method to meet specific strength,
deformation and volumetric criteria.
Sand Asphalt is a surfacing material consisting of a hot-mixed, hot-laid, plant mixture of
natural sand and, in some cases, mineral filler and crushed fine aggregate, bound with
straight-run bitumen.
Gap-Graded Asphalt is a hot laid, plant mixture of gap-graded aggregate, filler and straightrun bitumen, used for pavement surfacing.
Binder Course is the lower layer when two-course asphalt concrete is used as a surfacing. It
usually differs from the upper, wearing course, in having larger sized aggregate, a slightly
lower bitumen content, lower stability and greater voids.
Sand Bitumen is a base material consisting of a cold, mixed-in-place combination of sand (or
clayey sand) and either bitumen emulsion or cut-back. This material is intended for use in
areas with little or no gravel deposits.
Dense Bitumen Macadam is a hot-laid, hot-mixed recipe bituminous mixture consisting wellgraded aggregate, filler and straight-run bitumen, normally used for base construction.
Dense Emulsion Macadam is a cold laid, plant mixture of well graded aggregate, filler and
bitumen emulsion, used for base construction. The specifications are very similar to dense
bitumen macadam.
Prime Coat consists of low viscosity, usually cutback bitumen, applied to an absorbent
surface, usually the top of the base, which prime purpose is to help bind it to the overlaying
bituminous layer.
.
Tack Coat is a light application of bituminous binder applied to a bituminous or concrete
surface in order to glue this surface to the overlying, normally bituminous course.
1.3.5 Traffic
Private cars (cars) are all passenger motor vehicles seating not more than 9 persons,
including the driver.
Light Vehicles are all goods vehicles of not more than 15kN unladen weight.
Buses are all passenger motor vehicles seating more than 9 persons, including the driver.
Medium Goods Vehicles are all two-axle goods vehicles of more than 15kN unladen weight.
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Part 3 - Materials and Pavement
Heavy Goods Vehicles are all goods vehicles having more than two axles.
Commercial Vehicles include buses and goods vehicles of more than 15 kN unladen weight.
Equivalent Standard Axle (ESA) is a concept enabling the damaging effect of a range and
number of different axle loads to be considered in the structural design of a pavement. The
equivalent standard axle imposes a load of 80 kN (8,200 kgf) and other axles are correlated
to this by the following equation:
 80
ESA  L
4.5
where ESA is the equivalent standard axle, L is the axle load in kN divided by the standard
80kN axle, and 4.5 the exponent representing the relative damage
Design Period is the time period over which the proposed pavement must carry the predicted
number of equivalent standard axles without the need for major rehabilitation work, except
for maintenance. At the end of this period the pavement should still be in a sufficiently good
condition that strengthening will result in a further period of satisfactory traffic-carrying.
Traffic Classes are the predicted cumulative numbers of equivalent standard axles divided
into the following classes:
Traffic Class Cumulative Number of
Standard Axles (ESA)
T1
0.003 to 0.25
T2
0.25 to 1
T3
1 to 3
T4
3 to 10
T5
10 to 25
T6
25 to 60
T7
60 to 100
1.3.6 Abbreviations
AASHO
American Association of State Highway Officials, which became AASHTO
AASHTO
American Association of State Highway and Transportation Officials
AADT
Average Annual Daily Traffic
ADT
Average Daily Traffic
ACV
Aggregate Crushing Value
ALD
Average Least Dimension
ASL
Above Sea Level
ASTM
American Society for Testing and Materials
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BS
British Standard
CBR
California Bearing Ratio
COMESA
Common Market for East and South Africa
CR
Crushing Ratio
DCP
Dynamic Cone Penetrometer
ESA
Equivalent Standard Axle
FI
Flakiness Index
GM
Grading Modulus = [300-(% passing 2mm)-(% passing 0.425mm)-(% passing
0.075mm)]/100
HF
Hubbard - Field
KEBS
Kenya Bureau of Standards
ISO
International Standard Organization
LAA
Los Angeles Abrasion
LL
Liquid Limit
MC
Moisture Content
MDD
Maximum Dry Density
OMC
Optimum Moisture Content
PL
Plastic Limit
PI
Plasticity Index
PM
Plasticity Modulus = (PI * % passing 0.425mm sieve)
SADC
Southern African Development Community
SG
Specific Gravity
SS
Standard Specification for Road Construction
SSS
Sodium Sulphate Soundness
TS
Tensile Strength
UC
Uniformity Coefficient = Ratio of Sieve size through which 60% of material
passes to Sieve size through which 10% of material passes
UCS
Unconfined Compressive Strength
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Vibrating Hammer
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1.3.7 Comparison of BS and ASTM Sieve Sizes
BS sieve
Aperture
size
75mm
63
50
37.5
28
20
14
10
6.3
5
3.35
2
1.18
600µm
425
300
212
150
75
63
ASTM
D422
Aperture
Size
3 inch
2½ inch
2 inch
1½ inch
¾ inch
⅜ inch
#4
#6
#8
# 10
# 16
# 20
# 30
# 40
# 50
# 60
# 70
# 100
# 200
# 230
75mm
63.5
50.8
38.1
19.05
9.52
4.75
3.35
2.36
2.00
1.18
850 µm
600
425
300
250
212
150
75
63
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2 Traffic
2.1 General
Deterioration in paved roads caused by traffic is a function of the magnitude of the individual
wheel loads and the frequency with which they are applied. For pavement design purposes,
therefore, it is necessary to know not only the total number of vehicles using the road but
also the axle loads. Traffic loading is normally expressed in terms of ‘equivalent standard
axles’, ‘ESA’, a concept developed following the AASHO Road Test carried out in the USA in
the late 1950s. An axle carrying 8.16 tonnes was arbitrarily defined as a ‘standard axle’, to
which axles of different weights were correlated to derive equivalence factors, thereby
obtaining an expression of the damaging effect. Thus:
 80
ESA  L
4.5
Where ESA is the equivalent standard axle, L is the axle load in kN divided by the standard
80kN axle, and 4.5 the exponent representing the relative damage. This equation was
derived by Liddle (1962) for the test conditions at the time. Although Liddles’ formula is safe
only up to axle weights of 130kN (13 tonnes), nevertheless, in the absence of anything
better, current practice is still to use this equation for greater axle weights. A more secure
practice would be to determine the proportion of axle weights greater than 130kN and then
to adjust the traffic category accordingly (see later).
There is now considerable evidence that the 4.5 exponent varies according to the pavement
type, thickness, balance (how the strengths of the constituent layers compare with one
another), and subgrade strength. Values between 3 and 5 have been determined from
research in South Africa with a Heavy Vehicle Simulator (Van Zyl et al, 1984). For the sake
of simplicity, and also since most current pavements in Kenya are of the same type, the 4.5
value is retained.
A tandem axle may inflict slightly more or slightly less damage than two separate axles
depending on various factors but, again for simplicity, it is recommended that they are
treated separately in the calculation of ESA.
The ultimate objective in design is thus to determine the cumulative number of ESA in the
design period. This is achieved in a number of operations:

the axle load distribution of the traffic is evaluated

the axle loads converted into ESA

the initial daily number of ESA calculated, and

an annual growth rate over the design period selected.
2.2 Present Kenya legislation
The Kenya Roads Board (KRB), website (www.krb.go.ke/), states that there is an estimated
185,000km of roads in Kenya, defined in Table 2.1:
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Table 2.1: Kenya Road Classification
Type
Class
Description
Length,
km
Authority responsible
National
A, B, C
Main highways
14,000
Roads Dept, Ministry of Roads
& Public Works
District
D, E
Secondary roads
49,000
District Roads Committees
Unclassified
Rural
100,000
Urban
City & town
Urban
14,000
Special
Purpose
Park & Game Park &
Reserve
Reserve
Game 8,000
City,
Municipal
Councils
&
Town
Kenya Wildlife Service, Forest
Department
Low volume roads (<200vehicles per day) comprise over 80% of the road length and roughly
carry about 40% of the traffic, in terms of vehicles per day. These roads will be generally
constructed either of gravel or earth and some will also have an all-weather surfacing.
The legal limits currently in force in Kenya, according to Legal Notice No 118-Traffic
Amendment Rules 2008, are listed in Table 2.2:
Table 2.2: Vehicle Axle Load Legal Limits
Axle Group
Legal Limit (kg)
Error allowance
Allowable Axle Load
Single Steering
8,000
-
8,000
Single Rear
10,000
400
10,400
Tandem Rear
16,000
600
16,600
Triple Rear
24,000
800
24,800
The Maximum Gross Vehicle Weight of a vehicle is defined in Table 2.3:
Table 2.3: Maximum Permissible Gross Vehicle Weights
Vehicle Type
Legal Limit (kg)
Vehicle with two axles
18,000
Vehicle with three axles
24,000
Vehicle & semi-trailer with total of three axles
28,000
Vehicle & semi-trailer with total of four axles
34,000
Vehicle & drawbar trailer with total of four axles
36,000
Vehicle & semi-trailer with total of five axles
42,000
Vehicle & drawbar trailer with total of five axles
42,000
Vehicle & semi-trailer with total of six axles
48,000
Vehicle & drawbar trailer with total of six axles
48,000
No vehicle with more than six axles is permitted unless special exemption is granted.
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These recommendations compare well with those of SADC and COMESA which are listed in
Table 2.4.
Table 2.4: SADC and COMESA Vehicle Weight Limits
Economic Load Limits (tonnes)
Grouping Single Axle
Tandem
(8 tyres)
Steering Drive
(2 tyres) (4 tyres)
SADC
8
10
18
COMESA 8
10
16
Tridem
GVM
(12 tyres)
24
24
56
53
By comparison, in the UK, the maximum permitted weight of an articulated vehicle
increased by more than a third between 1983 and 2001. Today goods vehicles with 6 axles
can weigh 44 tonnes, articulated combinations can be up to 16.5m long while drawbar
combinations (and bendy buses) can be up to 18.75m long. Many stakeholders are
convinced these increases have been beneficial in terms of safety the environment and the
economy. Many others are equally convinced that they have had the opposite effect.
In Sweden and Finland vehicles of up to 60 tonnes and 25.25m length are permitted and
several other countries in Europe are considering permitting them with the Netherlands at an
advanced trial stage.
Enforcement of axle load limits is undertaken by Roads Dept. of the Ministry of Roads &
Public Works and operations are carried out on a 24/7 hour basis. There are five permanent
weighbridge stations located along the Mombasa to Nairobi and Malaba road at Mariakani,
Athi River, Gilgil, Webuye and Isebania. In addition there are weighbridge stations at
Mombasa Port, Mtwapa, Namanga, Nairobi, Maai Mahui, Kisumu and Malaba. All vehicles
greater than 7 tonnes gross weight must be weighed and non-compliant vehicles charged
and prohibited from using the road until in compliance. The KRB states a compliance rate of
80 to 90%. However, it is understood that on many routes the overloading may exceed these
values, especially if there is no monitoring. It is therefore recommended that for any new
project an axle load survey is carried out to obtain an accurate assessment of the design
traffic.
An additional factor leading to increased stress on the pavement is tyre pressures, which
have increased significantly in recent years. The tyre pressure used in the AASHO road test
in 1962 was 0.48MPa (70psi) whereas a study carried out in Kenya in 1987 recorded the
mean value as 0.7MPa (102psi). Additionally, tyre configurations have changed, whereby
modern single tyres or ‘super single’ tyres (wider than singles) have replaced the original
twin tyres on trucks, each with different damaging effects. These factors are presently not
considered in the computation of ESA.
2.3 Evaluation of Traffic for Design Purposes
2.3.1 Traffic Counts
The loads imposed by private cars and light goods vehicles with axle weights < 1.5tonnes do
not contribute significantly to the structural damage of a paved road and thus, for design
purposes, can be ignored. However, for economic and congestion forecasting, the total
traffic is determined and routine traffic counts are carried out annually by the Ministry of
Transport & Communications at a number of census points. They distinguish between cars,
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light goods, buses, medium goods and heavy goods vehicles. Where such results are
available, the initial daily traffic can be estimated by extrapolation.
A standard type of vehicle classification scheme is presented in Table 2.5.
Table 2.5: Vehicle Classification Scheme
Category
Type
Description
1 Light vehicles
1a
Motorcycles
Motorcycles with/without side cars
1b
Passenger cars
Cars seating up to nine passengers
1c
Small buses
Matatus, minibuses seating up to 30 passengers
1d
Light Goods
2 Medium and heavy vehicles
2a
Large buses
Buses and coaches seating more than 30 passengers
2b
Medium goods
2 axles, twin tyres on rear axle, >1.5 tonnes unladen
weight, <8.5 tonnes gross vehicle weight
2c
Heavy goods
3 axles
2d
Heavy goods
4 axles or more, trailers included, >3 tonnes unladed
weight or >8.5 tonnes gross vehicle weight
3 Others
Tractors, road rollers, or vehicles with 5 or more axles
depending on survey requirements
Where traffic census data is not available or is insufficient, specific traffic counts are required
at key points and axle load surveys carried out to determine the initial daily traffic and
possible seasonal variations. The recommended survey period is one week, for 24 hours at
least on two days to determine the nighttime flows, and the counts are classified into the
abovementioned traffic classes. Times when there are especially increased or decreased
traffic flows should be avoided. Automatic counters can be used for greater accuracy
because the survey can be conducted over a longer period and detect seasonal variations
caused by the weather or harvest time for example but, unless very sophisticated and
expensive types are used, obviously cannot distinguish vehicle types. Details of automatic
counters are contained in TRL ORN 40 (2004).
2.3.2 Axle Load Surveys
Axle load surveys are required to estimate the ESA. Most of the ESA will be carried by the
medium and heavy vehicles and Fig 2.1 proposes a scheme to distinguish the different types
of these vehicles in the execution of an axle load survey.
Figure 2.1: Commercial Vehicle Types
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The most common method of carrying out an axle load survey is to weigh a sample of
vehicles at the roadside using portable weighpads. It is possible to weigh about 60 vehicles
per hour using this method. If the traffic flow is too high a sample should be selected for
weighing. Weigh-in-motion equipment is popular but it is less accurate, requires regular
calibration and more expensive.
It is important that weighpads are regularly calibrated by either the manufacturer using a
proving ring or in the field with a vehicle of known weight.
On many roads it will be necessary to consider whether the axle load distribution of the
traffic in both directions is the same and significant differences can occur for example on
roads connecting docks, quarries, heavy industrial works and mining areas. In Kenya the
Mombasa-Nairobi-Uganda highway is a good example. Survey results from the more heavily
trafficked direction should be used for pavement design purposes.
2.3.3 Evaluation of Axle Loads
The axle loads will be obtained by multiplying the average daily number of commercial
vehicles by the appropriate Equivalence Factor and then summing the ESA for all the vehicle
types. In Table 2.6 the effect of road width and vehicle flow is considered for design
purposes.
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Table 2.6: Calculation of Commercial Traffic
Carriageway
Width, m
Traffic,
Commercial
Commercial
considered
Vehicles
per
day
Traffic
to
be
Single, ≤ 7
Total commercial traffic in BOTH
DIRECTIONS
Single, > 7
Total commercial traffic in MOST
HEAVILY TRAFFICKED LANE
Dual
< 2000
Total commercial traffic in ONE
DIRECTION
Dual
> 2000
A special study of the
distribution will be necessary
traffic
Notes
1. On single carriageway roads, the offside wheeltracks of commercial vehicles tend to
follow the central part of the road, the more so as the carriageway becomes narrower
and the traffic lighter. Where the carriageway width is 7m or less it is assumed that
the central section of the road is used by over 70% of the commercial vehicles and,
in this case, the sum of the ESA in both directions is taken to allow for the overlap.
2. On dual carriageway roads, the inside, slow-traffic lanes will usually carry at least
80% of the commercial vehicles, as long as the flow does not exceed 2000
commercial vehicles per day. If it is more, then special studies will be needed to
estimate the proportion of commercial vehicles using each dual carriageway lane.
2.3.4 Estimating the Cumulative Number of Standard Axles
To estimate the total number of ESA for the pavement design, it is necessary to forecast the
annual traffic growth rate and decide the length of the design period, as described below:
2.3.4.1 Forecasting the Annual Growth Rate
This is a difficult exercise but it may help to separate traffic into the following three
categories and estimate how each category could grow in the future:
Normal Traffic: traffic which would pass along the existing road in ordinary circumstances
whose growth could be based on national historical trends, fuel sales or any other specific
local circumstances.
Diverted Traffic: traffic that changes from another route but still retains the same origin and
destination
Generated Traffic: additional traffic that is generated in response to the improvement of a
road.
Guidance can be obtained from the following factors: historical growth, economic trends,
geometric capacity of the road, increases in vehicle numbers and loading and social
realities. Typical growth rates range from 2 to 15% per annum, averaging about 4% per
annum.
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2.3.5 Length of Design Period
Design period should not be confused with design life. At the end of the design period the
road pavement will not be completely worn out or have deteriorated to the point that reconstruction is needed but only require to be strengthened to carry traffic for a further period.
During the design period, it is accepted that routine maintenance (eg shoulders and drainage
system maintenance, vegetation control, patching and sealing) and periodic maintenance
(surface dressing, asphalt overlays, slurry seals) will be carried out.
The aim is to minimize the total expenditure on the pavement, including the initial
construction cost and subsequent maintenance or strengthening costs discounted to the
present day value. This raises the question of stage construction.
Stage construction offers economic advantages and initial design periods should not exceed
15 years, even if longer overall lives are anticipated. It also provides an opportunity to
choose the structural characteristics of the second stage in the light of actual conditions,
which may differ substantially from the original conditions.
The cumulative number of ESA, T, for the chosen design period, N (in years), is then
obtained from the following:
T  365t1
1  i N
1
i
Where:
t1 is the average daily number of standard axles in the first year after opening, and
i is the annual growth rate expressed as a decimal fraction
2.4 Traffic Classification
The traffic classes listed in Table 2.7 adequately account for all traffic categories likely to be
carried by the bituminous roads of Kenya.
Table 2.7: Traffic Classes
Class
T1
Cumulative Number
ESA
0.003 to 0.25 million
T2
0.25 to 1 million
T3
1 to 3 million
T4
3 million to 10 million
T5
T6
T7
10 million to 25 million
25 million to 60 million
60 million to 100 million
of Description
Very light traffic; very few heavy vehicles.
These roads can be defined as ‘Low
Volume’ (<200vpd) and are the transition
from gravel to paved roads; they may use
non-standard materials and may also have
all-weather surfacings
Light traffic; mainly cars, pick-ups and small
trucks with <10% heavy commercial
vehicles.
Moderate traffic; 10% to 20% heavy
commercial vehicles
High traffic volume and/or >20% heavy
commercial vehicles
Very high traffic volume and/or many laden
commercial vehicles
Very high volume of heavily laden
commercial vehicles
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The proportion of axle loads greater than 130kN should be determined and, if this proportion
is greater than 50% of the total axle loads, consideration should be given to increasing the
traffic class by one for the purposes of design.
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3 Natural Environment
3.1 Climate
Climate has a fundamental influence on road materials and performance. Straddling the
Equator from 40N to 40S with a land area of 580,000 km2 (85% of the area of France), Kenya
has a tropical climate, ameliorated by relief. It is hot-humid at the coast, warm-temperate
inland and hot-dry in the north and northeast parts of the country. There are two rainy
seasons: one between March and June, and the other between October and November. The
temperature remains high throughout these months except in the uplands. The rainfall is
sometimes heavy and falls in the afternoon and evening. The hottest period is from February
to March and coolest in July to August.
From the coast on the Indian Ocean the low plains rise to central highlands, thence to the
shores of Lake Victoria at 1130m ASL. The highlands are bisected by the Great Rift Valley
but are also the site of the highest point in Kenya, Mount Kenya, which reaches 5,199 m
ASL. Mount Kilimanjaro which at 5,895m ASL is the highest point in Africa can be seen from
Kenya just south of the Tanzanian border.
The design of drainage systems largely depends on the expected climatic conditions. The
choice of roadmaking materials will also be influenced by climate: in this respect, the
following areas have been demarcated:


‘wet’ areas (mean annual rainfall greater than 500mm), where the use of plastic
pavement materials, defined as having a PI >50, should be avoided if possible.
Bituminous surfacings should be as impervious as possible. Shoulders should be
impermeable or properly sealed. Great attention should always be paid to both
internal and external drainage
‘dry’ areas (mean annual rainfall less than 500 mm), where higher plasticities can be
accepted for pavement materials and open-textured base materials can be used.
Difficulties may occur with cement-treated materials, because of the rapid
evaporation of water hindering the hydration of cement and the tendency of the
treated material to crack extensively as a result of shrinkage and volumetric changes
caused by the daily temperature variations. Drainage and protection against erosion
should not be neglected as short but heavy storms are likely to occur even in the
driest areas.
The diversity of climate is illustrated by Figs 3.1 and 3.2.
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Figure 3.1: Kenya: Temperature maxima-minima
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Figure 3.2: Rainfall maxima-minima
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3.2 Geology
Kenya can be subdivided into three basic regions:



the coastal fringe of Mesozoic and Tertiary strata
the plains containing ancient soils derived from the weathering of the Pre Cambrian
basement gneisses and granites, and Recent sandy or gravelly soils derived from the
re-working of these deposits, and
the highly weathered to relatively unweathered extrusive igneous rocks
accompanying the volcanism associated with the development of the Rift Valley
trough fault system which commenced in early Tertiary times and continues
intermittently to the present. The extrusive rocks consist of volcanic ash and tuff and
lava, generally of basic alkaline composition, which have been subject to profound
tropical weathering, producing a variety of fertile soils, for example the red coffee
soils, and black cotton soils, and other subgrades of variable suitability for road
construction.
Fig 3.3 illustrates the distribution of the main geological regions.
A consequence of the variable geology is that there is a wide range of road-making materials
available in Kenya. Many different sorts of gravels occur, including lateritic gravels, quartzitic
gravels, calcareous gravels, coral limestone, etc. Various types of sand and silty or clayey
sands are also found. Kenya also has abundant resources of hard stone lava aggregate and
gneiss and granite stone although the degree of weathering of these materials is variable
and often profound.
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Figure 3.3: Geology
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3.3 Demography
The current population of Kenya (late 2009) is around 40 million who are concentrated in the
warm-temperate and fertile volcanic uplands, and along the coast as shown on the attached
map. The main road network is thus to be found in this region, and is the area where most
road building and road maintenance activity occurs. Apart from these, a main highway
connects the port of Mombasa to the capital Nairobi and beyond to Uganda and other
countries in central Africa and is thus of prime strategic importance.
Fig 3.4 shows the population distribution.
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Figure 3.4: Demography
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4 Earthworks
Construction of new roads, and sometimes also reconstructed roads if they are widened,
invariably requires the movement of soil and rock prior to building the road pavement.
Cuttings and embankments will be constructed to obtain a satisfactory alignment on most
roads and the following discusses the main factors to be considered.
4.1 Cuttings
In most regions of the world cuttings are made in materials in different stages of weathering.
This is particularly true in tropical regions where the higher temperatures increase the rate of
chemical reaction, and if rainfall is significant, the result is often a quite profound and
variable weathering profile. Cuttings in weathered rock and soil are generally unstable
because of accumulation of water in the material and slips occur when this accumulation of
water reduces the cohesion of the soil and increases its mass.
Wherever a cutting is required, the following factors will affect its design and cost:




Type, volume and position of the materials to be excavated
Level and flow of water table and springs
Stability of the slopes
Drainage and protection against erosion
4.1.1 Type, volume and position of the materials to be excavated
The type of material excavated governs the construction methods, the use to which the
material can be put, its suitability as subgrade material and the slopes that can be safely
constructed.
From both economical and technical viewpoints it is important to determine with reasonable
accuracy the respective volumes of rock, ‘rippable’ material and ‘diggable’ material occurring
in each cut. This is not easy to determine, and it may not be possible until the construction
phase, but an approximation can be achieved with boreholes: rotary percussion are the
quickest and cheapest.
Tropical weathering generally results in the occurrence of two types of materials: residual
soils and weathered rock, which together with the fresh unweathered rock, if it is reached by
the cutting, makes three material types. The depth and degree of weathering is usually very
variable, both vertically and horizontally and the properties of the residual materials can vary
over short distances at the same level.
The depth to rock is clearly important not only because of its effect on the cost of the cuttings
but also because the presence of rock can provide a surface on which a perched water table
can exist. Depending on the type of rock and its structure, springs could also occur.
The design CBR of the roadbed (= subgrade) in a cutting should be taken as the lowest
realistic CBR value encountered within the material depth.
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4.1.2 Level and flow of water table and springs
Water tables and springs may be permanent, seasonal or (in the case of water tables)
perched: all types can occur in Kenya. In any case, their presence and characteristics must
be determined (although this is sometimes not easy to do) as they will affect the method of
excavation, the stability of the cut slope and the drainage system required.
4.1.3 Stability of the slopes
The analysis of slope stability is usually based on measurements of soil density, moisture
content and soil strength together with calculations of soil stresses using slip-circle analysis.
However, this type of analysis assumes that the soil mass is uniform, which is rarely the
case and it is more common for failures to occur along vertical planes of weakness, or other
pre-existing planes of weakness, such as joint or bedding planes, even if the rock is
completely or profoundly weathered.
In most cases slope angles are determined by experience and those that have generally
been found to be satisfactory, where there is no water seepage or external loads present,
are shown in Table 4.1:
Table 4.1: Slope Angles for Earthwork Materials
Material
Sand (cohesionless)
Silty sand/silt
Residual (red) soil
Weathered rock
Fresh rock
Vertical : Horizontal
1:2
1:1
1.5:1 if depth <4m
1:1 if depth >4m
2:1 to 4:1
5:1 to 10:1
It is advisable that any cutting greater than 5m height, or if the water table situation is
problematical, should be studied by a specialist. This may well require a detailed site
investigation and associated laboratory testing, resulting in recommendations for elaborate
soil stabilization techniques, outside the scope of this manual.
4.1.4 Drainage and protection against erosion
Control of ground water in the cut slopes is important for it is essential to disperse surface
water from the road formation at all stages of construction. Adequate drains must be
constructed at the toe of the cut slope to carry away surface water flowing off the cut slope;
otherwise it could prejudice the stability of the road formation. Cut-off drains, constructed at
the top of the cutting to prevent water from above the cutting adding to the run-off on the
slope itself, must be properly lined and maintained otherwise they will exacerbate the
instability of the cut slope.
Control of slope erosion is sometimes difficult to reconcile with slope stability. For example,
in the red clays, which are common in Kenya, a cut slope of 1:1 (see Table 4.1) would be
extensively eroded where its height exceeded about 5m. Two solutions are possible. Either
the slope is cut at 1:1 and planted with grass (preferably at the beginning of the first rainy
period) or the slope is cut at 1.5:1 and provided with 2m wide benches at every 4m vertical
spaces.
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For aesthetic and safety reasons a low angle slope is normally considered more desirable
than a near vertical slope. The need to balance the amount of cut and fill material may also
have an influence on slope angles.
In deep cuttings, where the pavement is laid soon after completion of the cutting,
consideration should be given to heave, especially if the foot of the cutting is still in residual
soil.
4.2 Embankments
Embankments will be needed when the vertical alignment of the road has to be raised above
the level of the existing ground either to satisfy design geometric standards or to prevent
damage by surface or ground water. Most embankments are low, between 0.5m to 1.5m
high; heights of 5m or more may, however, be used on major highways. Wherever an
embankment is required, the following factors will affect its design and cost:




Foundation conditions
Acceptable fill material
Slope stability
Placing and compaction of fill
4.2.1 Foundation Conditions
The residual soils widespread in Kenya are not usually compressible and any settlement that
does occur is likely to be substantially complete by the time the embankment is constructed.
Nevertheless, when an embankment has to be built on wet, compressible soil, such as soft
clay, detailed investigations are necessary to determine the most suitable construction
method, the rate of construction and any special measures required. Usually, either the soft
clay is removed and replaced with coarse rockfill or it is substantially consolidated before the
road pavement is constructed.
The consolidation will involve either pre-loading with a higher (heavier) embankment or the
installation of vertical sand drains or a combination of both. Normally 90% consolidation will
have to be achieved before placing the pavement layers. If the consolidation option is
chosen, the rate of dissipation of construction pore pressures in the soft, saturated
foundation material must be investigated and a suitable construction rate decided. This is
very important, especially if a high pre-loading embankment is proposed, in order to avoid a
shear failure during construction. Even if the proposed embankment is only a few metres
high, a full geotechnical investigation is necessary to determine the magnitude and rate of
settlement and the likely pore pressures generated during construction. Piezometers can be
installed in the foundation material to determine its pore pressures, enabling faster safe rates
of construction if the forecasts have been pessimistic or to prevent failure if optimistic.
Conventional oedometer consolidation tests using specimens of undisturbed samples from
the horizontal plane normally give accurate predictions of the amount of settlement for a
layer of soft, saturated clay loaded by an embankment. However, the time of settlement
predicted by this method is usually much longer than in practice because in most normally
consolidated clays the drainage path in the horizontal direction is many times more
permeable than in the vertical direction. Oedometer tests with specimens cut from
undisturbed samples from the vertical plane will give an accurate prediction of time of
settlement under an embankment load.
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4.2.2 Acceptable fill material
Almost all material types, from sandy clays to broken rock, can be used for embankment
construction, the main limitation being the ease with which they can be handled and
compacted. Usually, the material will be obtained from cuttings or borrow pits close to the
embankment. Material of low plasticity is preferred because it will pose fewer problems in
wet weather. If more plastic material is used, it must be shaped and compacted quickly so as
to shed rain water. If the embankment is higher than 6 metres it is desirable to reserve
material of low plasticity for the lower layers.
Materials generally unacceptable for fill are as follows:





containing more than 5% of organic matter, such as topsoil, swamp material, wood etc
having a swell of more than 3%, such as black cotton soil
having a plasticity index more than 50: however, some residual red clays with a PI > 50
may successfully be used. These soils, when compacted in embankments to a greater
density than found in situ, develop considerable shrinkage and suction forces associated
with seasonal wet and dry periods. These forces are large and can give rise to
longitudinal cracks to repeatedly be formed through to the surface of any road pavement
containing rigid or semi-rigid layers.
having a moisture content greater than 105% of the optimum moisture content of BS
Light (AASHTO T99), and
having a CBR < 2%
Embankments have nevertheless been constructed with these ‘unacceptable’ materials, eg
Embakasi near JK International Airport. Special precautions, described in Chapter 11, are
then required. A fully flexible pavement will be the most suitable on these embankments but
if rigid or semi-rigid layers are necessary (lean concrete, concrete, cement or lime stabilized
gravels), the problem of cracking may possibly be overcome by incorporating a layer of
polythene sheet at the top of the subgrade earthworks and laying a thin layer of sand or
crushed fines, before the subbase.
Rockfill can be used providing that boulders no greater than 0.2m3 (600mm size) are used
and that this material is not placed within 600mm of formation level.
If it is possible the best materials, either from cuttings or borrow pits, should be reserved for
the upper layers of fill.
4.2.3 Slope stability
Side slopes for embankments up to 8m height, resting on non-saturated soils, are normally
constructed between 1:1.5 and 1:3, (vertical:horizontal) as follows:
Table 4.2 Slope Angles for Embankment Materials
Material
Sands, cohesionless
Other materials
Recommended
slope
angle, vertical:horizontal
1:3 if height ≤ 1m
1:2 if height > 1m
1:3 if height ≤ 1m
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1:2 if height >1 but < 3m
1:1.5 if height >3 but < 10m
Embankments higher than 10m or founded on soft, wet materials should be subject to
individual specialist analysis.
4.2.4 Placing and compaction of fill
Compaction increases the density of a material by expelling air from the voids and bringing
the individual particles in closer contact. This increases the shear resistance and reduces
settlement. Therefore, soils in embankments and cuttings are usually compacted using
special equipment, such as rollers, tampers and vibrators; and the success of compaction
depends on the soil type and in situ moisture content, the type of compaction equipment and
the energy applied. It is therefore essential that laboratory tests are carried out beforehand
to determine the dry density/moisture content characteristics of the candidate soils and to
define the achievable densities.
Uniformity of compaction is of prime importance in preventing uneven settlement. Although
some settlement can be tolerated, it must be minimized, especially on the approaches to
bridges and culverts where adequate compaction is essential.
For construction on level ground, all soft and organic material must be removed and hollows
filled to obtain a level surface to receive the fill. Any backfilling required to achieve a level
surface shall be compacted to a dry density equivalent to 93% of BS Heavy (AASHTO
T180), corresponding to about 100% of BS Light.
For construction on sloping ground, where the slope is greater than 1:3 (vertical:horizontal),
horizontal benches shall be cut into the sloping ground. Immediately on completion of this
operation, the whole of the area to receive the fill shall be compacted to 93% of BS Heavy
(AASHTO T180) to a depth of 150mm. The time between preparing the area and placing the
fill shall be kept to a minimum to conserve the moisture.
Fill shall normally be placed in layers of compacted thickness up to 250 mm. Thicker layers
may be permitted only where trial sections have proved that the required compaction can
readily be achieved over the full layer depth.
The minimum layer thickness shall be twice the maximum particle size of the fill material.
Normally, the layers of fill material shall be compacted throughout to a minimum of 93% of
BS Heavy (AASHTO T180), except for the uppermost 150mm which shall be compacted to a
minimum of 95% of BS Heavy (AASHTO T180).
The British Standard Vibrating Hammer Test, BS 1377, Part 4 (1990) shall be used for noncohesive soils and a minimum level of 93% of maximum density shall be specified for the
lower layers and a minimum of 95% of maximum density for the upper layers.
For very high fills, higher compaction may be required to minimize settlement.
The moisture content of the material shall be adjusted so that the above specified
compaction levels are attained. In situ moisture contents below the Optimum Moisture
Content of BS Heavy can be accepted, provided that the compaction equipment and method
are sufficient to achieve the specified compaction level. In arid areas, compacting the
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2009
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material in a very dry state can be effective and economical. However, research has shown
that dry-compacted material has more air voids than the equivalent material compacted at
the optimum moisture content and can be loose even at high density, needing to be
confined. Nevertheless, dry-compacted non-plastic materials have given good performance
in the arid areas on N Kenya.
Normal laboratory compaction tests cannot be accurately carried out on materials containing
a high proportion, approximately 25% or more, of particles greater than 40mm size. On such
materials the minimum density and appropriate moisture content required shall be
determined from site compaction trials.
In the UK compaction requirements are usually specified by means of a method specification
which eliminates the need for in situ density tests. Where fairly homogenous materials are
used, the compaction requirements may consist of a method specification with the
parameters being fixed after full scale compaction trials:




Maximum thickness of compacted layer
Characteristics of the compacting equipment
Number of passes for each roller
The permissible range of moisture content
When rockfill is used it shall be placed at the bottom of the embankment. The largest sizes
of rock shall be placed in layers of maximum compacted thickness of 1m. The interstices
shall then be filled with smaller rocks, spalls and approved finer material. The whole layer
shall be compacted until the interstices are completely filled or until the required settlement
is obtained. Heavy vibratory rollers are generally the most suitable machines for compacting
rockfill.
It is most important that the specified compaction is achieved over the whole width of the
embankment. Loose material left on the slopes may absorb water and endanger the stability
of the slopes.
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2009
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5 Drainage and Erosion Control
5.1 Drainage of Surface Water
This chapter deals with the drainage of surface and ground water, and methods of protecting
slopes and ditches from erosion. Cross drainage (culverts) is dealt with in Part VI of the
Road Design Manual (Bridge Design).
5.1.1 Side Ditches
The design of these ditches is covered in Part I of the Road Design Manual (Geometric
Design of Rural Roads) where standard cross sections for different terrain and
gradient/capacity curves are given.
5.1.2 Cut Off Ditches
It is usually desirable to construct a cut-off ditch at the top of cutting slopes to prevent water
flowing down the face. The preferred type, consisting of a combined ditch and bank, is
detailed in Part 1 of the Road Design Manual (Geometric Design of Rural Roads). The
moderate slopes of 1 vertical: 2 horizontal used in this document have been chosen to allow
the inevitable movement of pedestrians and livestock with as little damage to the ditch as
possible.
5.1.3 Discharge Channels
Depending on topographic conditions it is sometimes necessary to collect water at the top of
either a cutting or an embankment and discharge it down the slope. For this purpose
discharge channels shall be constructed and lined with masonry, concrete or metal. The
usual dimensions are 400 mm wide by 400 mm deep. If half-round channel elements are
used the diameter should normally be 500 mm.
5.1.4 Collection of Water in Embankments
On embankments, where water is to be discharged down the side slopes in discharge
channels, it is necessary to lead all water to the tops of these channels. This can be
achieved by some form of kerbing or a recessed channel.
The kerbing can be formed from masonry, precast concrete units or in-situ concrete. The
channel can be formed from precast concrete or metal channels, with an internal diameter in
the range 300 to 400 mm.
For safety reasons these features should be placed outside the edge of the surfacing. Where
a crash barrier is installed the kerbing or channel should be installed immediately in front of
the supports, on the traffic side.
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5.1.5 Embankment Toe Ditches
At the base of embankments, toe ditches may be necessary to remove water from the
vicinity of the embankment or to prevent erosion of the fill. They shou1d be designed on
similar principles to side ditches mentioned in Section 5.1.1 above.
5.2 Drainage of Ground Water
Ground water may be encountered in the following situations:

in cuttings, a water table with a level above or near formation, or springs, and

in low-lying or poorly drained flat areas, a water table near formation, likely to affect
the subgrade by capillary rise.
5.2.1 Drainage Remedies
5.2.1.1 Choice of proper alignment
The best expedient for the prevention of drainage problems is carrying out a proper survey
of the areas concerned and selecting both vertical and horizontal alignments so that the
formation is as far away as practicable from water tables and springs. In particular, in lowlying or poorly drained areas, it is necessary that the road be raised by means of an
embankment to avoid surface flooding.
5.2.1.2 Subsoil drains
Longitudinal subsoil drains can be used to lower a water table. These will normally consist of
porous concrete, open jointed or perforated pipe laid in a trench with a surround and backfill
of free-draining material, e.g. graded crushed stone (maximum size : 60 mm), clean coarse
gravel or sand. The pipe size will depend on the expected flow of water but will generally not
be less than 100 mm internal diameter. The depth of the trench will depend on the level of
the water table and the permeability of the soil but normally it should be at least 1metre
deeper than the formation level and 500 mm wide.
In some cases where it is necessary to prevent surface water from entering subsoil drains,
the upper 500 mm of the trench shall be backfilled with impermeable clayey material.
If the surrounding ground is likely to squeeze or wash into the free-draining material, filter
protection is required. This can be achieved by placing filter material as free-draining
material in the trench.
Filter materials shall comply with the following requirements:
5*S15<F15<5.S85
Where:
 F15 is the sieve size, in mm through which 15% by weight of the filter material passes.

S15 is the sieve size, in mm through which 15% by weight of the natural soil passes.
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585 is the sieve size, in mm in which 85% by weight of the natural soi1 passes.
It is important that the pipe be surrounded by filter material to prevent fines from clogging the
openings.
A non-woven gee-fabric of an approved type may be placed around the draining material to
prevent silt or fine particles from being washed into it. It may also be useful to place nonwoven gee-fabric around the pipe. The effective pore size of the fabric should comply with
the above filter criteria.
Where the flow of water is small and where non-woven geo-fabric is placed around the
draining material, it may be unnecessary to place a pipe.
Where pipe drains are used, inspection chambers with silt traps shall be constructed every
100 m along straight sections and at every change in direction. These will enable the pipe to
be rodded or flushed out.
5.2.1.3 Blanket drains
Blanket drains are used to remove seepage water appearing in the base of cuttings or in the
subgrade. The blanket shall consist of a filter layer in contact with the soil, and a coarser
collector layer. Non-woven gee-fabric may also be used, to prevent fines from blocking the
draining layer. Protection by filter layers or non-woven geo-fabric may be required on both
sides of the blanket drain.
5.2.1.4 Seepage Remedies
If during construction unanticipated local seepages or springs are encountered in cuttings
they may be controlled by either a counterfort drain or sub-horizontal well. In it simplest form
a counterfort drain consists of an excavated “slot” or deep trench running into the cut slope,
which is then backfilled with free-draining material and in large cases a porous pipe.
The filter criteria already stated will apply and some arrangement must be made to lead
away the intercepted water. Geo-fabric can also be used as already described.
Sub-horizontal wells are formed by drilling into the cut slope at a slight upward angle to
intercept water-bearing strata. The hole is then lined with a slotted or perforated pipe to keep
it open and to carry the water out. Usual diameters range from 50 to 100 mm and lengths
may reach 50m.
5.3 Erosion Control
Erosion problems may occur on the side slopes of embankments or cuttings, gravel
shoulders or at any other point where surface run-off concentrated or a spring occurs. The
obvious remedies are therefore well-designed surface and sub-surface drainage features
and appropriate slope angles for the soils and rocks present. This last measure is
problematic as there is no standard test to assess “erodibility”. The best guidance would be
obtained from observations of actual road sections, assuming these exist.
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2009
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Various surface protection systems can be used in conjunction with the above.
5.3.1 Protection of Slopes
5.3.1.1 Topsoiling and Grassing
Sprigs of indigenous “runner” type, grass may be planted on slopes by one or two methods:



The slope shall be covered with a layer of fine topsoi1 free of stones greater than
50mm. The minimum thickness should be 75mm. The layer shall then be planted
with grass
Sprigs of grass shall be planted at approximately 200 mm centres in pockets of
topsoil, 75 mm deep.
Planting should be carried out at the beginning of a rainy season.
5.3.1.2 Surface treatments with seeds and fertilizers
When difficulties are anticipated in establishing a healthy growth of grass on a sterile soil, a
mixture of grass seeds and fertilizer may be applied. This can be done either as a wet or dry
process. In the former process grass seed, fertilizer, mulch material and water are mixed to
form slurry which is then sprayed onto the ground. In the dry process grass seed and
fertilizer are mixed and applied to the ground, followed by watering and possible application
of mulching material.
5.3.1.3 Gravel or stone blanketing
Erodible materials may be protected by placing coverings of gravel or stone blankets. The
blanketing material should have a maximum size of 40 mm and he placed in an even layer of
at least 75 mm.
5.3.1.4 Fascines
Placing fascines or branches over the most vulnerable areas, generally combined with
some form of grass planting, will help stabilize the slope until it is covered by grass or other
vegetation.
5.3.1.4.1 Serrated slopes
Serrated slopes aid in the establishment of vegetation. Serrations may be constructed in any
material that is rippable or that will hold a vertical or subvertical face for a few weeks, until
vegetation becomes established.
5.3.1.4.2 Other protective works
More costly types of protection, such as stone pitching (possibly grouted), gabions, masonry
or placing of concrete may also be used, but, in general, they are economically justified only
where the overall slope stability has to be improved.
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5.3.2 Protection of Ditches and Channels
5.3.2.1 Critical Length of unlined ditches
The critical length of unlined ditches must be determined, with regard to erosion control. The
critical length is defined as the maximum length of unlined ditch, in which water velocities do
not give rise to erosion.
The maximum velocity of water can be calculated from the slope, shape and dimensions of
the ditch, volume of water and from the roughness coefficient of the material. Knowing the
maximum permissible velocity for each type of material, the maximum length of ditch in this
material can then be determined. The recommended maximum permissible velocities for
different types of material are as follows:
Table 5.1: Maximum velocity of water flow
Material
Max.
permissible
velocity (m/s)
Fine sand
Silt – Coarse sand
Silty Clay – fine gravel
Stiff clay
Coarse Gravel
Soft rock – Conglomerate
Hard rock – Masonry - Concrete
0.3*
0.4 – 0.6*
0.5 -0.8*
0.9 – 1.3
1.2 – 1.7
1.8 – 2.5
3.0+
5.3.2.2
*Where the materials are
grassed, the maximum
permissible velocity is of
the order of 1.5 m/s if a
good cover is provided and
1.1m/s if a sparse cover is
provided.
Methods of protection
Sections of ditch beyond the critical length must be protected from erosion by lining. The
following methods may be used:







Grassing
Turfing
Stone pitching (possibly grouted)
Placing of masonry
Concreting
Reducing the gradient and constructing steps (the steps must be paved)
Placing velocity breakers
5.3.2.3 Sedimentation Control
If water velocities are too low sedimentation may occur. Ditches and drains should therefore
be given sufficient gradient everywhere, in so far as topography and erosion control will
permit. Sedimentation velocities for a few types of material are approximately the following:





Silt
Fine sand
Coarse sand
Fine gravel
Gravel
0.08 m/s
0.15 m/s
0.20 m/s
0.30 m/s
0.65 m/s
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2009
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6 Subgrade
The aim of the design process is to protect the bearing capacity of the in situ subgrade
material in order that the road pavement will be able to fulfill its service objective over the
design period. The bearing capacity and quality of the subgrade (or roadbed or fill) is of
prime importance in the selection of pavement type and is improved by overlaying it with
layers of material to achieve an integrated and structurally balanced system.
6.1 Subgrade Classes
For practical purposes the design subgrade bearing capacity or strength is expressed as the
‘California Bearing Ratio’, or CBR, in line with general practice. It has been proved for
Kenyan soils that there is good correlation between the CBR and the elastic modulus. A
survey of Kenyan subgrade soils showed that they can be grouped into six bearing capacity
classes, S1 to S6, shown in Table 6.1, corresponding to values obtained on materials of the
same type along homogeneous sections of road. The CBR range reflects natural variations
in the soil and the normal scatter of test results:
Table 6.1: Subgrade Classes
Soil Class
CBR
Range
S1
S2
S3
S4
S5
S6
2
3 to 4
5 to 7
8 to 14
15 to 29
30+
Modulus,
MPa (of
median
value)
15
25
50
80
125
>250
Notes:
1. No accommodation for CBR values < 2 has been made, because it is inappropriate
to lay a pavement on soils of such poor bearing capacity. Such weak soils are
saturated expansive clays, saturated fine silts or compressible (swampy) soils, e.g.
mud, soft clay, etc. These materials will, where possible, either be improved by
chemical or/and mechanical stabilization and re-classified; or they will be removed
and other cover applied.
2. The use of S1 soils as direct support for the pavement should be avoided and, where
practicable, such poor quality soils should be excavated and replaced, or improved,
or covered with an improved subgrade.
3. The CBR range of S5 is fairly wide. This is because Class S5 is either gravelly
material or unsoaked soil, the CBR values of which always show considerable
scatter. Furthermore, the difference in the pavement thickness required is
comparatively small when the subgrade bearing strength varies from the lower to the
upper limit of this Class.
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4. S6 covers all subgrade materials having a CBR > 30 and which comply with the
plasticity requirements for natural materials for subbase. In such cases, no subbase
is required. No class of higher bearing capacity has been considered as such natural
subgrade materials are rare and as a roadbase is always necessary to provide a
homogeneous and uniform layer. However, the reconstruction of roads sometimes
results in a new structure being constructed on an old road, the surface of which is
considered as subgrade and whose residual strength is often considerably greater
than CBR 30.
5. Where the subgrade CBR values are very variable the designer should balance the
cost of having very short sections of different subgrade categories against a
conservative design taking account of the worst conditions encountered over longer
sections.
6.2 Classification of Kenyan Subgrades
The materials listed in Table 6.2 cover almost all the natural subgrade materials found in
Kenya, classified according to bearing strength:
Table 6.2: Kenyan Natural Subgrades
Bearing Strength Class
After 4 days At OMC
soak
(BS Light )
Black cotton soils
S1
S5
Micaceous silt (weathered rock)
S1
S3
Other residual silt (weathered rock)
S2
S4
Red clays (coffee soils)
S3
S5
Sandy clays from volcanics
S3/S4
S5
Ash and pumice*
S3/S4
S5
Silty loam from gneiss or granite
S4
S5
Calcareous sandy soil
S4
S5
Sandy clay on PreCambrian basement
S4
S5
Clayey sand on PreCambrian basement S4/S5
S5/S6
Dune sand
S4
S4/S5
Coastal sand
S4
S5
Weathered lava
S4/S5
S5/S6
Quartz gravel
S4/S6
S5/S6
Soft weathered volcanic tuff
S4/S6
S5/S6
Calcareous gravel
S4/S6
S5/S6
Laterite gravel
S5/S6
S6
Coral gravel
S5/S6
S6
1. Material Type
* The ash and pumice soils tend to have a low field density, thus a correspondingly very low
maximum dry density than anticipated from the measured CBR value. Such soi1s, with a
Standard Compaction MDD less than 1.4 mg/m³ should not be used in construction unless
improved.
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6.3 Determining the Subgrade Strength
6.3.1 Recommended Subgrade CBR Test Procedure
The strength of the subgrade depends on the type of material, its density and the prevailing
moisture content. For each type of material, it is therefore necessary to determine the
relative compaction that could be obtained in-situ and the maximum moisture content likely
to occur in the subgrade.
In order to obtain a complete knowledge of the relationship between density, moisture
content and CBR, a “6 point” CBR test should be carried out on a representative sample of
each type of subgrade material encountered. The tests are conducted in the following way:
The material shall be compacted at 3 different levels of compaction. The samples shall be
prepared at the moisture content expected at the time of field compaction. At each level of
compaction, one CBR shall also be measured on one soaked specimen. The time of soaking
will depend on the anticipated wettest conditions. The amount of water absorbed during
soaking and the eventual swell shall also be measured.
The above method enables an estimate to be made of the subgrade CBR at different
densities and thus helps in determining the relative compaction required. It also indicates the
loss of strength which soaking may cause. A full particle size analysis should also be done
on each representative sample.
6.3.2 Subgrade Compaction Requirements
The upper 150mm of the subgrade shall be compacted to a dry density corresponding to a
minimum of 95% of BS Heavy (or AASHTO T180). The lower 150mm shall be compacted to
a minimum of 93% of BS Heavy, equivalent to about 100% of BS Light (or AASHTO T99)
These criteria apply to cuttings where there is no improved subgrade, and on all
embankment fills.
In cuttings where an improved subgrade is placed, the upper 150mm of the cutting surface
material, prior to placing the improved subgrade layer(s), shall be compacted to a minimum
of 93% of BS Heavy and the lower 150mm to a minimum of 90% of BS Heavy, equivalent to
about 95% of BS Light.
All improved subgrade, compacted to a dry density of 95% of BS Heavy, shall have a
minimum soaked CBR of either 7% or 15%, as described later in this Chapter.
The British Standard Vibrating Hammer Test, BS 1377, Part 4 (1990) shall be used for noncohesive soils and a minimum level of 93% of maximum density shall be specified for the
lower layers and a minimum of 95% of maximum density for the upper layers.
The maximum compacted thickness which shall be laid at one time is generally 200 mm.
The moisture content shall be adjusted if possible in order that the required relative
compaction is obtained, and at the time of compaction it shall not exceed 105% of the
Optimum Moisture Content of BS Heavy.
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Dry compaction may be possible in arid areas with gravelly or sandy materials but research
in N Kenya has shown that they remain loose even at target densities and need to be
confined (by the pavement layers). Dry compaction of clays is not normally satisfactory
because the swell of such materials is abnormally high when allowed to absorb water.
It is advantageous to obtain relative compactions higher than the above figures, since
compaction not only improves the subqrade bearing strenqth, but also reduces permeability.
Increasing the subgrade bearing strength provides a better platform for the construction of
the pavement layers. This applies, in particular, to clayey sands, silty sands and granular
materials, the coarse particles of which are hard and do not crumble under heavy
compaction.
6.3.3 Estimating the Subgrade Moisture Content
The moisture content of the subgrade soil under the road pavement at any given time will
depend on the following factors:






local climate
depth of the water table
type of soil
topography and drainage
permeability of the pavement materials
permeability of the shoulders
A study of Kenyan subgrade moisture conditions revealed the general relationships between
mean annual rainfall, soil type, drainage conditions and subgrade moisture content which
are given in the Table below.
Table 6.3: Relation between rainfall, soil type and subgrade moisture
Mean
Annual
Rainfall,
mm
> 500
(‘Wet’)
< 500
(‘Dry’)
Water
Table
Soil Type
Deep
Clays
Silts
Drainage
Subgrade
Content
or Impermeable
pavement,
reasonable
surface
drainage
Permeable pavement, poor
surface drainage
Deep or Sands
or
none
sandy clays
Moisture
Averages less than
OMC; maximum 3% >
OMC
Often exceeds OMC;
maximum saturated
Well
below
OMC;
maximum
equal
to
OMC
Note: 1 OMC at BS Light (AASHTO T99)
2 Permeable pavements are pavements constructed with open-textured materials or
deteriorated pavements showing surfacing and/or base cracking
6.3.4 Determining the Subgrade Design Strength
In the absence of a direct and accurate evaluation of the ultimate subgrade moisture
content, the subgrade strength shall be determined as follows:
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
if the mean annual rainfall is more than 500mm, subgrade strength shall be
determined from the 4 day soaked CBR at the specified compaction level of BS
Heavy (AASHTO T180. In saturated conditions it may be necessary to adopt the
OMC of BS Light (AASHTO T99) compaction

if the mean annual rainfall is less than 500 mm, the subgrade strength may be
determined as the CBR at the OMC of BS Heavy. However, such a design shall only
be permitted only where it has been established that no prolonged soaking may
occur and, for this purpose, consideration shall also be given to factors such as
permeability of the natural ground and the topography, ie the ability of water to drain
rapidly under all circumstances.
6.4 Subgrade Requirements for Pavement Design
6.4.1 Materials Suitable for Pavement Support
Materials directly supporting the pavement shall normally comply with the following:



CBR ≈ 15% at specified compaction, normally 95% of BS Heavy (AASHTO T180)
Swell < 2% at 100% MDD (Modified Compaction) and 4 days soak
Organic matter < 3% (percentage by weight)
Thus, all situations where the natural subgrade is S4 or less will require placement of an
improved subgrade. The nature and arrangement of the improved subgrade will depend on
the CBR of the natural subgrade and the available materials but the intention is to reinforce
the natural subgrade with improved subgrade layers of CBR 7 and CBR 15 in the manner
described graphically in the catalogues presented in Chapter 9.
Class S1 soils (CBR 2 or less) will thus either require stabilization or removal and
replacement with better quality material.
Class S5 (CBR 15 to 29) and S6 (CBR 30 or more) soils will not require an improved
subgrade.
6.4.2 Improved Subgrade
Placing an improved subgrade not only increases the bearing strength of the pavement
support but also:





protects the upper layers of earthworks against adverse weather conditions
(protection against soaking and shrinkage),
facilitates the movement of construction traffic,
permits more effective compaction of the pavement layers,
reduces the variation in the subgrade bearing strength, and
prevents pollution of open-textured sub-bases by plastic fines from the natural
subgrade.
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2009
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6.4.3 Lime Treated Subgrade
Treatment of subgrade soils with lime is encouraged because otherwise they may have to be
removed and disposed. It may be effective in the following cases:

where the soils are excessively clayey and no better material is economically
available; treatment with hydrated lime may be a cost-effective solution.

where the soils are excessively wet and cannot expeditiously be dried out; treatment
with quicklime may allow construction to proceed and provide a markedly stronger
subgrade.
Specifications for lime treatment are given in Chapter 7.
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2009
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7 Pavement Materials
Pavement materials consist of subbases, bases and surfacing. Recommendations for their
properties are described below.
7.1 Subbases
The functions of the subbase are to act as a construction platform for the upper pavement
layers and as a separation layer between the subgrade and the roadbase. In certain
circumstances it may also act as a drainage layer, especially in concrete roads. The
selection of a suitable subbase material will, therefore, depend on the design function of the
layer and the anticipated moisture conditions, both at construction and in service.
7.1.1 Natural Materials
Natural materials in Kenya suitable for subbases can be lateritic, quartzitic or calcareous
gravels, some forms of soft stone, coralstone (on the coast), clayey and silty sands, and
conglomerate. When used as subbase they shall invariably have low plasticity and should
comply with the other requirements in Table 7.1.
Table 7.1: Subbase: natural materials: specifications
Material Properties
CBR,%, 4
Swell
Atterberg Limits
(KS 999: Part 2)
LL, max %3
PI, max %, or
LS, max %
Grading Modulus2
Particle Size
Field density
Layer restrictions
Notes:
1.
2.
3.
4.
Material Class
Min 30 (soaked)
Max 1% at BS Heavy
General material
Coral gravel
Wet1
Dry
Wet
Dry
45
50
45
55
16
20
18
24
8
10
9
12
Min 1.2
Max size: ⅔ of compacted layer
Min 95% of BS Heavy
Min thickness compacted layer:
100mm
Max thickness compacted in one
layer: 200mm
‘Wet’ and ‘Dry’ refers to rainfall zones corresponding to >500mm and <500mm per year respectively
Grading Modulus is [300-(% passing 2mm)-(% passing 0.425mm)-(% passing 0.075mm)]/100
Atterberg Limits: LL, Liquid Limit; PI, Plasticity Index; LS, Linear Shrinkage
CBR values measured at specified field density (normally 95% of BS Heavy)
Materials derived from weathered igneous rock, particularly basic rocks like basalt, phonolite
and dolerite, which are common in Kenya, often contain decomposed minerals and can be of
very poor quality. Their use should be avoided if possible; at least before using for the first
time they should be subject to a petrographic analysis by a specialist to determine the
proportion of weathered minerals. If the proportion of weathered minerals is greater than
50% they should definitely be avoided. Weathered micaceous rocks, such as schists and
some gneisses, are likely to give rise to similar difficulties.
‘Soft stone’ can also be used for subbase. A poor quality soft stone shall be assessed and
used in accordance with the requirements for natural gravel.
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7.1.2 Graded Crushed Stone
Graded crushed stone may be used as subbase material where no suitable natural gravel
can be found. The material requirements, traffic limitations and construction procedures are
summarized in Table 7.3. It shall comply with the following criteria:
Table 7.2: Graded Crushed Stone: Specifications for Subbase
Material Properties
Compaction, Vibrating
hammer
SG of compacted layer
Stone cleanliness
Stone toughness
Stone durability
Stone particle shape
Grading, sieve size,
mm
75
63
50
37.5
28
20
10
6.3
2
1
0.425
0.075
Particle Size
Compaction moisture
content
Layer restrictions



Material Class
GCS2
Average 96% of MDD
No result < 94% of MDD
Average dry density min 82% of SG
No result < 80% of SG
SEV (AASHTO T176) min 30
PI max 6 (wet area): max 8 (dry area)
ACV max 32, or
TFV min 90 (dry) and 70(wet)
Water Absorption max 2%
(if in doubt then max Mg2SO4
Soundness 18%; KS 1238-20)
Flakiness Index max 35%
Envelope 0/40
Envelope 0/60
100
95-100
100
85-100
90-100
75-95
75-95
60-87
60-90
50-80
35-75
30-67
25-63
23-58
15-45
13-40
8-35
7-32
4-23
4-20
0-12
0-10
Max size shall be ⅔ of compacted layer
Between 80-105% of OMC (Vibrating
hammer)
Min thickness compacted layer: 100mm
(0/40); 150mm(0/60)
Max thickness compacted in one layer:
200mm
‘Crusher-run’ should be used as much as possible; the grading envelopes given
cover the crusher-runs usually obtained. For softer stones a grading at the design
stage which is coarser than the envelope may be acceptable.
Material shall consist of crushed stone, free of clay, organic and any other
deleterious material
Grading after compaction shall be considered: compaction may cause further
crushing and produce additional fines in the case of soft stone
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Generally, the fines (% passing 0.425mm sieve) shall be only slightly plastic: in dry
areas, the plasticity may be further relaxed (PI<8)
Graded crushed stone should always be kept moist during handling, transporting and
laying and should not be stockpiled in heaps higher than 5 m, in order to avoid
segregation
Care must be taken to ensure that the layer edges are properly compacted
No visible movement under a steel wheeled roller applying at least 5000 Kg/metre
width of roll.
7.1.3 Stabilised Natural Materials
Natural gravels, sands and clayey sands, which do not meet the subbase requirements
given in Table 7.1, may be stabilised with cement or lime. The choice of stabilizer depends
on the percentage of fines (material passing 0.425 mm sieve) and the PI; reference is made
to Section 7.2.3, Table 7.4.
The materials suitable for stabilisation and the stabilized material shall comply with the
criteria given in Table 7.5, category CS.
Curing shall be carried out by covering the surface with approved plastic sheeting, moist soil,
straw and/or keeping the surface damp by frequent applications of a light spray of water.
7.1.3.1 Cement bound granular subbases
These materials are intended to perform as rigid monolithic pavement layers to increase the
dynamic stiffness of the pavement. For traffic classes T6 and T7 a DBM base is required:
this is a rigid layer which needs a rigid layer to be constructed on, otherwise it will break.
Typically, they are graded crushed stone, conforming to the requirements in Table 7.3, to
which is added 2% to 3% cement in a plant mixer. The strength of this material should
conform to CB1 or CB2 in Table 7.6.
7.2 Bases
The main function of the base is to act as the load-spreading layer of the road pavement.
Therefore, only strong materials will be suitable. Bases fall into two categories: unbound and
bound. Unbound bases, such as natural gravels and crushed stone, rely on their intrinsic
internal friction to develop the necessary bearing capacity. Bound bases have a binder,
either bitumen or cement or lime, which is used to strengthen them and enhance their ability
to reduce the traffic stresses on the layers below.
7.2.1 Natural gravel
Natural gravels meeting the requirements in Table 7.3 are very scarce in Kenya. Lateritic
gravels are not suitable owing to their poor nodule hardness and high plasticity. Weathered
rocks are of even poorer quality. Only quartz gravels and coral gravels are potentially
satisfactory.
It may be advantageous to stabilize them mechanically, by mixing in sand to reduce the
plasticity, or stone (crushed or not) to provide hard, coarse, angular particles. An addition of
up to 30% of sand or stone is regarded, practically and economically, as a maximum.
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Normally the specified field compaction will be 95% of BS Heavy: in practice, compaction up
to 97% of BS Heavy may be considered. However, the bearing strength will be significantly
increased by higher compaction only if the coarse particles are hard enough to resist heavy
compaction without being crushed or pulverized.
Natural gravels are suitable only for Traffic Classes T1 to T5, for they are prone to attrition
and their properties are too variable.
Table 7.3: Specification for Natural Gravel as Base
Material
Properties
CBR,%, at specified field density
Swell
Atterberg Limits
Particle Strength
Grading, sieve size, mm
50
37.5
28
20
10
5
2
1
0.425
0.075
Field Density
Layer restrictions
Material Class
Min 80
Max 0.5% at BS Heavy
General
Coral gravel
LL max 30
LL max 35
PI max 8 (LS max 4) PI max 10 (LS max 5)
PM max 90
ACV max 35, or
TFV min 75 (dry) and min 50 (wet)
Envelope 0/40
100
95-100
80-100
60-100
35-90
20-75
12-50
10-40
7-33
4-20
Min 95% of MDD BS Heavy at 80-105% OMC
Min thickness compacted layer: 125mm
Max thickness compacted in one layer: 200mm
7.2.2 Graded Crushed Stone
Graded crushed stone (GCS), either basaltic rock, gneiss or granite, is the most widely used
base material in Kenya. Recommended specifications are presented in Table 7.4:
Table 7.4: Crushed Stone Specifications for Base
Material Properties
Compaction, Vibrating
hammer, KS 999-4
SG of compacted layer
Stone cleanliness
Stone toughness
Stone durability
Material Class
GCS1
Average 98% of MDD
No result < 96% of MDD
Average dry density min 85% of SG
No result < 82% of SG
SEV (AASHTO T176) min 30
Fines non plastic
ACV max 30, or
TFV min 110 (dry value) and 75(wet
value)
Water Absorption max 1%
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Stone particle shape
Grading, sieve size,
mm
50
37.5
28
20
10
6.3
2
1
0.425
0.075
Particle Size
Layer restrictions






Part 3 - Materials and Pavement
(if doubt then max Mg2SO4 Soundness
18%: KS 1238-20)
Flakiness Index max 30%
0/20
0/30
0/40
% passing
% passing
% passing
100
100
90-100
100
90-100
75-95
90-100
65-95
60-90
60-75
40-70
40-75
40-60
30-55
30-63
30-45
20-40
20-45
15-30
15-32
15-35
13-27
10-24
10-26
4-10
4-10
4-12
Max size shall be ⅔ of compacted layer
Min thickness compacted layer: 125mm
Max thickness compacted in one layer:
200mm
GCS is not considered suitable for Traffic Classes T6 or T7
For Traffic Class T5 the GCS must be entirely crushed: for T4 and lower the GCS
may be semi-crushed
The grading generally required is 0/40 mm but for T4 and T5 a finer grading is
required in order to minimize segregation and provide sufficient stability
GCS should always be kept wet during handling, transporting and laying and should
not be stockpiled in heaps higher than 5 m, to avoid segregation.
Special care must be taken to ensure that the layer edges are always properly
compacted, by providing an extra width or specific lateral abutment
Adding material from another source to achieve the grading is permissible providing it
is passing 5mm sieve, a maximum content of 15%, non plastic and free of
deleterious material
7.2.3 Stabilized materials
7.2.3.1 General
Since there is an indigenous cement and lime industry in Kenya, it can be advantageous to
treat otherwise unsuitable natural materials with cement or lime. It will be only appropriate,
however, when the cost of the treatment is less than the cost of the removal of the
unsuitable material plus replacement by suitable material. Stabilization enhances the
properties of road materials in the following ways:



retention of strength when saturated with water
increased resistance to erosion
increased effective elastic moduli of the layers above the stabilized layer
Possible problems are associated with these desirable effects:


environmental and traffic stresses can cause stabilized layers to crack
cracks can reflect through the surfacing allowing water to enter the structure
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the stabilization reactions are reversible if the stabilized material is exposed to the
atmosphere, resulting in decrease in the strength of the stabilized layers
the stabilization procedure requires skill; for instance, the operation must be carried
out relatively quickly, especially with cement stabilizer, and the resulting mixture of
the stabilizer and natural material must obviously be as homogeneous as possible.
The balancing of these advantages and disadvantages generally limits the amounts of
admixed stabilizer to between 2% and 6%.
There is a significant difference between improved materials and stabilized materials.
Improvement consists of treating materials with a small amount of lime (and cement
sometimes) in order principally to reduce the plasticity and improve the engineering
characteristics. Stabilization consists of treating materials with a sufficient amount of lime or
cement, so that their bearing capacity is significantly increased.
Some soils do not stabilize well. Soils containing more than 1% of organic matter are one
example. Also, if montmorillonite is present amongst the clay minerals, its large volume
changes with changing moisture content may disrupt the stabilized soil.
An indication of the suitability of the soil for stabilization is provided by the Initial
Consumption of Lime test (BS 1924 1990) which determines the amount of lime, or cement,
required to complete the neutralizing reactions and initiate the stabilizing reactions whereby
strength-forming calcium silicates and aluminates are progressively created. Normally, the
amount of stabilizer used in the contract is the ICL value + 1%. Soils containing >0.3% of
sulphate should be avoided for stabilization as there have been instances of deleterious
reactions between the sulphate and the strength-forming calcium silicates and aluminates.
7.2.3.2 Choice and Quality of Stabilizer
The choice of stabilizer depends on the plasticity of the natural material, see Table 7.5:
Table 7.5: Choice of Stabiliser
% Passing 75µm sieve
Less than 25%***
More than 25%
PI
< 6, or
PI x (% passing 75µm) <
60
6 to 10
> 10
< 10
10 to 20
> 20
Best Stabiliser
Cement*
Cement preferred
Cement or lime
Cement preferred
Cement or lime
Lime preferred**
* Lime requires the presence of clay minerals for reaction to take place and is therefore
suitable only for materials with high PI.
** Cement can be used to stabilize materials of high PI provided they are treated
beforehand with approx 2% of lime, which reduces plasticity and improves workability.
*** Desert sands do not usually stabilize well with cement. They are more effectively
stabilized with bitumen (see later).
Cement shall conform to KS EAS 18-1 ’Cement: Composition, Specifications and Conformity
Criteria, Part I. Lime shall conform to KS 02-97 1982 ‘Specification and Methods of Test for
Building Limes’. The water used shall contain less than of 2000ppm sulphate and 1000ppm
of chloride. In addition, unless cement and lime are properly stored and used in a fresh
condition, their activity will be substantially reduced. Even if properly stored they will suffer a
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progressive loss in strength with time: after 3 months, 20%; after 6 months, 30%; after 1
year, 40%.
7.2.3.3 Design Criteria
Typical design criteria are presented in Table 7.6:
Table 7.6: Design Criteria for Stabilised Materials
Material properties
After stabilisation:
CBR, min (95% MDD BS
Heavy)
UCS, MPa
Before stabilisation:
Atterberg Limits, max values
LL
PI
LS
Aggregate strength TFVdry ,
KS 1238-12
Grading, Sieve Size, mm
50
37.5
20
5
2
0.425
0.075
Organic content BS 1377: Part 3
Sulphate (SO3) content BS
1377: Part 3
Layer Thickness
Compaction level






Material class
CB1
CB2
CS1
CS2
70
40
3.0 to 6.0
1.5 to 3.0
1 to 2
Max 1
25
8
4
Minimum
50 kN
30
12
6
-
25
-
25
-
-
-
100
100
85 – 100
80 – 100
60 – 90
55 – 90
30 – 65
25 – 65
20 – 50
15 – 50
10 – 30
10 – 30
5 - 15
5 - 15
Maximum Maximum
0.5%
1.0%
Maximum 0.3%
Maximum 1.0%
Max particle size 1/2 of compacted layer
thickness but not >50mm.
97% of BS Heavy
95% of BS Heavy
It is recommended that materials should have a Uniformity Coefficient of at least
5. (Uniformity Coefficient = Ratio of Sieve size through which 60% of material
passes to Sieve size through which 10% of material passes)
For cement the Unconfined Compression Strength (UCS) shall be measured on
150mm cubes after 7 days airtight moist curing and 7 days soaking in water at
27 ± 2˚C in accordance with BS 1924: Part 2. The compaction degree of the
specimens shall be 97% MDD of BS Heavy Compaction.
For each source of material to be stabilised the UCS shall also be determined
on specimens cured airtight for 14 days. The ratio of UCS measured after 7
days curing and 7 days soaking and the UCS measured on specimen cured for
14 days shall not be less than 75%.
For lime the curing period is 21 days, moist cure, followed by 7 days soaking,
unless otherwise instructed by the Engineer.
The strength of stabilised subbase, CS, can optionally be measured by the CBR
test.
Testing shall be carried out according to BS 1924, Part 2
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7.2.3.4 Practical Considerations
Plant mixing is the most effective way of homogenization of stabilizer and material but the
plant must be close to the site to avoid delays. However, it is a costly operation compared to
mix-in-place methods and only materials relatively low in plasticity can be treated (with a
Plasticity Modulus <700) in this manner. It is estimated that the variation of stabilizer content
with plant mixing compared to mix-in-place is ± 10% and ± 20% respectively.
If cement stabilizer is used, mixing, compaction and finishing shall be completed within 2
hours and the treated layer shall be protected against evaporation within 4 hours. If lime
stabilizer is used, the respective times allowed are 4 hours and 8 hours.
Protection and curing shall normally be achieved by the application of a bitumen seal coat. A
prime coat is normally inadequate to prevent evaporation. Other methods that can be used
are plastic sheeting, moist soil, straw or by keeping the surface damp by spraying.
No vehicle shall be allowed on a cement treated layer for the first 7 days after compaction.
If two or more layers are constructed it is important to prevent carbonation occurring at the
surface of the bottom layer. The layer thickness should be between 100mm and 200mm.
7.2.4 Lean Concrete
As its name implies, this is a weak concrete containing about 5% to 7% of cement
(compared to 12% to 13% cement for proper concrete) and having a compressive strength
of 10MPa to 15MPa at 7 days. Aggregate parameters are listed in Table 7.3, the quality of
the aggregate the same as crushed stone for base. Ordinary Portland Cement can be used
but there are many varieties of cement produced in Kenya and it may be worthwhile
considering the use of slow setting and/or low heat of hydration cements to allow more time
for construction and to offset the shrinkage problems.
It is paver-laid, in thick layers of minimum 150mm, with the same conditions for finishing and
protection as stabilized materials, discussed above.
Both cement bound granular subbases/bases and lean concrete are laid without joints. They
inevitably crack to relieve the stresses generated during cement hydration and, unless the
material is covered with a substantial thickness of upper base and surfacing, the cracks will
reflect through to the surface and allow the weather to penetrate. Since the main purpose of
these layers is to enhance the strength of flexible pavements on weak foundations, the
cracks are a disabling feature.
7.2.5 Sand Bitumen Mixes
Comprehensive guidance is provided by Southern African Bitumen Association (Sabita) of
South Africa, including some information provided free on the internet: www.sabita.co.za.
Bitumen stabilization is appropriate in hot, dry areas, such as NE Kenya. Sandy soils
predominate in these areas and, indeed, may be the only readily available material. Water is
not needed at any stage in construction. Also, it is often the case that the bearing capacity of
the subgrade is high, requiring only thin bases. Bitumen stabilized mixes are usually not
robust and suitable only for Traffic Classes T1 or T2, providing there are few heavy
commercial vehicles.
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Bitumen stabilized bases always require bituminous surfacings to prevent abrasion by traffic
and to protect the bitumen from the harsh environment. A surface dressing is the most
appropriate because of the thick bitumen film applied although other surfacings such as a
Cape Seal or Otta Seal may also be suitable.
The sand used should be well graded, preferably non-plastic and a typical target grading is
shown in Table 7.7:
Table 7.7: Sand Specification for Sand Bitumen Mixes
Sieve Size,mm
5
2
1
0.6
0.425
0.075
% passing
100
90 – 100
90 – 100
80 – 90
70 – 80
10 – 20
However, most natural desert sands are single-sized and, as such, are unsuitable by
themselves for bitumen stabilization because of the lack of mechanical stability. It may be
necessary to find sands with different gradings and mix to obtain a more satisfactory
grading. The quantity of fines (<0.075mm) should be limited to the lower end of the grading
in Table 7.7 because the higher the proportion of fines the greater the percentage of bitumen
required for the mix stability.
Bitumen content will range between 3% and 6%, by weight of dry sand, and the more
viscous the bitumen the higher the stability of the mix. Use of penetration grade bitumen (say
Pen 60/70 or 80/100) will produce the highest stabilities but will require heating the sand as
well as the bitumen. Probably the extra energy required to heat the sand is comparable to
the additional cost of using cutback bitumen (say MC 30) or bitumen emulsions (say, anionic
A2 or A3), and then waiting for the flux to evaporate before compacting the mixture. An
approach successfully adopted in South Africa is to use a 60% anionic emulsion, applied at
a 1% to 3% rate (residual bitumen 0.6 to 0.8%) combined with 1% of cement. It is
conjectured that the emulsion initially benefits compaction and then the combined effect of
the bitumen and cement contributes to a long term gain in strength.
Bitumen-sand mixtures can be adequately compacted with 6 to 10 tonne steel rollers. It is
not necessary to test the field densities to which they are compacted but it is important to
check regularly the grading and bitumen content. The Hveem Stabilometer is probably more
suitable than the Marshall test in indicating the influence of sand grading and bitumen
content on the mechanical properties of these mixtures. Marshall design criteria are given in
the Table 7.8:
Table 7.8: Marshall design for Sand Bitumen Mixes
Marshall Parameters
Traffic Class
T1
T2
1kN
1.5kN
Marshall Stability at 600C
(min)
Marshall Flow Value at 600C 2.5mm 2mm
(max)
The bitumen and sand should be mixed in a pugmill type of mixer. The equipment needed is
quite basic for small scale operations. In addition to the mixer and trucks for transportation, a
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bitumen heater (if penetration grade is used) and a steel-wheeled roller are required. With
larger operations, more control is needed: for example, loading hoppers and elevators, large
mixers and a spreader-finisher for laying the material are required.
7.2.6 Dense Bitumen Macadam
Bituminous premixes are produced in plant using aggregates of good quality, hot-mixed,
transported to site and laid and compacted while still hot. The mixes must be designed to
provide high deformation resistance, high fatigue resistance and good durability while being
sufficiently workable during construction to allow satisfactory compaction. The exact
requirements depend on the application, whether as a base or surfacing.
In Kenya dense bitumen macadam, also known as close-graded bitumen macadam, is
normally used for base for heavily trafficked roads of Traffic Class T5 or greater. It is a
‘recipe premix’, ie a mix of bitumen and aggregate which has been proven by experience to
be satisfactory, rather than a mixture which has been designed from mechanical testing
procedures, such as the Marshall test procedure. It is particularly appropriate for having high
stiffness and resistance to deformation but, since the air voids content can be in excess of
5%, it needs a surfacing for protection against the environment.
7.2.6.1 Aggregates
The aggregates used for DBM (and for all bituminous premixes) must be clean, hard and
durable, similar in quality to crushed stone for roadbase. They consist of coarse aggregate,
with particle size greater than 2.36mm; and fine aggregate, with particle size between
2.36mm and 0.075mm. (The definition of fine aggregate is that of the Asphalt Institute; note
that AASHTO define coarse and fine aggregate with reference to the 4.75mm sieve). Premix
also contains a small proportion of filler, consisting of rock fines, cement, or lime.
The coarse and fine aggregates and filler are combined into various grading envelopes
depending on their use. Recommended envelopes and aggregate properties for wearing and
base, or binder, course are listed in Table 7.9:
Table 7.9: Aggregate Specification for Dense Bitumen Macadam
Material Properties
Stone cleanliness
Material Class
Coarse Aggregate
< 5% passing 0.075mm sieve
Stone toughness
Stone durability
ACV max 28
Water Absorption max 2%
(if in doubt then max Mg2SO4
Soundness 18%)
Stone particle shape Flakiness Index max 25%
Adhesion to bitumen Coating > 95%
Grading of
0/30, % passing 0/40, % passing
aggregate mix
sieve size, mm
sieve size, mm
50
100
37.5
100
95-100
28
90-100
70-94
20
71-95
-
The Republic of Kenya – Ministry of Roads 53
Fine Aggregate
SEV (AASHTO T176) min 45
(for material passing 4.75mm
sieve)
Mg2SO4 Soundness max 20%
Coating > 95%
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2009
14
10
6.3
2
1
0.300
0.150
0.075
Layer restrictions
Part 3 - Materials and Pavement
58-82
56-76
44-60
44-60
26-40
25-40
20-33
20-33
7-21
7-21
4-15
2-8
2-8
Min thickness compacted layer: 60mm (0/30), 75mm (0/40)
Max thickness of compacted layer: 100mm (0/30), 125mm (0/40)
The filler material should consist of cement, lime or other mineral dust and be non-plastic,
with 100% passing the 0.425mm sieve and a minimum of 75% passing the 0.075mm sieve.
Bulk density in toluene can range from 0.5 to 0.9g/l.
For 0/30 DBM and lower the following Marshall requirements in Table 7.10 are
recommended:
Table 7.10: Recommended Marshall criteria
Design Traffic
(M ESA)
Min Stability
(kN at 600C)
Flow (mm)
Compaction level
(no. of blows)
T1 to T2 T3 to T4 T5
T6 to T7
3.5
6.0
7.0
9.0
2 to 4
2x50
2 to 4
2x75
2 to 4
2 to 4
To refusal To refusal
The bitumen used must be of Penetration grade and meet the requirements in Table 7.11:
Table 7.11: Bitumen Specifications
Test
Min or
Max
Penetration at 250C
Softening Point (0C)
Flash Point (0C)
Solubility in Trichlorethylene, %
TFOT heating for 5hr @ 1630C
 Loss by mass, %
 Penetration (% of original)
 Ductility at 250C
Min
Min
Max
Min
Min
Test
Method
(ASTM)
D5
D36
D92
D2042
D1754
Penetration Grade
80/100
42 to 51
219
99
60/70
46 to 56
232
99
40/50
49 to 59
232
99
0.8
50
75
0.5
54
50
0.5
58
-
Normally in Kenya either 60/70 or 80/100 Penetration grade bitumen is used: however,
40/50 penetration grade bitumen has been used successfully elsewhere in warm climates.
In asphalt production two factors are important:


the minimum mixing temperature should be used to achieve complete coating of the
aggregate but high enough to enable compaction to be completed, and
the mixing temperature must not be elevated above the allowable range to
compensate for long delivery journeys or because of cold weather;
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Control of mix temperature should be based on bitumen viscosity but as a general guide the
maximum temperature limits shown in Table 7.12 for mixing and delivery to the paver should
be adhered to:
Table 7.12: Bitumen Temperature Maxima-Minima
Operation
Mixing (ASTM D 946)
Max.(ºC)
Delivery to paver
(ºC)
Grade of bitumen
40-50
60-70
170
165
80-100
160
150-170
140-160
145-165
Compaction should start at as high a temperature as possible without causing undue
distortion of the mix under the rollers and completed before the mat has cooled to less than
about 90ºC. If necessary, asphalt delivery vehicles should be insulated and covered to
prevent excessive cooling during long journeys or during delays, especially on sites subject
to cold and windy conditions.
No formal design method exists to determine the optimum composition of 0/40 (and some
0/30) basecourses) because the maximum particle size for the Marshall test is 25mm.
Therefore, it is important that trials should be carried out to determine suitable mix
proportions and procedures. Durable mixes require a high degree of compaction and this is
best achieved by specifying density in terms of the maximum theoretical mix density or,
alternatively, by using a modification of the Percentage Refusal Density test with extended
compaction time (BS 598, part 104 (1989). A comprehensive discussion is contained in
Appendix G of TRL Road Note 19.
Mixing times and temperatures should be set at the minimum required to achieve good
coating of the aggregates and satisfactory compaction. Temperatures between 1300C and
1500C are recommended for the mixing plant and between 1200C and 1500C for laying. The
maximum compacted thickness for one layer is 200mm.
The highest bitumen content commensurate with adequate stability should be used.
For the heaviest trafficked roads it is necessary to design the DBM to refusal density
following the procedure outlined for asphaltic concrete below.
7.2.7 Dense Emulsion Macadam
Dense emulsion macadam (aka close graded emulsion macadam) is a cold mixed, cold laid
plant mixture of well graded aggregate and bitumen emulsion and its principal use will be as
a pavement overlay material.
The requirements for aggregate and filler are basically the same as for DBM. The emulsion
can be either slow-setting anionic A3 or slow-acting cationic K3. The amount of residual
bitumen should normally be between 3 and 5% by weight of the dry aggregate.
Dense emulsion macadam may be a cheap alternative to dense bitumen macadam.
Although its strength is only slightly less than that of dense bitumen macadam, no heating or
drying of aggregate and no heating of the binder are required. Moreover, the mixture can be
laid by grader. Dense emulsion macadam should be laid in layers of compacted thickness
not exceeding 150 mm to permit the evaporation of water. Full details of the mix design for
dense emulsion macadam are given in Part V of the Road Design Manual. Dense emulsion
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macadams require heavy compaction to be continued at intervals until all movement ceases;
this may not be achieved until a period of some days after laying.
7.3 Surfacings
7.3.1 Prime Coat
A prime coat is an application of low viscosity bituminous binder to an unbound surface,
usually an unbound or a cement/lime-bound surface, in order to promote and maintain
adhesion between the roadbase and a bituminous surfacing.
MC 30 and MC 70 are the most suitable binders. MC 30 can be used for practically all types
of materials. MC 70 is suitable only for open textured materials, such as graded crushed
stone. The depth of penetration should be between 3 and 10mm and the quantity sprayed
should be dry within two days. The rate of application will depend on the texture and density
of the material to be primed. It is usually between 0.8 and 1.2litre/m2. It is good practice to
dampen the surface to be primed as this facilitates the penetration of the binder
Priming a cement-treated layer with cut-back can cause slight surface disintegration,
because of interference with the cement hydration. If difficulties arise, priming should be
replaced with a bitumen emulsion tack coat, although the absorbance of an emulsion is not
as good as cutback bitumen.
If the prime coat has to be trafficked before the surfacing is placed, it should be blinded with
clean, non-plastic natural sand, crusher dust or fine aggregate.
Prime coats applied to saline surfaces are subject to salt blistering. Reference is made to
Chapter 11.3 for further details.
7.3.2 Tack Coat
The prime function of a tack coat is to glue a new bituminous surface to an underlying
bituminous surface. Tack coats should be very thin, otherwise they will act as a lubricant
rather than a glue (especially in hot climates) and unnecessarily increase the bitumen
proportion in the overlying asphalt. It is best to use a bitumen emulsion, spread thin to
approximately 0.2 to 0.8 l/m2. All tack coats should be applied to a cleaned surface shortly
before laying the next bituminous layer but allowing sufficient time for evaporation of cutter
or run-off of emulsion water. Rapid curing cut-backs (RC 250, 800 or 300); medium curing
cut-backs (MC 250, 800 or 3000); quick-breaking emulsions (Al or Kl-70); or A3 Anionic
emulsion diluted with water l:l. MC 30 & MC 70 prime cut-backs are not suitable for tack
coats.
7.3.3 Surface Dressing
7.3.3.1 General
Surface dressing, or chip seal as it is otherwise known, is a very effective and versatile
technique. It consists of the spraying of a bitumen film followed by the application of a layer
of aggregate chippings. Thus, surface dressing does not impart structural strength to the
pavement or improve the riding quality but it does provide a waterproof seal and can restore
surface texture. It can be used as the principal seal for surfacing lightly trafficked roads or
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used as a maintenance process to re-seal all types of roads. The lives of all bituminous
surfacings are extended by periodic applications of surface dressing.
The main property of surface dressings that lends itself to such versatile behaviour is the
bitumen film, which is much thicker in surface dressing than bituminous pre-mixes. On
heavily trafficked roads where the pre-mixes are compacted to refusal, the bitumen content
is even lower than normal and in these situations surface dressing is applied to protect the
pre-mix from early weathering.
Surface dressing can be applied to new and rehabilitated roads carrying up to about 1000
vehicles per day. The technique consists of applying a prime coat on the finished base
followed usually by a double surface dressing, ie a second application of bitumen and
aggregate on top of the first. The advantage of this is that any deficiencies in the first
application will be covered by the second application. The quality of a double surface
dressing will be greatly improved if traffic is allowed to run on the first dressing for a period of
at least two weeks. This permits the formation of a firm interlocking mosaic for the
introduction of the second layer. However, this attribute may be compromised by traffic or
animals contaminating the first seal.
For maintenance purposes, a single seal (or other types of seal, see later) is usually laid.
An intermediate type of surface dressing, shown to be appropriate for heavy and/or fast
traffic is known as ‘racked-in’ where a heavier bitumen application is used than for a single
surface dressing (but less than a double seal). A layer of large chippings is then applied
followed immediately by an application of smaller chippings to ‘lock up’ the larger chippings
and form a stable mosaic.
Yet another type of surface dressing, known as a ‘pad coat’ is used when the hardness of
the surface allows very little embedment, such as a cement stabilized road base or a dense
crushed rock roadbase. A first layer of 6mm chippings are applied which will provide the
basis for a second layer of 10mm or 14mm chippings.
The success of surface dressings depends primarily on the adhesion of the aggregate
chippings to the road surface; the chippings must therefore be clean and free from dust.
Inaccurate rates of spread of both bitumen binder and aggregate, poor quality materials and
poor workmanship can drastically reduce the effective life of a surface dressing.
The substrate must have a uniform surface texture and should be cleaned to remove
extraneous material, and should be dry, before surface dressing is applied. If it is not uniform
the bitumen binder will permeate into open-textured areas but remain on the surface in
close-textured areas. On existing roads, potholes should be repaired and trafficked for about
a month before the surface dressing is applied.
Detailed, up-to-date advice on surface dressing is contained in Overseas Road Note 3, 2nd
Edition, published by TRL Ltd in 2003. The main points relevant to Kenyan conditions are
summarized below.
7.3.3.2 Bitumen binder
7.3.3.2.1 Selection
Penetration grade bitumens, cutback bitumens and bitumen emulsions can all be used for
surface dressing. The bitumen must be capable of being sprayed, ‘wet’ the surface of the
road in a continuous film, adhere to the chippings and be strong and durable enough to hold
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the chippings against the traffic forces at the prevailing temperatures. In Kenya either a
cutback binder, MC 3000, or a cationic bitumen emulsion, K1-60 or K1-70, are preferred, MC
3000 being used where road temperatures exceed 350C. Penetration grade bitumen is more
readily available and it may be necessary to modify it on site by ‘cutting back’, or diluting it,
with diesel or kerosene. Diesel is preferable because it is less volatile and normally between
2% and 10% is necessary to modify 80/100Pen bitumen to the required viscosity range.
In new construction it is essential to apply a prime coat to bind the surface and ensure good
adhesion between base and surfacing. The prime coat is usually fluid cutback bitumen,
either MC 30 or MC 70. Emulsion bitumens are not suitable.
Polymer-modified bitumen is often used (but not yet in Kenya) in order to improve the binder
performance where the road surface experiences high stresses. Examples are thermoplastic
rubbers and crumb rubber derived from waste car tyres. Latex rubber is also used to modify
emulsion bitumens.
7.3.3.3 Aggregate Chippings
Hard, tough, clean crushed aggregate chippings of roughly cubical or sub angular shape
with single nominal sizes 6, 10, 14 and 20mm are required. Table 7.13 presents these
requirements, reproduced from KS 02-1228 1994. The methods for the tests defined below
are contained in KS 1238 2003.
Table 7.13: Specification for Aggregate Chippings for Surface Dressing
Material
Properties
Traffic, vpd
Toughness, ACV, max %
Durability (by Na2S04)
Surface Dressing Aggregate
>6000
16
2000-6000 500-2000
20
23
Max 12%
<500
26
Shape (Flakiness), max,%
Cleanliness, % passing
0.075mm sieve
Polished Stone Value*
Grading, sieve size, mm
28
20
14
10
6.3
5
3
2
0.5
20
20
25
14/20
100
85 – 100
0 – 30
0–7
0–2
-
60
10/14
100
85 – 100
0 – 30
0–7
0–2
-
25
<0.5%
6/10
10
85 – 100
0 – 30
0 – 10
0–2
-
50
3/6
100
85 – 100
0 – 30
0 – 10
0–2
* The Polished Stone Value test (KS 1238-15 2003) of the chippings is important if the primary purpose of the surface dressing
is to restore the skid resistance of the road. The PSV required is related to the nature of the road site and the speed and
intensity of the traffic.
Where only the weaker aggregates are available, use of a rubber-shod roller is preferred to a
steel-shod roller in order to minimize chipping fracture.
Dusty chippings are a problem, particularly in hot climates, because it adversely affects
adhesion to the bitumen. The chippings should either be washed in the stockpiles or,
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preferably, pre-coated before spreading. This is particularly beneficial if traffic conditions are
severe. Pre-coating should be carried out using either of the following methods:


Using a pre-coating fluid comprising either bitumen emulsion or a mixture of diesel or
kerosene containing 20% of Pen 60/70 bitumen and applied as a light coating, or
Using Pen 60/70 bitumen at the rates shown in Table 7.14: coating by this method
may be carried out in a conventional asphalt plant
Table 7.14: Bitumen Coating Rate for Surface Dressing Chippings
Nominal Size of
Chippings, mm
6
10
14
20
Target Bitumen Content,
% by mass
1.0
0.8
0.6
0.5
The selection of the nominal chipping size to be used depends on the traffic volume and
substrate hardness. The interaction of these two variables is resolved in Table 7.15:
Table 7.15: Selection of Surface Dressing Chipping Size
Surface
Type
Very hard (concrete)
Hard (aged asphalt surface)
Normal (aged surface dressing)
Soft (new asphalt surfacings)
Very soft (bleeding surface dressings)
Approx no. of commercial vehicles
per lane per day
2000-4000 1000-2000
200-1000
10
10
6
14
14
10
20*
14
10
**
20*
14
**
**
20*
<200
6
6
10
14
14
* Care should be taken with 20mm chippings that no loose chippings are left on the road prior to opening when
open to traffic
**Unsuitable conditions for surface dressing
7.3.3.4 Design
7.3.3.4.1 Average Least Dimension and Bitumen Application Rate
Obtaining the optimum rate of spread of bitumen binder in an even film on the road surface
is the most important factor in ensuring the success of a surface dressing. The determination
of the application rate is based on the Average Least Dimension (ALD) of the chippings. This
is the average thickness of a single layer of chippings when they have settled in their final
interlocked position. It can either be directly determined by measuring a representative
sample of about 200 chippings according to the method described in Chapter 14 or indirectly
determined from the nomograph in Fig 7.1 using the Flakiness Index and Median Size of the
chippings (=the sieve size through which 50% of the chippings pass).
Figure 7.1: Determination of Bitumen Application rate
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7.3.3.4.2 Weighting factor
The ALD of the chippings is used with a weighting factor to determine the spray rate of the
bitumen binder. The weighting factor, F^, is determined by summing the four variables listed
in Table 7.15:
Table 7.16: Weighting Factor for Bitumen Spray rate
Total Traffic
Very light
Light
Medium
Medium-heavy
Heavy
Very heavy
Climate
Wet & cold
Tropical
Temperate
Semi-arid
Arid
Vpd/l
0-50
50-250
250-500
500-1500
1500-3000
3000+
Factor
+3
+1
0
-1
-3
-5
+2
+1
0
-1
-2
Existing Surface
Untreated/primed base
Very lean bituminous
Lean bituminous
Average bituminous
Very rich bituminous
Factor
+6
+4
0
-1
-3
Type of Chippings
Round/dusty
Cubical
Flakey
Pre-coated
+2
0
-2
-2
Using ALD and F^ in the following equation will determine the basic binder spray rate:
R  0.625  F ^*0.023  0.0375  F ^*0.0011ALD
Where:
F^ = Weighting Factor
ALD= average least dimension (mm)
R = Rate of Spread of bitumen (kg/m2)
Alternatively the two values can be used in the design chart given in Fig 7.2.
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Figure 7.2: Determination of Binder Application Rate from ALD and Weighting Factors
The intercept between the F^ line and the ALD line is located and the rate of spread of
binder is read off directly at the bottom of the chart.
For slow traffic or climbing lanes the rate of spread should be reduced by approximately
10%. For fast traffic or downgrades steeper than 3% the rate of spread of binder should be
increased by approximately 10%.
As regards the application of emulsion, K1-60, will run off the road if the rate of application
exceeds about 1.2 1/m2, on a smooth primed base, and 1.51/m² on a chip seal.
Consequently, with emulsion, the application rates should be calculated as follows:
1. Determine the total amount of residual bitumen required, by using Chart S1c.
2. Calculate the total amount of emulsion required.
3. Split this amount into two sprays for single surface dressing and three sprays for
double surface dressing, so that no run-off will occur and the upper spray rate is
minimized. (Because of its low viscosity emulsion flows down and fills the voids
between chippings).
7.3.3.4.3 Chipping Application Rate
An estimate of the rate of application of aggregate chippings, assuming a loose density for
the chippings of 1.35 Mg/m3, can be obtained from the following equation:
Chipping Application Rate = 1.364*ALD
A more precise and practical method of estimating the chipping application rate is to spread
a single layer of chippings on a tray of known area. The chippings are then weighed and the
process repeated ten times, the average being calculated, and then being increased by 10%
to allow for ‘whip-off’. This value can be finally refined by observing if any binder remains
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exposed after spreading, or if chippings rest on top of one another, indicating too low or too
high a chipping application rate respectively.
For double seals, aggregate chippings are generally half the size of the first seal.
Recommended combinations are: 14/20 + 6/10; 10/14 + 3/6. Where triple seals are required
(for instance if traffic is very high) the procedure for the first two seals is the same as for
double surface dressing with the third seal being crushed rock fines.
Adhesion agents are available for adding to binders to improve the ‘affinity’ between the
aggregate and the binder and help minimize the damage to surface dressings that can occur
in wet weather. These agents can enhance adhesion between chippings and binder even
when they are wet. The effectiveness and the amount of additive needed can be determined
by the Immersion Tray Test, fully described in ORN 3. Fresh hydrated lime can be used to
enhance adhesion when about 12% by mass of bitumen is added and it also retards bitumen
hardening. It should not be added to cationic bitumen emulsions which already contain an
adhesion agent.
7.3.4 Slurry Seals and Cape Seals
A slurry seal is a mixture of fine aggregates, Portland cement filler, bitumen emulsion and
additional water (ASTM, D 3910, 1996). When freshly mixed they have a thick creamy
consistency and can be spread to a thickness of 5 to 10 mm. Slurry mixes are best made
and spread by purpose made machines.
Both anionic and cationic emulsions may be used in slurry seals but cationic emulsion is
normally used in slurries containing ‘acidic’ aggregates (ie most aggregates other than
limestone), and its early breaking characteristics are also advantageous when rainfall is
likely to occur. Suitable aggregate gradings, reproduced from ASTM D 3910, are given in
Table 7.17.
Type 1 is suitable for crack sealing and correcting fretting damage. Type 2 is suitable for
filling more serious surface damage and Type 3 is suitable for sealing new and undamaged
AC surfaces. Crack sealing will not be effective if the cracks penetrate down to the full depth
of the AC layer.
The coarse aggregate should be entirely crushed and sound aggregate (normally basalt in
Kenya) in order to give optimum skid resistance.
Table 7.17: Slurry Seal Types
Sieve size
No.
mm
⅜”
4
8
16
30
50
100
200
9.5
4.75
2.36
1.18
0.600
0.300
0.150
0.075
Type of seal
Percentage passing sieve size
1
2
3
100
100
100
90-100
70-90
90-100
65-90
45-70
65-90
45-70
28-50
40-60
30-50
19-34
25-42
18-30
12-25
15-30
10-21
7-18
10-20
5-15
5-15
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The ASTM standard must be referred to for full details of test methods and mix design. Tests
include;




‘consistency’, to determine optimum mix design
‘set time’ to determine the time to initial set
‘curing time’ to determine initial cohesion of the slurry and resistance to traffic
‘wet track abrasion test’ to measure the wearing quality of the seal
There would be great advantage if the bitumen emulsion was made with a polymer modifier
or rubber additive such as latex. This would ensure early strong bonding between the
bitumen and the aggregate and make the mix more stable in hot weather.
Slurry seals are normally applied on top of a single surface dressing to make a ‘Cape Seal’,
which has a smoother texture than surface dressing alone and is more durable. However, to
avoid bleeding, the quantity of bitumen used in the surface dressing should be less than that
normally required. Cape Seals are more expensive than double surface dressings, and
require specialized machinery and expertise to apply. They are probably not economic for
low volume roads.
Table 7.18: Cape Seal specification
Chipping Size in Surface Dressing, mm
20
14
10
Coverage (m2/m3)
130 to 170
170 to 240
180 to 250
7.3.5 Otta seal
Named after a location in Norway where it was first applied, an Otta Seal is a graded
aggregate or gravel containing all sizes including filler used instead of single sized chippings.
It is thus more economical in its use of stone. There is no formal design procedure but
recommendations based on case studies have been published. An Otta Seal may be applied
as a single or double layer and experience has shown them to be satisfactory for 10 years’
service or more on roads carrying up to 300 vpd.
The grading of the aggregate is based on the predicted traffic level. For light traffic (<100
vpd) a ‘coarse’ grading is used while for heavier traffic a ‘dense’ grading should be used.
Grading envelopes are recommended in Table 7.19:
Table 7.19: Otta Seal Specifications
Sieve size, mm
19
16
12
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
Percentage passing
Dense
100
79 – 100
61 – 100
42 – 100
19 – 68
8 – 51
6 – 40
3 – 30
2 – 21
1 – 16
0 – 10
The Republic of Kenya – Ministry of Roads 63
Coarse
100
77 – 100
59 – 100
40 – 85
17 – 46
1 – 20
0 – 10
0–3
0–2
0–1
0–1
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The selection of bitumen binder reflects the aggregate quality but normally cutback bitumen
MC 800, MC 3000 or Pen 200/150 are used. Spray rates are selected empirically and thus it
is essential that pre-construction trials are carried out. Typically, spray rates for single seals
are between 1.6 and 2.1m2 but adjustments are normally necessary and the Design Guide
should be consulted (NPRA, 1999).
An important aspect of Otta Seal construction is the need for extensive rolling for two to
three days after construction. The rolling forces the binder upwards, coating the aggregate
and initiating the process continued by traffic, of forming a pre-mix appearance to the
surface. After-care can be lengthy and involves sweeping back dislodged aggregate to be
further compacted by traffic.
7.3.6 Sand Seal
In situations where aggregate for surface dressing is unobtainable or too expensive, sand
can be used as a replacement. Sand seals are less durable than surface dressing because
of abrasion by traffic but they can provide a satisfactory surfacing for traffic levels up to 100
vpd.
The sand should be clean and coarse, with maximum size of 6mm, containing no more than
15% finer than 0.3mm and a maximum of 2% finer than 0.15mm. The sand should be
applied at a rate of 6 to 7 m3/m2. The bitumen binder, which either be a cutback or an
emulsion, should be spread at a rate of 1 to 1.2 kg/m2, depending on the type of surface
being sealed.
7.3.7 Fog Spray
A light spray of bitumen emulsion is ideal for improving early retention of chippings in a new
surface dressing. The road surface is usually dampened before spraying. The emulsion must
break completely before traffic is allowed onto the surfacing and it may be necessary to dust
the surface with crusher dust or sand beforehand. If the emulsion is diluted with water, to
obtain a 45% bitumen content to ensure the bitumen will flow around the chippings, the
suitability of the water must be established by mixing small trial batches.
The spray rate for the diluted emulsion will depend on the surface texture of the new
dressing but the best results will be achieved if the residual bitumen in the fog spray is
treated as part of the design spray rate for the surface dressing. The spray rate is likely to be
between 0.4 and 0.8 l/m2 and it is important to avoid over-application of bitumen which will
otherwise result in reduced skid resistance.
7.3.8 Thin Surfacings
An effective way of providing good surface texture and skid resistance would be to surface
an AC wearing course or binder course mix with a dense but specialist thin bituminous layer.
This would have several advantages;



the AC can be made entirely with basalt aggregate;
a denser AC mix could be used if necessary, although the seal would prevent
premature aging of the bitumen in the lower layer;
the thin surfacing can be designed to be durable and to have good skid resistance
without increasing the risk of plastic deformation;
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it should be possible to add a second thin layer, when required for maintenance
purposes, without the need for milling; and
in built-up areas there will be less interference with kerb heights.
There are several types of thin surfacing materials that would be suitable for use on roads in
Kenya. However, it is probable that the use of modified bitumen and high standards of
manufacture and construction would be required. The most suitable surfacings are likely to
be proprietary materials.
There are many proprietary thin surfacings. Nichols et al (2002) reported on several such
materials that have been laid in the UK with favourable results. Three categories types of
thin surfacings were described;





paver-laid surface dressings: ultra-thin surfacings developed in France
thin AC: generally with polymer-modified bitumen
thin Split Mastic Asphalt (SMA); generally unmodified bitumen with fibres
multiple surface dressing: polymer modified bitumen and aggregate applied
separately
micro-surfacing: thick slurry surfacing generally with modified bitumen
Table 7.20 lists the wide choice of surfacings currently approved for use in the UK. For
Kenya it will be important to have materials that are relatively insensitive to hot conditions
and use of modified bitumen would meet this requirement. The use of a coarse grading for
the underlying asphalt layer would assist in resisting embedment of aggregate in the surface
layer.
Table 7.20: Selection of Proprietary Surfacings currently available
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Category
Name
UK producer
Paver-laid
surface
dressing
Safepave
Fibre-reinforced
Safepave
UL-M
Hitex
Axoflex
Tuffgrip
Colrug
Thinpave
Viapave
Masterflex
Associated Asphalt Limited
Thin AC
Thin stone
mastic asphalt
Multiple
surface
dressing
Masterpave
Axofibre
Viatex
Steelpave
Smatex
Premier Pave
Nashpave
Masterphalt
Smartpave
Duratex F
Surphalt
Finatex
Jean Lefèbvre (UK) Limited
Aggregates Industries UK
Limited
Lafarge Aggregates Limited
Hanson Quarry Products
Europe
Colas Limited
Aggregate Industries Limited
RMC Aggregates (UK)
Limited
Tarmac Limited
Tarmac Limited
Lafarge Aggregates Limited
RMC Aggregates (UK)
Limited
SteelPhalt
Aggregates Industries UK
Limited
Foster Yeoman Limited
Tarmac Limited
Tarmac Limited
Hanson Quarry Products
Europe Ltd
Tarmac Limited
Total Bitumen
Total Bitumen
7.3.9 Asphalt Concrete
7.3.9.1 General
The road user mainly requires an asphalt concrete premix surfacing to provide a satisfactory
riding quality and impart a sufficient skid resistance under all weather conditions. The design
engineer requires a premix surfacing to protect the underlying pavement layers from ingress
of water and the abrasive and disruptive actions of traffic and have a maximum
maintenance-free life.
There are two generic types of asphalt premix surfacing:
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Interlocked aggregate mixes, such as asphaltic concrete, which derive stability from
the aggregate interlock, obtained by careful adjustment of the mix grading, and from
the cohesion provided by the bitumen, and
Mortar type mixes, such as gap-graded asphalt or sand asphalt, which derive stability
from the cohesion of the fines-filler-bitumen mortar.
Asphalt concrete is the bituminous surfacing of choice in Kenya. There is some doubt
concerning the stability of gap-graded asphalts in hot climates, and even in temperate
climates their use is declining in favor of alternative premixes, such as thin surfacings,
allegedly more durable and resistant to deformation.
7.3.9.2 Design
The design of asphaltic concrete mixtures assumes that the particle size distribution
(=grading) of the aggregate should produce the highest possible density in the aggregate
fraction of the mixture. This is achieved by producing a continuously graded aggregate
following the ‘Fuller Curve’ (derived by Fuller & Thompson in 1907), according to the formula
below:
P  100 * d / D 
0.45
where P is the percentage of aggregate passing sieve size d, and D is the maximum size of
aggregate in the mixture.
Table 7.21 shows sieve sizes raised to the power 0.45 and Fig 7.3 shows the maximum
density grading as a straight line.
Table 7.21: Sieve sizes raised to 0.45 power
Sieve size
(mm)
37.5 25.4
19
12.5
9.5
4.75 2.36 2.00 1.18
0.6 0.425 0.3
0.15 0.075
To power 0.45 5.11 4.29 3.76 3.12 2.75 2.02 1.47 1.37 1.08 0.80 0.68 0.58 0.43 0.31
Figure 7.3: Maximum density grading
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100
Maximum density line
90
Passing sieve size (%)
80
Nominal
maximum
size
Maximum
size stone
70
60
50
40
30
20
10
0
Sieve size, mm (raised to 0.45 power)
A compacted blend of crushed aggregates will give a maximum density if the particle size
distribution follows the Fuller curve. However, this minimises the Voids in the Mineral
Aggregate (VMA) that can accommodate both bitumen and the necessary air voids after
compaction. This type of mix will be very sensitive to proportioning errors and it is best
practice to modify the distribution away from the maximum density line. This is especially
important when designing heavy duty asphalt concretes for the highest traffic classes.
Mixtures of aggregate of the chosen grading and bitumen of the selected Penetration grade
are evaluated using the rather complex and long-winded, and poorly reproducible, Marshall
procedure. This procedure was elaborated by Bruce Marshall in the 1950s to determine the
optimum strength for various aggregate-bitumen mixtures and an indication of the density to
which the asphalt should be compacted on the road, although it did not explain that there
were two phases of asphalt compaction; a) during the laying of the asphalt and b) during the
passage of traffic over the design life.
The stability of these mixes depends crucially on the air voids contained in the compacted
mixture (Voids in the Mix = VIM) and, it has been found by experience, if VIM falls below 3%,
the mixture becomes very susceptible to plastic deformation. The original Marshall design
generated asphalt mixtures and defined their stability and flow at a standard temperature in
order to estimate road compaction in the wheelpaths after a few years’ trafficking, up to a
maximum very approximate 1 million esa, a reasonable value for the traffic at that time.
However, traffic loads have since increased well beyond this value and the Marshall design
has had to be modified by increasing the compaction effort in the procedure. In practice this
would require the samples to be compacted with an inordinate number of blows such that
the Marshall method becomes impractical.
Alternatively, a more forceful method of compaction is used by employing the Percentage
Refusal Density Test (PRD) (BS 598 Part IV 1989). Asphalt mixtures are compacted to
refusal and the mixture still containing 3% air voids is selected. In practice, field compaction
is required to achieve 95% of this refusal density, with no value <93%.
There are three general types of grading envelope;
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(i)
(ii)
(iii)
Part 3 - Materials and Pavement
those that span the maximum density ‘Fuller’ line;
those that lie mostly below the maximum density line; and
those that lie mostly above the maximum density line.
Selection of an appropriate grading depends upon the level of traffic loading in terms of
equivalent standard axles, and in some cases where vehicles have very high axle loads.
Mixes made with an aggregate grading which spans the maximum density line need to be
designed and made with care because the grading can be made to lie very near to the
maximum density line and, therefore be sensitive to proportioning errors in the asphalt plant.
The surface finish is also likely to be smooth with little surface texture and far from ideal for
wet weather skid resistance.
Mixes made with an aggregate grading which lies above the maximum density line contain a
high proportion of sand size aggregate. This type of mix can be ‘tender’ and require care
during compaction. The surface texture will tend to be very fine, giving poor wet weather
skid resistance. Because of the high sand content air voids in the mix are more likely to be
discontinuous than in the coarser mixes, even at the same total air voids content. Such
mixes should potentially be the most durable and could be appropriate for roads carrying
light traffic.
Mixes made with an aggregate grading which lies below the maximum density line should be
the most suitable mixes for heavy traffic. These aggregate gradings should provide enough
voids within the aggregate matrix (VMA) to enable asphalt mixes to carry sufficient bitumen
to coat the aggregate and also enable good workability, at the same time retaining adequate
VIM under heavy traffic.
The tendency is to use coarser gradings as the traffic loading increases and a very coarse
mix is probably most appropriate for climbing lanes and other severely loaded sites. The
difficult balance between mix resistance to deformation and durability must be reached and it
may be necessary to seal this type of mix with a surface dressing.
7.3.9.3 Grading and other properties
The aggregate and filler properties are similar to those for DBM, Table 7.9. Properties for the
various premixes are given in Table 7.22:
Table 7.22: Properties of Asphalt Concrete
Type I (High Stability: stiff, rut-resistant, for T5 and
above )
Sieve Wearing Course
Binder Course
Size
(mm)
0/14
0/10
0/6
0/20
0/14
0/10
28
100
20
100
90-100 100
14
90-100 100
75-95
90-100 100
10
70-90
90-100 100
60-82
70-90
90-100
6.3
55-75
60-82
90-100 47-68
52-75
60-82
4
45-63
47-67
75-95
37-57
40-60
45-65
2
33-48
33-50
50-70
25-43
30-45
30-47
The Republic of Kenya – Ministry of Roads 69
Type II (Flexible,
for T4 and below )
Wearing Course
0/14
100
90-100
70-95
55-85
46-75
35-60
0/10
100
90-100
62-90
50-80
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1
23-38
0.425 14-25
0.300 12-22
0.150 8-16
0.075 5-10
Bitumen
Grade**
Bitumen
Content, nom.
Marsh. Stably.,
75 blows, kN
Flow Value,
mm
Voids in Mix, %
Voids, %, at
Refusal Density
Void in Mineral
Aggregate, %
Voids filled with
bitumen, %
Fine Aggregate
(<2mm)
Filler*
Layer Thicknes
Mixing Temp
Laying Temp
Compaction
Equipment
Part 3 - Materials and Pavement
23-38
33-50
14-25
20-33
12-22
16-28
8-16
10-20
5-10
6-12
60/70 to 80/100
18-32
20-35
11-22
12-24
9-17
10-20
5-12
6-14
3-7
4-8
60/70 to 80/100
20-35
12-24
10-20
6-14
4-8
25-45
14-32
11-27
6-17
3-8
80/100
25-50
14-33
11-27
6-17
3-8
5.5-7.0
4.5-6.0
5.0-6.5
5.5-7.5
5.5-7.5
6.0-8.0
5.0-6.5
Min 9
Min 9
4 to 7
2 to 4
2 to 4
2 to 5
3 to 5
Min 3*
4 to 10
Min 3*
3 to 8
Min15
(at design VIM)
65-73
Min13
(at design VIM)
±5% (of the value
determined at JMF optimum
bitumen content)
Sand Equivalent >40; Sulphate Soundness <12
Cement, lime, limestone; non-plastic; passing 0.425mm 100%; passing
0.075mm >75%; Bulk density in toluene 0.5-0.9g/ml.
Maximum filler:bitumen ratio 1.2
0/10 25mm; 0/20 50mm
Bitumen 130 to 1500C (60/70); 120 to 1400C (80/100)
By Paver min. temperature 1300C (60/70); 1250C (80/100)
Density (min) 96% of Lab design Marshall
Min temperature at end of compaction 800C (60/70) 700C (80/100)
Steel wheel rollers: 5 to 7kg/mm of roll width
Pneumatic tyred rollers; min 2 tonnes per wheel
*The purpose of the filler is to extend the bitumen (ie fill the voids) and to make it stiffer. Too much filler (beyond 1.2 times
bitumen content) makes the mixture difficult to work.
**Bitumen properties are given in Table 7.11.
Type I asphaltic concrete is a fairly stiff type of mix designed to resist rutting and high
stresses. Type II asphalt concrete is a more flexible mix, designed to resist comparatively
high flexural deformation. It must be placed in a thin layer, maximum 50mm.
The desired rigidity (or flexibility) will be obtained by the proper combined choice of the
following factors: Penetration grade of bitumen, degree of crushing of coarse aggregate,
angularity of sand, mix grading, amount of filler, amount of bitumen, filler to bitumen ratio
and Voids in total Mix (VIM).
The production of flexible asphalt concrete Type II will generally require the use of an
appreciable proportion of rounded sand and a comparatively high amount of bitumen 80/100.
It is important to bear in mind that the mix composition should strike a balance between the
requirements of stability and durability. In particular, it is not desirable to achieve Marshall
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stabilities much higher than the minimum values recommended since this would produce a
lean mix prone to rapid hardening of the bitumen and fatigue cracking.
Because of the rapid ageing of bitumen generally observed in Kenya, the use of bitumen
harder than 60/70 grade is not recommended. It is advisable to use the softest grade of
bitumen and the highest bitumen content compatible with the achievement of the
recommended minimum stability, maximum flow value and minimum voids requirements.
Overheating of the aggregate and of the bitumen must be avoided, as it causes oxidization
of the bitumen.
Asphalt Concrete Type I, because of its low voids content, is very sensitive to mix variations.
Furthermore, as the bitumen filling is thin, the mix is somewhat difficult to work and compact.
This type of mix requires very strict control of production, laying and compaction.
It is suitable for the heavy traffic classes.
Asphalt Concrete Type II is suitable only for medium and light traffic.
7.3.9.4 Design for Heavy Traffic: Refusal Density (Superpave™)
Superpave™, or Superior Performing Asphalt Pavements, is a procedure which was
developed in the 1990s in the USA from the Strategic Highway Research Programme as an
improved system for specifying bitumen binders, aggregates, developing asphalt mixture
design and analyzing and establishing pavement performance prediction. It purports to be a
performance-based specification system. However, to be able to follow it completely requires
a substantial investment in new equipment and products, currently outside the range of the
budgets of most developing (and some developed) countries’ budgets. For example, Table
7.23 lists the equipment required to investigate bitumen properties.
Table 7.23: Superpave equipment for bitumen performance characterisation
Property
Aging
Temperature/
viscosity
Equipment
Rolling Thin Film Oven
Standard
AASHTO T240
Pressure Aging Vessel
AASHTO PP1
Rotational Viscometer
Dynamic Shear
Rheometer
Purpose
Investigate aging during
construction
Investigate aging over
pavement life
Bitumen performance during
handling
Visco-elastic properties
applicable to rutting and
fatigue
Aggregate characteristics required were subdivided into ‘consensus’ and ‘source’ properties
but mostly this amounted to the same situation as before, which is to be expected.
Instead, elements of Superpave™ have been adopted, the most significant of which is a
means of compacting the asphalt trial mixes in the laboratory in a more appropriate manner,
and beyond the normal Marshall level of compaction, in order to replicate conditions on the
heaviest trafficked roads. To do this, a piece of equipment known as a gyratory compactor is
required. This compacts the trial mix in a manner more akin to the action of traffic than the
Marshall hammer, and is much quicker. Presently in Kenya most major projects include it in
the equipment list and it is used and compared alongside the PRD Test (BSI, 1989).
Eventually the gyratory compactor will replace the PRD Test because:


it can be adapted for large moulds if larger sized aggregate is used
it gives good density distribution through the depth of a sample
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it can mould Marshall samples, and
it is more representative of field compaction than the impact-type Marshall
compaction.
For the heaviest trafficked roads, T5 to T7, it is necessary to design the asphaltic concrete to
a greater degree of compaction than is practical by usual Marshall design. A number of
gradings should be investigated so that a workable mix is identified containing a minimum of
3% Voids in the Mix (VIM) at Refusal Density. It is important to use a strong aggregate
because there is the danger that the more robust compaction will damage the aggregate and
change its grading: so, if possible, the aggregate should come from a source known to give
good results. These mixes will most likely have the grading characteristics displayed in
general in Fig 7.4 and listed in Tables 7.24 and 7.25:
Figure 7.4: Generalised Superpave grading
Table 7.24: Superpave: Grading Control Points
Nom Max Sieve Size
Size, mm mm
37.5
0.075
2.36
25.0
37.5
50
25.0
0.075
2.36
19.0
25.0
37.5
19.0
0.075
2.36
12.5
19.0
25.0
12.5
0.075
Control Point
% passing
0
6
15
41
90
90
100
100
1
7
19
45
90
90
100
100
2
8
23
49
90
90
100
100
2
10
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2.36
9.5
12.5
19.0
28
90
100
Part 3 - Materials and Pavement
58
90
100
-
Table 7.25: Superpave: Grading ‘Restricted Zone’ Boundaries
Sieve Size
within
restricted zone
4.75
2.36
1.18
0.6
0.3
Minimum and maximum boundaries of sieve size for nominal
maximum aggregate size (minimum-maximum % passing
37.5
25.0
19.0
12.5
34.7-34.7
39.5-39.5
23.3-27.3
26.8-30.8
34.6-34.6
39.1-39.1
15.5-21.5
18.1-24.1
22.3-28.3
25.6-31.6
11.7-15.7
13.6-17.6
16.7-20.7
19.1-23.1
10.0-10.0
11.4-11.4
13.7-13.7
15.5-15.5
Nominal Maximum Size is one sieve larger than the first sieve to contain more than 10% of aggregate. Where possible the
largest particle size should be less than 25mm so that the requirements of the Marshall test design can be complied with.
Mixes identified for compaction trials shall be made to the laboratory design bitumen content
and two other bitumen contents of +0.5% and +1.0% additional bitumen. Cores will be cut to
determine the density of the compacted material. The core will then be reheated to 145 ±50C
in the appropriate mould and compacted to refusal with the vibrating hammer test. The cores
from the compaction trial must have a density equivalent to 95% of the refusal density. The
compaction trials are aimed to identify a workable mix which can be made to a bitumen
content which gives no less than 3% VIM at refusal density.
Trials shall also be carried out with the Gyratory Compactor, if available, to determine the
number of revolutions that are equivalent to the refusal density using the vibrating hammer.
For this heavy duty asphalt the maximum Marshall Stability should be 18,000N after 2x75
blows and at compaction to refusal shall have 3% VIM.
Having established the suitability of the aggregate source, several gradings should be
tested, including that used for the Marshall test, to establish the bitumen content and VIM at
refusal density and a bitumen content corresponding to a VIM of 3% should be selected.
Compaction trials should be carried out to establish the workability of the premix.
The temperature of the bitumen and aggregates on mixing should be 1100C ±30C above the
softening point of the bitumen. Compaction of the premix should commence as soon as it
can support the roller and should be completed before its temperature falls below 900C.
Rolling should continue until the voids measured in the completed layer are in accordance
with the requirement for a minimum compacted density of 98% of the Marshall optimum or a
minimum mean value of 95% of refusal density (with no value less than 93%). Mixes
designed to 3% VIM at refusal density and compacted during construction to a mean density
of 95% of refusal density will have 8% VIM. Mixes of this type should be very resistant to
secondary compaction by traffic but will be permeable and allow the ingress of water. They
should be trafficked and the bitumen hardened. Then the asphalt should be sealed with a
single surface dressing containing 14/10mm chippings. This is to protect the bitumen of the
heavy duty asphalt from premature oxidation and aging. The period of hardening will depend
on the traffic level and should be such that the chippings do not become embedded in the
wearing course.
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Minimum thicknesses for the individual layers are as follows: 37.5mm mix, 65mm; 25mm
mix, 60mm; 19mm mix, 50mm; 12.5mm mix 40mm.
7.3.10
Gap-graded Asphalt
Gap-graded asphalt has a certain range of particle sizes missing from the total aggregate
grading. It generally consists of aggregate of a fairly uniform size blended with sand and
filler. As there is very little stone-to-stone contact in the compacted mix, the stability is
derived from the cohesion of the sand-filler-bitumen mortar.
Although it has been successfully used on a section of Nairobi – Mombasa road it is not
robust in terms of deformation resistance, as would be expected. Even in temperate
climates on heavily trafficked roads it has a tendency to rut quickly and therefore probably is
suitable only for medium to light traffic. Its main virtues are its flexibility, its fatigue
resistance, and its durability, due to the good distribution of the voids structure and the
rounded shape of most of the fine aggregate.
The gap-graded mixes specified are designed for thin wearing courses (25-50mm). These
mixes are impermeable, because of the comparatively high bitumen content and the good
distribution of the voids structure. The coarse aggregate is limited to 55% by weight. “Low
stone content” mixes (less than 40%) are easy to work and compact and are very tolerant to
mix variations, whereas “High stone content” mixes (more than 45%) become sensitive to
changes in bitumen content.
Gradings and other properties are listed in Table 7.26:
Table 7.26: Gap-graded Asphalt Specification Criteria
Coarse Aggregate Grading
Sieve size
(mm)
20
14
10
2
% passing
100
38 to 100
0 to 69
0 to 2
Mix Design Requirements
Marshall Stability (S), N
Flow, (F), mm
Marshall Quotient, Q, S/F
Voids in Mortar of Mix, %
Filler/Bitumen Ratio
Fine aggregate + Filler
Grading
Sieve size
% passing
(mm)
1
100
0.425
70 to 97
0.300
49 to 93
0.150
16 to 58
0.075
0 to 20
Min
Max
3000
9000
2
6
2
3
9
0.9
1.3
7.3.10.1 Mix design method
1. Prepare mixture of fine aggregate and filler to give 1:6 ratio by mass of filler to fine
aggregate retained on 0.075mm sieve. Determine the optimum bitumen content
which gives maximum Marshall Quotient.
2. By adding coarse aggregate and reducing bitumen content the Marshall Quotient
may be increased to the specified value. 20% coarse aggregate will increase quotient
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by factor of approx. 1.5, 30% by 1.9, 40% by 2.4 and 55% by 3.0. For an increase in
coarse aggregate the revised bitumen content is given by:
NBC  OBC 100  S  / 100  2.3S  / 100
where:
NBC = Nominal Bitumen Content
S = % of coarse aggregate added
3. Prepare mixes in accordance with BS 594 at bitumen content from (2) to check
specified requirements.
Gap-graded mixes typically have the following grading and compaction characteristics:
7.3.11
Sand Asphalt
Sand asphalt consists of natural sand plus, in some cases, mineral filler and a small
proportion of crushed fine aggregate, bound with Penetration bitumen.
The recommended grading is listed in Table 7.27:
Table 7.27: Sand Asphalt Grading
Grading of Aggregate + Filler
Sieve size, mm
% passing
10
100
6.3
95-100
2
70-100
1
47-95
0.425
20-75
0.300
15-60
0.150
8-30
0.075
4-12
Marshall properties are Stability, 3000 tp 9000 N; Flow 2 to 6mm; Voids in Mix 5 to 10%.
Sand Asphalt is, suitable for wearing course in a thin layer (max. 50 mm).
Because of its relative richness in binder and the good distribution of the voids structure,
sand asphalt is impermeable, flexible and has a good fatigue resistance.
As its resistance to rutting is not very high, sand asphalt is suitable only for light and medium
traffic, ie Classes T1 and T2.
7.4 Other Materials
7.4.1 Reclaimed Asphalt Pavement (RAP)
7.4.1.1 Current Practice in Kenya
It can be economically viable, and environmentally desirable, to reclaim worn-out asphalt
and granular pavements, re-process and re-use them in the rehabilitated road.
Implementation of the technology, however, requires significant investment in specialist skills
and equipment, appropriate Governmental pressure to carry it out and the formulation of
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suitable specifications and/or methodologies to control the process. Details of the latter are
presented in Appendix H of ORN 19 2002 and summarized here.
Presently in Kenya, the technology is in its infancy. On the major road projects of the
Northern Corridor route, RAP is produced by milling the old road surface, and either mixed
with fresh aggregate and cement to make ‘cement-improved subbase’ and/or mixed with
additional crushed stone to make an ‘upper subgrade’. It could be that the strength of these
re-processed layers is manifestly greater than the normal requirement in order to ensure that
the life of the road will be long.
The stockpiling of RAP prior to re-processing it is a critical part of the procedure. It is
important that the variability of the RAP is well controlled. Stockpiled RAP tends to
agglomerate and a crust forms, depending on the hardness of the bitumen and the ambient
temperature. To offset this tendency, it has been found that the larger the stockpile the
better. RAP readily absorbs moisture and it is best stored under roofing in an open-sided
building.
7.4.1.2 RAP in Capping Layers
A capping layer would only be used in the reconstruction of a road pavement where (in the
unlikely event) the in situ subgrade CBR was <5%. The existing UK specification requires
RAP to meet the grading specification in Table 7.28. The capping layer can consist of 100%
of RAP providing the bitumen content is <10%. The recycled material can be laid to a
maximum thickness of 200mm providing the required density is obtained. This is 95% of BS
Heavy, BS 1377, Part 4, 1990. Alternatively, the material can be laid by method
specification.
Table 7.28: Grading Specification for RAP
Bs Sieve Size
Mm
125
90
75
37.5
10
5
0.6
0.063
% passing sieve size
100
80-100
65-100
45-100
15-60
10-45
0-25
0-12
7.4.1.3 RAP in Subbase Layers
The quality of the aggregate in the RAP should at least meet the requirements for this layer.
Fresh aggregate can be added to modify the grading and the compacted layer of the
blended material should be acceptable providing the bitumen is hard enough not to hinder
compaction and enable the required moisture content, which is between the optimum
moisture content and -2% of optimum moisture content achieved with the BS Vibrating
Hammer test, BS 1377, Part 4, 1990.to be met. This is also subject to a Trafficking Trial,
whereby the compacted RAP is laid on a prepared trial area constructed t a specified
standard and then trafficked with a loaded truck until 1000 esa have been applied. The mean
deformation in the wheelpaths must be <30mm for the material to be acceptable.
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7.4.2 Modified Bitumens
Until now, Kenya has not used modified bitumens, which is surprising because they have
been in existence for more than 15 years. It may be because of bad experience or cost, or a
combination of reasons. The main problem is that the field is a very specialized one;
investigations have to be carried to identify the modifier best suited to the bitumen of choice
because the beneficial effects are not guaranteed.
Unmodified bitumen has weaknesses: it becomes rapidly more ‘fluid’ at high temperatures,
facilitating rutting in asphalt and embedment in surface dressing: and in the intense light of
the tropics it rapidly ages and hardens, accelerating cracking. Numerous proprietary
products are commercially available to ameliorate these characteristics. The products can be
categorized as follows:
7.4.2.1 Polymers
It is possible to create additives which are claimed to enhance the good properties of
bitumen whilst suppressing the bad properties. Ethyl vinyl acetate (EVA), styrene butadiene
styrene (SBS) and styrene butadiene rubber (SBR) are three such examples. They all
increase the viscosity, and visco-elastic stiffness, of bitumen at high temperatures and
increase fatigue life.
7.4.2.2 Rubbers
It is claimed that natural rubber can be added to bitumen to enhance its resilience. Much of
the motivation for this has been the desire to recycle used vehicle tyres and the benefits are
still uncertain.
7.4.2.3 Chemical additives
Elemental sulphur and manganese have both been used as additives. Sulphur becomes a
liquid at a temperature greater than ≈1200C and is added to improve workability at high
temperature. Manganese is used to increase bitumen viscosity and stiffness but with the
disadvantage that bitumen becomes more brittle.
7.4.3 Cold Bituminous Mixes
Under normal conditions bitumen will only adhere to aggregate if the two materials are
heated sufficiently to drive off all water and fluidize the bitumen, an energy-intensive and
thus expensive procedure. Thus there is a strong motivation to invent a mechanism whereby
a form of bitumen workable at ambient temperature will mix with cold, wet aggregate, with
the added benefit of safety of working.
7.4.3.1 Emulsion Bitumen
Bitumen emulsion is a suspension of bitumen in water, manufactured by turning hot bitumen
into fine droplets by introducing it into a rapidly circulating drum and combining it with an
emulsifying agent and water. The emulsifying agent enables the bitumen and water to coexist as a relatively stable fluid, with the bitumen comprising between 40% and 70% of the
total volume.
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7.4.3.2 Foamed Bitumen
The bitumen foam is produced by the rapid mixing of hot bitumen, air and water. The water
content within the bitumen is from 2% to 5% of bitumen volume but, transformed into steam,
the foam expands into many times the volume of the bitumen itself, and then it can be
readily mixed into the aggregate. The ratio of peak foam volume to original bitumen volume
is known as the ‘expansion ratio’ and the time until this peak volume is halved is known as
the ‘half-life’. The expansion ratio is typically between 5 and 20 while the half life is typically
between 10s and 40s.
In contrast to bitumen emulsions, which can be stored for months, foamed bitumen has to be
used quickly, within tens of seconds: the advantage it has over emulsion is that it requires
less water. The presence of the water and the need to drive it off presents the principal
challenge to the successful use of both forms of cold mix.
Once compacted into the pavement the process of strength gain (‘curing’) begins. Since both
cold mixes contain water, they inevitably contain less bitumen and, in terms of volume, there
is insufficient bitumen in the mix eventually to coat the aggregate particles. The presence of
water during compaction of the cold mix means that the mix is left with a high void content
once the water has evaporated, thus reducing stiffness and fatigue resistance compared to a
well-compacted dense asphalt. Mitigating measures can be taken but the best one is to
change to a more open grading with a higher void content at full compaction. This permits a
higher bitumen percentage to be used and encourages the water to evaporate, at the cost of
a reduced stiffness.
7.4.4 Block Paving
Block paving consists of an interlocking mosaic of small blocks, in Kenya usually made from
fine concrete or from cut stone (usually by hand, so the material is not too hard). The blocks
are placed on a substrate of compacted medium-coarse sand. Beneath the sand is the base,
which can be a granular material and it is important that this is of sound quality otherwise the
whole structure will easily deform. The blocks themselves may have the properties of
concrete but it is the discontinuities between them and the substrate which have the crucial
effect on performance. They are commonly seen in city centres and are used for aesthetic
reasons and also for resistance to deformation. It is usually the quality of workmanship and
not the quality of materials and design that determines their longevity.
The common standard size of block is 200 x 100mm, by 60mm thickness. The substrate is
usually 30 to 50mm of sand or sand-cement mortar for heavy duty applications and sand is
also used to fill the interstices between the blocks. The thickness of sand is important: if it is
either too thin or too thick failure can occur; and also obviously the quality of sand in terms of
its angle of internal friction. It is also crucial that the strength and uniformity of the substrate
is even, otherwise individual blocks will either become loosened or will fracture. However, it
is also very important that the filling between the blocks and the block pattern itself are well
constructed. The pattern known as ‘herring-bone’ is one of the most effective at generating
the necessary block interlock: Fig 7.5 illustrates typical patterns.
Use of block paving is not recommended in situations such as the corners of streets
because the twisting forces of turning vehicles is very disruptive to the individual blocks.
Figure 7.5: Typical patterns for concrete blocks
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7.4.5 Geosynthetic materials
Unbound materials have nil tensile strength and sometimes it is necessary to provide this by
means of geosynthetic reinforcement. A successful fabrication can make a very significant
improvement to the deformation resistance of an unbound material. On other occasions
geosynthetic material is used as a separator to avoid intermingling of soil material with
pavement layers; and also sometimes as additional strengthening to an asphalt overlay or
inlay..
7.4.5.1 Reinforcement
7.4.5.1.1 Subgrade
The reinforcement membrane imparts strength by interacting with the shear strength of the
material being reinforced. The reinforcement must therefore achieve interlock with the
individual particles. Thus, for soils with particle sizes of the order of µm (micrometers), a
membrane of appropriate texture must be used; whilst for granular materials a geogrid is
more suitable.
Together with the stress reinforcement through particle-to-particle interlock there is also a
contribution due to the friction of the soil particles with the membrane. The main contribution
of the membrane, however, is to withstand the tensile stresses that may be suffered by the
soil. Obviously, the support is greatest in the plane of the membrane and decreases away
from it, however, as yet, there is no method of predicting the zone of influence of the
membrane.
7.4.5.1.2 Pavement
Geogrids are increasingly being used to enhance the life of an asphalt inlay or overlay by
slowing down the rate of crack formation and by providing additional shear resistance
against rutting, especially in high stress sites. A secondary benefit is that cracks are
prevented from opening even after penetrating the full depth of the pavement. Geogrids are
manufactured from steel, glass fibre, plastic and also asphalt. They are also placed into a
layer of sprayed bitumen, applied at an approximate rate of 1litre/m2 and in this situation act
as a sealant, preventing water from entering even after cracks have formed. They also have
a role as separators, preventing reflective cracking.
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The geosynthetic used must therefore be matched with the purpose and the material being
reinforced. It is the responsibility of the engineer to assess the need for and the
effectiveness of the varied products available in the marketplace.
7.4.5.1.3 Separator
Materials used for this purpose are called geotextiles, and are classed either as ‘woven’ or
‘non-woven’, and can be manufactured of plastic, glass fibre or plant-based material. The
main property is that they should permit water to flow freely yet not soil. It is important to take
account of the rupture strength possibly being exceeded by large particles or construction
traffic.
7.4.6 Hand-Packed Stone
In Kenya this form of construction is commonly used for bus bays and parking lots. Large
stones, usually of 15cm size and consisting of lava aggregate, are placed directly onto the
natural ground. The gaps between the stones are then infilled with rock fines and small
chippings, sometimes vibrated in with a hand operated machine, until the fines completely
cover the stones and present a level surface. The integrity of the surface then depends how
well the large stones have been packed together on the natural ground, how well the fines
have been vibrated in, and how strong the natural ground is to start with.
7.4.6.1 Stone quality
Generically, a hand-packed stone surface comprises a closely placed layer of broken stone
pieces that are wedged into place with stone chips hand rammed into the interstices; the
remaining voids are filled with fines. The hand packed stone layer is normally bedded on a
thin layer of coarse sand, with grading as follows: 90-100% passing 4.75mm; 0-15% passing
0.300mm; 0-2% passing 0.150mm. The surface relies on the development of mechanical
interlock between the discrete unbound particles for its strength and is thus highly dependent
on the provision of adequate edge support without which the layer would progressively ravel
from the edge inwards under the passage of traffic. The hand-packed stone surfacing
disperses the stresses caused by traffic loading and acts as a barrier to erosion and
pulverisation of the roadbed that would otherwise result from tyre friction.
The large pieces of broken stone form an interlocking load-bearing matrix in conjunction with
the smaller stone chips that must be tightly wedged to firmly anchor the whole layer in place.
Thus, load transfer takes place at relatively few contact points and the crushing strength of
the stone will influence the rate of its degradation under traffic loading. The stone should be
tough and durable and obtained from breaking down unweathered hard rock and should
‘ring’ when struck with a geological hammer; a hollow, dull sound is indicative of
degradation. Crystalline igneous rocks such as granite and stable basalt are preferable. The
use of rounded cobbles or sedimentary rock with weakly bonded bedding planes should be
avoided. The finished surface is durable and fairly impermeable but, as the hand-packed
stone forms the trafficked surface, the riding quality is only moderate.
7.4.6.2 Roadbed
Where the soaked CBR of the roadbed material is ≥ 5% (95% BS Light) the recommended
thickness of the stone packed surface is 150mm. If the CBR value is even lower a thickness
of 200mm is recommended.
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7.4.6.3 Construction
The roadbed is shaped to a camber of 3-5% and compacted to refusal density at a moisture
content close to the optimum. After setting out the finished road line and level a 250mm wide
by 200mm deep trench is excavated to accommodate kerbstones along each edge of the
road. The minimum triaxial dimensions of kerbstones should be 400, 200 and 100mm. The
smallest face of each kerbstone is dressed so that it is flat and approximately perpendicular
to the longest axis.
The kerbstones, placed in the trench with their longest axis vertical and smallest face
uppermost, are firmly bedded and laid to the final road level. The trench is backfilled with
moist, well compacted excavated material to firmly anchor the kerbstones in position.
Supplementary drainage measures should be provided to prevent any ingress of water
through the surface from becoming trapped behind the kerbstone edge support where it
would otherwise penetrate the roadbed causing it to soften and loose strength and the handpacked stone surface to deform and ultimately fail. The salient features are illustrated in Fig
7.6.
Figure 7.6: Hand-packed stone construction
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A variant that has recently been trialled for labour-intensive roads in Kenya is to use ‘quarry
waste’, comprising moderately to highly weathered stone for the construction of low cost,
sealed roads. The quarry waste is placed on a road bed that has been compacted to refusal,
spread and then compacted to shape using a vibrating roller. A thin bituminous seal is then
placed on top.
7.4.7 Rumble Devices
Data regarding rumble devices and speed humps can be obtained by free download from:
http://www.dft.gov.uk/stellent/groups/dft roads/documents/divisionhomepage/032065.hcsp
7.4.7.1 Purpose
Features with a vibratory and audible effect can be used, usually in rural areas, to alert
drivers to take greater care in advance of a hazard such as a bend or junction. Although
rumble devices have been used, in places, with the aim of reducing speeds, the evidence
so far indicates that any speed reduction obtained will tend to be minimal, and will be
eroded with the passage of time. It is also known that in some locations drivers have learned
to accelerate over the devices to lessen the vibratory effect.
7.4.7.2 Types
Rumble devices come in a variety of different forms, which have been described as rumble
strips, jiggle bars, and rumble areas. Rumble strips and jiggle bars are similar in concept and
design, both comprising narrow strips of material laid transversely across the carriageway.
Normally rumble strips will be laid in a series of groups consisting of between two to five
strips per group. Spacing between the groups can vary. Fig 7.7 gives examples.
Rumble areas are generally constructed of coarse chippings, but can also be formed from
block paving or gravel filled cellular blocks.
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7.4.7.3 Noise
Rumble devices can generate considerable noise over a large area depending on the
topography and ambient noise levels. Rumble areas tend to be less noisy than rumble strips,
but a more expensive form of construction. Noise generated will vary from location to
location and depend on the pattern and type of device used.
In general, siting of rumble devices close to residential properties should be avoided. Some
authorities do not use rumble devices within 200m of residential properties. Where a conflict
seems likely to arise between safety gains and increased noise levels, consideration should
be given to whether the noise disbenefit outweighs the benefit of accident reduction.
Additionally consideration could be given to using a lower height device, though this may be
at the expense of overall effectiveness.
7.4.7.4 Regulation requirements
The UK Traffic Calming Regulations permit rumble devices up to 15mm in height, provided
no vertical face exceeds 6mm in height.
The requirement not to exceed 6mm for the vertical face is important. Heights greater than
6mm could create difficulties for two wheeled vehicle drivers, particularly cyclists. If materials
such as thermoplastic are used to form rumble devices they confer the advantage that any
faces formed are rounded.
7.4.7.5 Rumble Device layouts
Choice of the most appropriate layout to adopt depends largely on local circumstances. The
following should therefore only be considered as general advice, to be modified as the
particular location dictates.
7.4.7.5.1 Full or half width
Rumble devices can be constructed across part of a carriageway only, so that they only
affect drivers approaching a hazard. Existing evidence suggests that, particularly where
drivers can see a long way ahead, they may cross the centre line of the road to avoid the
devices. This obviously can be dangerous but also lessens the effectiveness of the rumble
devices. Extending the device across the full width of the carriageway will prevent this.
7.4.7.5.2 Cycle and drainage provision
To allow for drainage and help cyclists to avoid rumble devices it is advisable to provide a
gap, preferably in the range of 750mm to 1m, between the edge of carriageway and the
device.
7.4.7.5.3 Appearance
Rumble devices should be of a contrasting colour from the generality of the carriageway, so
that drivers can see them. White must not be used, to avoid confusion with road markings.
Rumble devices should also be clearly visible at night: where the colour of the construction is
relied on, rather than signing, the use of a suitable reflective material may be feasible.
7.4.7.5.4 Location
Rumble devices will be most appropriate in rural locations in advance of hazards such as
bends and junctions. There is some evidence to suggest that rumble strips should not be
used on bends with a radius less than 1,000m, because of possible danger to motorcyclists.
Rumble devices used in urban areas will generally be limited because of the noise they can
generate. If rumble areas are used to indicate the start of shared surface roads the overall
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height should be in the order of 5mm in order to reduce noise levels, and make them more
friendly for cyclists to cross.
7.4.7.5.5 Signing
Where rumble devices do not stand out visually from the rest of the road surface, authorities
should consider whether they should be signed. Where rumble devices are used at the
approach to a hazard such as a bend or junction they should where possible be sited in
obvious relationship to signing warning of the hazard. Where this cannot be achieved,
specific signing for the rumble devices should be considered.
7.4.7.5.6 Height
For normal use a height of 13mm is adequate for providing both audible and vibratory
warning, whilst achieving any speed reduction that might be obtainable. When used in
combination with other features, such as at gateways, lower heights may yield acceptable
results. In all cases it is important to ensure that vertical faces do not exceed 6mm in height.
For rumble areas a 14mm chipping size set in an epoxy resin has been used relatively
successfully and can comply with the maximum height of 15mm generally.
7.4.7.5.7 Pattern
The pattern to be adopted will depend on physical features and driver behaviour at the
particular location. Irregular spacing between groups or areas will help to break up the noise
patterns generated, which may make them more acceptable to any nearby residents.
Decreasing the space between groups or areas is generally the most effective. The number
of groups/areas and strips per group should be kept to the minimum. In the case of rumble
strips, about 50 strips divided into 2 to 4 groups will normally be sufficient. With regard to
rumble areas 4 to 6 areas will normally be adequate, though where these take the form of
narrow bands this number may need to be doubled. Normally, spacing between rumble
strips in the individual groups will be between 300mm and 500mm. Spacings below 400mm
are more suitable for roads having speed limits less than 40mph. On roads with higher
speeds, the closer spacing tends to allow vehicles to "float" over the strips. The pattern of
rumble devices should finish within 50m of any hazard it is associated with.
7.4.7.6 Materials and costs
Rumble areas are generally much more expensive than rumble strip schemes, particularly
those using thermoplastic material. Currently prices for rumble area schemes range from
$5000 to $15,000, depending on materials used. This needs to be set against the average
cost of a personal injury accident. Rumble strips have mainly been constructed in a
thermoplastic material. They vary from $1000 to $2000 per scheme. It is likely, however, that
they would need to be replaced more frequently than rumble areas.
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Figure 7.7: Examples of Rumble Strips
7.4.8 Speed Humps
Road humps are a common sight in Kenya and are an extremely effective means of keeping
vehicle speeds low. However, there appears to be little standardization in shape and, since
most are constructed of asphalt with a painted surface, after time they become invisible to
the motorist and therefore can be dangerous.
In the UK, regulations originally permitted the construction of humps ranging from 50mm to
100mm in height but subsequent experience indicated that a height of 75mm was optimal,
still resulting in significant speed reduction and substantially lessening the likelihood of the
grounding of vehicles, compared to 100mm high humps.
Both flat and round-topped kerb-to-kerb humps are effective: Fig 7.8 gives design examples.
Both types may be tapered at the sides to allow a drainage channel between the hump and
the kerb. At low speeds, vehicles can cross these humps without causing undue discomfort
to passengers or damage to the vehicle, but as speeds increase, they become progressively
more uncomfortable. Fig 7.9 shows the design of a later development, the sinusoidal hump,
with a shallower initial rise. Compared to the round and flat-topped humps, the sinusoidal
hump is more comfortable for vehicles but with an attendant lower speed reduction.
Humps may be used along single carriageway and dual carriageway roads providing there is
a 30mph speed limit and the road is not a trunk, special, or principal road.
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Figure 7.8: Design of Round and Flat Topped Speed Humps
Figure 7.9: Design of Sinusoidal Hump
Road humps and rumble strips shall be constructed after completion of the surfacing. A
prime coat should be applied to the surface, consisting of MC 70 cutback bitumen, applied at
a rate of 0.55l/m2, or as directed by the Engineer.
The material for the hump or rumble strip shall be as directed by the Engineer. It should be
of a different colour compared to the surfacing. If it is asphalt, it should be placed and
compacted into moulds and finally shaped to the required profile by rollers and tampers.
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8 Structural Design Method
8.1 Design Principles
8.1.1 Thicknesses and Materials Characteristics
No pavement structure can be designed independently of the characteristics of the pavement
materials. Indeed, every material has a different behaviour which is largely influenced by the
characteristics of the other pavement layers.
8.1.2 Design Period
The concept of design period should not be confused with that of pavement life. Each of the
pavement structures proposed has been designed to carry a certain cumulative traffic. When
the pavement has carried the design traffic, it will need to be strengthened so that it can
continue to carry traffic for a further period. The need for, and design of, pavement
strengthening is discussed in Part 4, Overlay and Asphalt Pavement Rehabilitation Manual.
In this respect, it is necessary that paved roads be regularly surveyed, so that strengthening
can be planned and implemented before extensive deterioration has occurred.
It is assumed that during the design period maintenance will be carried out. Maintenance
includes the shoulders and drainage systems, erosion and vegetation control, patching and
sealing. This maintenance is essential and its neglect will seriously affect the pavement
performance.
8.1.3 Stage Construction
An early decision that has to be made is whether it is best to initially design a strong pavement,
which will last throughout the design period without the need for strengthening, or to design
a weaker, and therefore more economic, pavement with the aim of strengthening it at some
intermediate stage to enable it to last the remainder of the design period (= Stage Construction).
Stage construction is suitable for medium and light traffic (Classes T1, T2, T3 and T4). However,
the pavements proposed for light traffic (Classes T1, T2 and T3) are generally minimum or nearminimum pavements and it is therefore impracticable to reduce them further. Stage
construction is most suitable for pavements carrying medium traffic (Class T4), when normal
construction includes 50 mm of premix as surfacing (see Chapter 9). Stage construction
would then consist of:


Construction of the full road base thickness, and application of double surface
dressing.
An overlay of 30 - 50 mm of premix after about 5 years. It is important that this overlay
is allowed for in the design.
Stage construction is not recommended for heavy traffic, especially overloaded axles
(Classes T5, T6 and T7), as the risks of premature deterioration are unacceptable for such
important roads. Moreover, it is difficult and costly to handle traffic during strengthening
operations.
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8.1.4 Safety Factor
It is apparent that the heavier the traffic, the more costly the pavement and hence the higher
the safeguard against failure should be. For example, under heavy traffic (Classes T5, T6
and T7), it is unwise to place sophisticated (and expensive) materials, such as lean concrete
or dense bitumen macadam, directly on heterogeneous and deformable natural materials.
The ratio of the upper pavement layer to the lower pavement layer should be within the
range 1.5 to 7.5. If a thick bituminous layer is placed directly on a layer of graded crushed
stone it is likely to suffer tension cracking from bottom to top. Addition of cement to the
graded crushed stone to enhance its stiffness is recommended. The exact amount of cement
should be determined in the laboratory in accordance to the required stiffness to be
achieved.
8.1.5 Minimising Base and Surfacing Thicknesses
The thicknesses of road base and surfacing, which are made of the most expensive
materials, should be kept constant and as low as possible, for each class of traffic.
8.2 Practical and experimental considerations
8.2.1 Use of Flexible Pavements
Flexible pavements are defined as pavements composed of a base made of fairly deformable
material, such as natural gravel, graded crushed stone or cement or lime-improved material with a
thin bituminous surfacing (surface dressing or not more than 50 mm of bituminous premix).
Experience has shown that such flexible pavements are perfectly suitable for light and
medium traffic (Classes T1, T2, T3 and T4), i.e. up to 10 million standard axles, provided that this
does not include a substantial proportion of overloaded axles as defined in Chapter 2.
For heavy traffic (Classes T5, T6 and T7), it is necessary to construct a semi-rigid pavement
with a base of bound material, such as dense bitumen macadam, lean concrete, cement
stabilized gravel and/or a thick bituminous surfacing (high stability asphalt concrete).
The most common mechanisms of deterioration of the proposed pavement structures are
rutting in the subgrade, fatigue cracking at the bottom of the bituminous layer and horizontal
cracking at the bottom of cement/lime treated layer.
8.2.2 Influence of Subgrade
8.2.2.1 Compressive strain criterion
Due to the difficulty in estimating subgrade soil parameters most of the current analytical
design methods use the so called “subgrade strain criterion”. This approach relies on the
assumption that the strength of a soil is directly related to its stiffness which might not be
always generally true. Various relationships have been proposed, usually based on real
evidence of performance, which is reasonable within the range of subgrades and pavements
encountered in gathering that evidence. In this manual no specific relationship has been
used, since each is site-specific. It might be reasonable to use them in comparative design
processes but they should not be assumed to be a permanent deformation law unless
proven to be applicable to the specific conditions intended.
It is widely accepted that the compressive strain in the surface of the subgrade is the
criterion that governs the total thickness cover required in the case of a flexible pavement. If
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the compressive strain is excessive, permanent deformation will occur in the subgrade, causing
deformation at the pavement surface.
The relationship between the maximum permissible compressive strain and the cumulative
number of standard axles is usually given by an empirical equation called the “subgrade
failure criterion” which relates the vertical strain at subgrade level to pavement rutting
performance. Many subgrade failure criteria appear in the technical literature but each failure
criterion is an inseparable component of a specific pavement design method and should not
be extracted and used outside the context it was developed for.
In the case of rigid and semi-rigid pavements, the deciding criterion is generally not the
compressive strain in the subgrade, but the horizontal tensile strain in the upper pavement
layers.
8.2.2.2 Subgrade modulus
In the deeper, less highly stressed layers of the pavement, the stress levels rarely reach
failure. This means that the stress levels never approach the failure line. To determine the
transient deformation which will occur under traffic loading we need to know the (shear)
strain response to the change in stress which results from the traffic. This response is
characterized by the stiffness of the subgrade, improved subgrade or sub-base. It is to be
noted that the stiffness decreases as the stress level increases i.e. as the stress gets closer
to failure conditions. The modulus taken into account should correspond to the moisture
content of the subgrade soil most of the time under the pavement, since the effects of repeated
loading are relevant.
At present, quantification relies on conventional soil index tests, the California Bearing Ratio
test (CBR) and the Plate Bearing test (PBT). The CBR does not give a very reliable
indication of material behaviour. It is an empirical test in which (in its laboratory embodiment)
a plunger is advanced once into a cylindrical pot of recompacted soil at a constant rate. The
load causing 2.5 and 5mm penetration is recorded and expressed as a percentage of the
load required for the same penetration in a certain standard material. This depth of
penetration is more than sufficient to cause rupture and large permanent deformation. This
loading regime may be compared with that imposed by pavement traffic which comprises
many repeated light loadings. The test provides information largely relating to soil strength
whereas the designer requires stiffness and permanent deformation
Furthermore very different CBR results are obtained from in situ and from laboratory
measurements on the same material. Design methods are usually imprecise as to test
conditions and whether in situ testing is required. For these reasons, the test cannot be
recommended and values obtained should be used with great caution. The saving grace of
the test, and doubtless the reason for its continued use, is its relative simplicity, speed and
low cost. It is interesting to note that it has not been used in pavement design in California
for many years.
The plate bearing test (PBT) overcomes many of these problems in that it essentially
measures in situ stiffness. However speed of loading effects is not matched by the test. The
PBT can therefore be used during construction as a means of checking that the actual
stiffness is greater than that required for the successful performance of higher layers. Often,
these direct or semi-direct means of determining the stiffness of the pavement layers will not
be possible and empirical or semi-empirical methods might be used. The subgrade moduli of
the most common types of soils at their equilibrium moisture contents should be determined
by direct measurements (e.g plate bearing tests). The design system should incorporate the
dynamic elastic modulus of the subgrade as one of the principal design parameters.
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8.2.2.3 Permanent deformation
The size of the permanent strain depends on how close to failure the stresses are during the
loading pulse. Although it may be very small in any one cycle, because its effect is
cumulative, over many cycles there may be a noticeable build up of permanent deformation.
A stress path may approach failure either because the applied stress is large, or because
the pore water pressure is high, thereby reducing the level of effective stress. Thus a welldrained aggregate sub-base or subgrade is less likely to experience rutting problems during
construction than a wet one.
8.2.3 The Behaviour of Pavement Materials
8.2.3.1 Unbound materials
Protection of the subgrade is the basic pavement design requirement, however, it is
important that the overlying layers do not deform excessively under repetitive loading. This is
not being considered explicitly and reliance is made on material specifications. It is to be
ensured that granular layers should have a suitably high angle of internal friction, giving a
suitably high shear strength leading to an increased stiffness
It is important to appreciate that the dynamic modulus of any unbound layer is not simply a
function of the component material, but is also dependent to a large degree on the stiffness
of the underlying material. The Shell Pavement Design Manual (1978) used the concept of
modular ratio limitations in successive unbound layers. In addition, the resistance to attrition
of each material has been evaluated and the consequent traffic limitations are given in the
Pavement Materials Charts in Chapter 7.
8.2.3.2 Hydraulically Bound Materials
A hydraulically bound material (cemented) will in most cases end up in a cracked state,
which means that its apparent stiffness will inevitably be less than that of the intact material.
Experience suggests that a cement bound base layer with an initial stiffness in the range
10000-20000 MPa can easily end up with an apparent in situ stiffness of no more than 5000
MPa. It is this apparent stiffness which affects the way the layer spreads load to underlying
materials and supports overlying layers.
The deciding criterion is usually the tensile stress at the bottom of the cemented layer.
8.2.3.3 Bituminous Bound materials – fatigue characteristics
When bound materials are used, the deciding criterion is usually the horizontal tensile strain at
the bottom of the bituminous base or surfacing. If this strain is excessive, the layer will crack.
The principal task is that of developing a fatigue characteristic for use in design and this can
only be achieved by calibrating against observed pavement performance. Any relationship
derived from laboratory testing should be assessed carefully. Laboratory test results on
bituminous materials do not replicate real conditions and also the set ups and procedures
adopted during testing might lack realism.
In this manual, the fatigue characteristics of bound materials have been estimated on the basis of
measured characteristics of the material and from theoretical considerations.
Bituminous have a visco-elastic nature and their stiffness modulus therefore depend on the
rate of application of the load and on the temperature. Temperature has a significant effect
on the stiffness as well as the fatigue and permanent deformation resistance of bituminous
mixtures. It is therefore quite obvious that accurate knowledge of the temperature distribution
in the pavement should be available in order to allow realistic analyses of the stresses and
strains in asphalt pavements to be made. Assuming a constant temperature over the
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thickness of the asphalt layer is far from reality unless one is dealing with thin asphalt layers.
Furthermore the total asphalt thickness is commonly made of different types of asphalt
mixtures, especially in case the total thickness is larger than 100 mm.
8.2.3.4 Bituminous Bound materials – top-down cracking
The true mode of deterioration in a bituminous layer might be by surface cracking. It is
necessary to develop a calculation technique for strain originating at the surface. Although
there is no consensus some useful working approximations have been made and are based
on the assumption that surface strain is a function of asphalt compression.
It has been observed that for an identical granular pavement foundation but varying asphalt
thicknesses top-down cracking tends to predominate on thick pavements and that bottom-up
cracking predominates for thin pavements.
8.2.3.5 Pavement materials moduli
The above mentioned factors should be considered when assigning stiffness moduli to the
various pavement layers. It is known that rigid and semi-rigid pavement layers need
adequate support. It is suggested from empirical studies that, materials whose moduli are less than
10% of the succeeding rigid or semi rigid layer are unlikely to give that support. From consideration
of the moduli tabulated earlier, suitable support layers are given in Table tabulated below.
Table 8.1: Suitable Support Layers for Rigid or Semi-Rigid Pavement Layers
Rigid or Semi Rigid Pavement Layer
Stabilised material
Suitable Support Layers
Subgrade min CBR = 15%
Subbase gravel
Cement Stabilised material
Graded Crushed Stone (GCS)
Stabilised material
Dense Bituminous Macadam
Stabilised material
Graded Crushed Stone (GCS)
Lean Concrete
Stabilised material
Asphalt Type I
Dense Bituminous Macadam
Lean Concrete
Cement Stabilised material
Graded Crushed Stone (GCS)
Stabilised material
Asphalt Type II
Stabilised material
Sand Asphalt and Gap Graded Asphalt Stabilised material
Graded Crushed Stone (GCS)
8.3 Calculation of stress, strain, deflection and layer thickness
8.3.1 Calculation of stress, strain and deflection
For various pavement structures, an analysis of stresses and strains due to traffic loads
were made using an elastic multi layer system. In carrying out these analyses, the following
assumptions regarding the behaviour of the materials were made:



the pavement layers are composed of homogeneous and isotropic linear elastic
material
their horizontal extent is infinite
the layer thickness is uniform, and
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a load of uniform pressure is applied over a circular contact area
However, it should be noted that:

If granular layers are separated from the applied load by a significant thickness of
asphalt or concrete, it is reasonable to assume elastic material properties for them in
the subgrade stress or strain calculation. Multi-layer linear elastic analysis can then be
used, assigning an elastic modulus and Poisson’s ratio to each layer, and stresses
and strains calculated under the design load

It is logical to expect that the stiffness behaviour of asphalt to be very far from linear –
but not so. The point here is that the magnitude of strain taking place within the
aggregate skeleton is small compared to that common in an unbound material (i.e.
unbound layer not overlain by a thick bituminous structure) and in the small strain
region the stiffness of an unbound material is approximately constant. Furthermore
the loading rate is usually high enough to ensure that the bitumen is kept near the
elastic end of its visco-elastic behaviour spectrum. The combination is sufficient to
give an approximately linear stress-strain response. However, the actual value of
stiffness modulus will vary significantly with bitumen properties which in turn depend
on temperature and loading rate.
In the design procedure, the pavement is regarded as a three-layer system, if it comprises a
thin bituminous surfacing (surface dressing or thin bituminous surfacing), a four-layer
system, if it comprises a thick bituminous surfacing (more than 50 mm) and a five layer
system if an improved subgrade layer is considered.
The lowest layer, taken as semi-infinite, represents the subgrade including improved
subgrade, if any. The upper layers represent respectively the subbase, base and, if any, the
thick bituminous surfacing.
Layered analytical models are generally based on the work of Burmister (1943)
The calculation of stress, strain and deflection are computed with the following assumptions;



The design load is assumed to be uniformly distributed over one circular area giving
an applied pressure of 0.56MPa
Most of the pavement materials have a Poisson's ratio equal to 0.35 except for
hydraulically bound layers who are assigned a value of 0.25
(All layers are considered to have complete friction between
them (fully bonded)
The computer program ALIZE of the LCPC has been used to calculate the horizontal tensile
stress and strain at the bottom of each layer made of bound material, the vertical
compressive stress and strain in the surface of each layer, including the subgrade, and the
deflection at the surface of the pavement. Design axle loads up to 80 kN are considered.
8.3.2 Determination of layer thicknesses
In the case of flexible pavements, the total pavement thickness required has been
determined by a comparison between the compressive strain applied to the subgrade and
the maximum permissible strain which depends on the number of load applications.
In the case of bound materials, the thickness required for each individual layer has been
determined by a comparison between the tensile strain at the bottom of the layer and the
maximum permissible strain, as deduced from the fatigue law of the material.
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In addition, it has been checked that compressive strain on the subgrade is within a
permissible range.
8.4 Construction Principles
8.4.1 Minimum layer thickness
For each material, there is a minimum layer thickness below which proper laying and
compaction are not possible.
For granular materials, if D is the maximum particle size then the minimum practical thickness is
2.5 D for surfacing and base and 2 D for subbase layers.
In addition, irrespective of the material type, it is impractical to lay subbase and bases to
compacted thicknesses of less than 100 mm. For the different types of material considered, the
minimum practical thicknesses are as follows:
Table 8.2: Minimum layer thicknesses
Layer
Material Type
Minimum Thickness
(mm)
Subbase Natural Gravel
100
Clayey Sand
100
GCS2 0/40
100
GCS2 0/60
125
Base
Natural Gravel
125
Clayey Sand
100
GCS1 0/20
100
GCS1 0/30
100
GCS1 0/40
125
DBM 0/30
125
DBM 0/40
150
Surfacing Asphalt Concrete 0/20 50
Asphalt Concrete 0/10 25-30
Sand Asphalt
25-30
8.4.2 Minimum significant thickness increments
Considering the usual level and thickness tolerances within which the different layers have to
be constructed, it is clear that thickness variations of less than 25 mm are meaningless.
Consequently, the layer thicknesses of the structures proposed vary by minimum increments
of 25 mm.
8.4.3 Compliance with the specifications
All the materials are assumed to comply with the requirements given in Chapter 7 and all the
layers to be constructed in accordance with current specifications.1
1
References
Burmister, DM (1943): Theory of Stresses and displacements in layered systems and application to
the design of airport runways. Proc. Highway Res. Board, 23, Washington DC, pp 126-148
Autret, P et al (1982): ALIZE III Practice. 5th Int. Conf. on Structural Design of Asphalt Pavements,
Delft University
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9 Standard Pavement Structures
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10 Pavement Shoulders, Drainage and Cross Sections
In an ideal world, the road shoulders would be an extension of the carriageway. Thus, there
would be no question that the shoulders were structurally adequate for purpose, would
support the sides of the carriageway and, provided they were sealed, would prevent
rainwater seeping into the edge of the carriageway and weakening it.
However, in the real world, in order to economise, the shoulders are often constructed of
different, usually inferior materials, are not sealed, leading to their differential erosion and
premature weakening of the carriageway.
10.1 Shoulders
Deterioration of paved roads often begins with edge-fretting, especially if the shoulders are
unsealed; the repair of such damaged roads is then difficult to carry out effectively. Thus, the
pavement shoulders should be considered as a fundamental part of the pavement which
functions are to:





improve road safety by providing better visibility and convenient hard standing for
temporarily disabled vehicles and police roadblocks
give added width to the carriageway for emergency use
provide lateral support to the pavement layers, especially if granular materials are
used for the base
facilitate removal of surface water from the road and,
protect the edges of the subqrade against soaking and facilitate the internal drainage
of pavement layers.
Shoulders should therefore have sufficient strength to carry occasional traffic, be impervious
to surface water, be properly shaped so as to shed water completely and be erosion
resistant.
It is always preferable to construct the base and subbase materials right across the
shoulders to the drainage ditches. This provides lateral support to a granular base and
simplifies the construction.
10.1.1
Bearing Capacity of the Shoulders
Use of the same pavement structure for the shoulders as for the carriageway simplifies
construction and ensures that the bearing capacity of the shoulders will be adequate for the
design life of the road. If this is not the case, site conditions will determine the strength
required for the pavement depending mainly on the likelihood of heavy traffic using the
shoulder.
Generally shoulders to bituminous roads should be constructed at least with material of
gravel wearing course quality, which is to a minimum strength of CBR 30% or, if stabilized
material is used, to CS standard. For the heaviest traffic (Classes T5 to T7), higher strengths
are required, and in this case the shoulders should definitely be constructed to the same
standard as the carriageway.
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For lightly trafficked roads (Classes T1 and T2), where no suitable gravel is available,
material with a minimum soaked CBR of 15% at 95% of BS Heavy can be used. In arid
areas the soaked condition may be relaxed to CBR of 15% at 95% of BS Heavy, at OMC.
10.1.2
Surfacing of Shoulders
A waterproof and durable bituminous surfacing must be used for roads where the shoulders
are paved. Prime coats alone are inadequate.
If the shoulder base material is a non-cohesive material such as graded crushed stone or
non-plastic gravel, the shoulders must be primed and sealed. The type of seal may be either
a single or double surface dressing, preferably with a sand seal cover, or an Otta seal with a
sand seal cover, or asphalt concrete Type 2 in the case of traffic classes T5 to T7.
If the shoulder base material is a cohesive material, such as plastic gravel or cement (or
lime) stabilised material, the shoulders may be left unsurfaced, except in the case of traffic
classes T5 to T7 where they should be surfaced as above. It is preferable, however, that all
shoulders should be sealed because, in addition to increasing their longevity, sealing
prevents the ingress of surface water at carriageway edges.
For lightly trafficked roads (Classes T1 and T2), shoulders can be left unsealed but protected
from erosion by topsoiling and grassing. After the final trimming and compaction, the
shoulders shall be topped with 20 mm of humus or topsoil and lightly rolled. Sprigs of
indigenous “runner type” grass should then be planted or alternatively the planting of seed
may be used. This type of protection is applicable to gravel (and earth) shoulders, in fairly
wet areas. The shoulder surface should then be about 20 mm below the carriageway edge.
Where shoulders are unsealed attention should be paid to the internal drainage of the
carriageway base because suitable gravel for the shoulders is likely to be impermeable, thus
preventing drainage from the base. If the carriageway base is constructed of permeable
materal, the following alternative measures are required: either


place a 75mm thick drainage layer immediately below the shoulder gravel, or
install a special drainage facility
Consideration may be given to protection of surfaces from high speed running by heavy
traffic. This may be done either by providing rumble strips at intervals or by using special
paver edging shoes to provide a distinct non-dangerous drop-off at the edge of the
carriageway to the shoulder level.
Kerbs at the edge of the carriageway are the best protection for edge-fretting but they are
expensive. Their use should be considered for all roads carrying heavy traffic subjected to
frequent entry and exit and stopping on shoulders, or in urban areas. However, there is a
small problem in that they create a discontinuity along the road edges, with possible water
ingress, and should only be used where a permeable base and subbase are used.
All surface treatments for shoulders should wherever possible be designed to give the
shoulders a contrasting texture or colour compared with the carriageway.
10.1.3
Prevention of cracks in the shoulders
Longitudinal cracks in shoulders are associated with differential settlement in earthworks or
pavement layers owing to road widening. Good techniques for earthworks and pavement
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construction, set out in the Standard Specifications for Highway Construction, minimize
these risks.
Longitudinal cracks in shoulders can also be caused by the presence of expansive soils in
the subgrade. Design and construction methods to counteract cracking caused by expansive
soils are set out in Chapter 11.
Transverse cracks can develop in shoulders due to either thermal movement in bituminous
layers or by shrinkage in stabilized layers. All that can be done if this occurs is to re-seal the
cracks during periodic maintenance.
10.2 Drainage
10.2.1
Drainage on the Road Surface and Shoulders
Rain falling on the road surface and shoulders must be conveyed rapidly to the side ditches.
For this purpose, road and shoulder surfaces are given a crossfall, the value of which
depends on the nature or the surface. The following crossfalls are recommended:




Bituminous and concrete road surfacings : 2.5%
Earth and gravel road surfaces
:3 to 4%
Gravel shoulders
: 4%
Primed or cement treated shoulders
: 6%
For minor roads constructed in hilly territory, frequent crossfall changes can lead to flat spots
on the finished pavement, which hold water. Cement stabilised bases are particularly
vulnerable to this fault as they cannot normally be re-trimmed and re-compacted within the
specified time limits. The crossfall on this type of road may therefore be steepened to 4%, at
the discretion of the designer
10.2.2
Drainage of the Pavement Layers
Effective drainage of granular pavement layers is essential for their good performance and is
ensured by attention to cross section details. In particular, ‘boxed-in’ pavements, where
water could be trapped in the pavement layers, must not be used. Measures to ensure
proper drainage of the pavement layers must be included in the design, particularly where
internal drainage could be impaired, possibly in the following circumstances:



where shoulders are designed with different materials to those of the carriageway
where kerbstones are extended into granular pavement layers, and
where unpaved shoulders comprising near impermeable materials are used.
10.2.3
Granular bases
Where a granular base and paved shoulders are used, the base and subbase layers must
be extended across the full width of the shoulders
10.2.4
Cemented or Bituminous bases
Where economically possible the base should be extended across the full width of the
shoulders.
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10.2.5
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Drainage of the Subgrade
Sufficiently deep open side drains or alternatively special facilities such as sub-surface
drains will ensure proper drainage of the subgrade. Particular attention to design and
construction details is required where rock occurs, which could trap water in the subgrade.
In soils, open side drains shall not be less than 0.5m deep, measured from the drain bottom
to formation level.
In cuttings in soils open side drains shall be not less than 1m deep, measured from the drain
bottom to formation level. This depth can be reduced to 0.5m if the subgrade is cement or
lime modified.
In cuttings in solid rock the required drainage measures depend on site conditions and shall
be decided in individual cases.
The need for sub-surface drains as alternatives to open drains depends on site conditions,
requiring careful consideration owing to their high cost. Construction in urban areas, the
presence of subsoil wells and in some types of cuttings are instances where these types of
drains are required.
10.3 Cross Sections
Normally, the cross section design for a road is determined by the geometric standards
applied to the project and includes any special circumstances such as problem soils.
Standard principles and designs are given in Parts I and II of the Kenya Design Manuals.
10.3.1
Edge Restraint
The edges of the roadbase must be given sufficient lateral support, so that they can support
heavy vehicles. This problem is particularly serious in the case of non cohesive materials,
such as graded crushed stone. Proper compaction of the edges is in any case difficult and
lower densities frequently result. Two alternative solutions are available:

Lay a base wider than the bituminous surfacing, so that the base edges are not
trafficked. The extra width shall be 200 - 300 mm on each side. It shall be primed and
sealed, together with the inner edge of shoulder (total width of edge seal: 400 - 600
mm).

Alternatively, concrete kerbs may be placed. However, they are expensive and are
justified only when graded crushed stone bases are used, or in urban areas.
Moreover, they create a discontinuity along the edges, with possible cracking and
subsequent water ingress. Kerbs along roadbases are therefore recommended only
where a pervious subbase is laid.
10.3.2
Recommended Cross-Sections
10.3.2.1 Normal shoulders (width ≥ 1.5m)
Resulting from the above considerations, recommended pavement cross-sections are shown
below:
Figure 10.1: Shoulder cross sections
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Type A normally applies to roads with bases comprising one of the following materials:






plastic natural gravel
cement or lime improved material
cement stabilized gravel
bitumen stabilized silty or clayey sand
dense bitumen macadam
lean concrete
Type B applies to roads with graded crushed stone or non-plastic gravel bases and
impervious subbases, when the base is not more than 150mm thick.
Type C applies to roads with graded crushed stone or non-plastic gravel bases and pervious
subbases. It may also be chosen for roads with cement or bitumen-treated bases, if the base
impermeability is uncertain or if extending the subbase is found to be a simpler construction
procedure.
Type D applies to roads with graded crushed stone or non-plastic gravel bases and
impervious subbases. Since the drainage layer and the upper shoulder must not be less
than 75 mm thick, this cross-section is suitable only when the base thickness exceeds
150mm.
Type E applies to cohesionless or low cohesion base materials with pervious subbases.
It should be noted that the list of pavement cross sections given in this Manual is not
exhaustive. The design engineer may, for technical and/or economic reasons, choose other
types of cross-sections, provided the basic requirements for drainage and edge restraint are
complied with.
10.3.2.2 Narrow shoulders (width < 1.5m)
The base and subbase should be extended right across the shoulders. This type of
pavement cross-section is referred to as “Type X”
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11 Problem Soils
This chapter describes particular subgrade soils which can be a problem to the highway
engineer in road construction. They are described as ‘Low strength soils’, ‘Expansive soils’,
‘Saline soils’ and ‘Organic’ soils.
11.1 Low Strength Soils
Soils with CBR < 3%, or < 2% in arid areas, occurring within the design depth, are defined
as ‘Low Strength Soils’. Before they can be included in the foundation structure they require
treatment which can include one or several of the following measures:




Removal and replacement
Chemical stabilisation with either lime and/or cement
Mechanical stabilisation, or
Raising of the vertical alignment to increase cover, thereby re-defining the design depth
Details regarding the treatment of such soils will vary according to soil properties, site
conditions, available equipment, alternative materials and parallel experience and will be
determined at the time of the project.
11.2 Expansive Soils
11.2.1
Definition
Otherwise known as ‘black cotton soil’ because of its characteristic appearance, the main
property of expansive soil is the significant volume changes it undergoes when wetted and
dried. When a road is sealed a strip of land is created under the road which is protected from
seasonal variation of rainfall. The centre of the strip will be subject to different moisture (and
therefore volume) changes. This can result in longitudinal cracking along the road edges if it
is founded on black cotton soil, which become more accentuated with time and progressively
extend towards the center of the road.
Figure 11.1: Moisture Variation in Expansive Soils
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11.2.2
Part 3 - Materials and Pavement
Distribution
It is believed that the primary source of residual expansive clay soils is the in situ weathering
of basic igneous, metamorphic and pyroclastic rocks, which occur in abundance in Kenya.
Thus, expansive soil is quite common and local knowledge is very useful to identify areas
where it can be a problem. Typically it forms in flat, poorly drained environments which
favour the formation of the expansive soil minerals.
11.2.3
Identification
Apart from their appearance other indicators of expansive soils are described in Table 11.1:
Table 11.1: Indicators of Expansive Soils
Soil Description
Soil Type
Consistency, when slightly moist to dry
Consistency, when wet
Structure
Colour
Typical Features of Expansive Soils
The more clayey, the more likely to be
expansive
Stiff or very stiff
Soft and sticky
Cracked surface and slickensided fissures
Usually dark but this is not always so
The shrinking and swelling property is caused by the preponderance of the clay mineral
montmorillonite. There is no quantitative test to determine the amount of montmorillonite
present but normal classification tests enable the severity of the expansiveness to be
established, as explained below.
It has been found that the ratio of Plasticity Index to clay fraction is more or less constant for
any one soil, but this constant varies depending on clay type. The correlation between PI
and clay type is termed ‘Activity’, where:
Activity = PI/clay fraction
To be consistent, the clay fraction is expressed as that portion of the soil sample passing the
0.425mm sieve, rather than the percentage passing 2µm. On this basis clays can be
classified into four groups as shown in Table 11.2.
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Table 11.2: Activity of clays
Description
Inactive clays
Normal clays
Active clays
Highly active clays
Activity
<0.75
0.75 to 1.25
1.25 to 2
>2
If the presence of active or highly active clays is established, further testing involving the
determination of the Shrinkage Limit (KS 999 Part 2: 2001) is advisable. The Expansiveness,
ex, is calculated from the following empirical formula:
ex = 2.4*Wp – 3.9*Ws + 32.5
where Wp = Plastic Limit * fraction passing the 425m sieve/100
and
Ws = Shrinkage Limit * fraction passing the 425m sieve/100
Expansiveness is then classified according to Table 11.2:
Table 11.3: Degree of Expansiveness of Expansive Soils
Expansiveness,
ex
<20
20 to 50
>50
11.2.4
Classification
Low
Medium
High
Remediation
Four possible treatments are possible to overcome the problem of expansive soils:




avoid by re-alignment
excavate and replace with non-expansive materials
stabilise with lime, or
minimise moisture changes by engineering measures
11.2.4.1 Re-alignment
This is only possible if the expansive soil is limited in extent.
11.2.4.2 Replacement
This is the simplest and most effective treatment but the cost and effect on the environment
by sidecasting large quantities of material must be assessed. Indeed, in Kenya the thickness
of black cotton soil is usually between 1 to 1.2m and it is underlain by weathered igneous
rock (phonolite in Nairobi area) which can be used as backfill. In practice it is sufficient to
remove the expansive soil to a depth of 1m. Even if some expansive clay remains it should
be adequately confined and protected from moisture changes. The backfill should be at least
of S2 quality and impermeable enough not to act as a drain.
Embankments should be constructed with suitable fill material as discussed later.
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11.2.4.3 Stabilisation
If proper mixing can be achieved, treatment of expansive soil with 4% to 6% of hydrated lime
is usually effective and provides the following improvements:





reduces the Plasticity Index to less than 20
increases considerably the Shrinkage Limit
reduces Swell to negligible values, and
increases the CBR to minimum of 10 (after 7 days cure) and 15 (after 28 days cure)
alters the grading by agglomeration of the clay particles similar to that of a silt
All these improvements render the treated soil easily workable and it can be assumed that it
will become of S4 quality. However, it is costly because a substantial thickness must be
treated (minimum 300mm) and therefore is advantageous where no suitable backfill or
improved subgrade material exists, or there are strong environmental objections to
sidecasting large amounts of expansive soil.
The mixing procedure is to add the lime in two or three increments, followed by intense
mixing by pulvimixer. The mixed soil should be left fallow for two to three days between each
mixing operation to enable the lime to take effect. Wet weather makes initiating this
operation impossible owing to the physical nature of the soil.
11.2.4.4 Engineering Measures for Construction on Expansive Soils
If none of the above measures can be avoided, special precautions are required to avoid
damage to the road structure caused by detrimental volume changes when building on or
with expansive soils.
Widening of shoulders is beneficial whenever economically feasible. The zone of seasonal
moisture (and volume) change is thus moved further away from the carriageway.
Side drains, if required, should be placed at a minimum distance of 4 to 6 m, depending on
road category (Category A roads requiring 6 m). Side fill consisting of expansive soil requires
protection from erosion by grasses but no trees should be planted or allowed to root on the
embankment slope.
Table 11.3 proposes alternative methods of construction over expansive soil:
Table 11.4: Construction on Expansive Soils
Expansiveness
Low
ex<20
Medium
ex20-50
High
ex>50
Alternative proposed construction over expansive soil
Paved Trunk Roads
Other Paved Roads
Sealed shoulders
Side slopes 1:6*
As normal design
See Fig 11.2
Sealed shoulders
Side slopes 1:6 minimum
Earthworks cover min 1m
Earthworks cover min 0.6m
See Fig
Excavate and replace 0.6m clay as Fig
Earthworks cover min 1m
Sealed shoulders
Side slopes 1:6 min
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Shoulder width min 2m
Alternative:
None
Part 3 - Materials and Pavement
Alternative:
Sealed shoulders
Shoulder width min 2m
Earthworks cover min 1m
Side slopes min 1:6
* Where the earthworks cover is > 2m the side slopes can be made maximum 1:4
Processing and compaction of expansive soils does not reduce their swell properties and
their strength is not significantly increased. Attempts to adjust the moisture content, eg to
achieve an optimum are time-consuming, impractical and unnecessary. Nominal rolling of
the roadbed is desirable to obtain a working surface for construction of pavement layers. Fill
materials used for replacement of expansive soil should meet the specifications for fill.
Plastic soils with minimum PI of 15 should be used when available at economic haulage
distances.
Figure 11.2: Alternative methods of construction on Expansive Soil
11.3 Saline Soils
The presence of soluble salts, ie NaCl, Na2CO3, NaHCO3, (but not gypsum, Na2SO4, which is
only slightly soluble) in pavement or earthwork materials, or more critically in the subgrade
and/or groundwater can cause damage to prime coats and thin surfacings. This is a
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significant risk in arid climates because of the migration of these salts to the surface as a
result of evaporation. Coastal areas, or possibly areas in the vicinity of the saline lakes in
Kenya, are most at risk from this mode of damage.
The content of soluble salt can be rapidly but indirectly determined by laboratory or field
determination of electrical conductivity. It is prudent to determine the precise configuration of
soluble salts by chemical analysis on a few samples and relate this to the conductivity. If the
tests are carried out on potential pavement materials, rather than subgrade, construction
water should be added at 1.5 times the required amount to obtain the OMC, to allow for
evaporation, before the sample is tested.
Prime coats are very vulnerable to the formation of blisters in the bituminous surfacing and
by fretting of the edges of the surfacing. If the soluble salt content, measured as % Total
Soluble Salt (TSS), exceeds approximately 0.3% in the upper 50mm of the road base, they
are susceptible to damage. Cutback prime is more vulnerable than emulsion prime.
Blistering damage is accelerated if the road is low-trafficked.
Surface dressing is more resistant to attack. Single and double surface dressings are not
susceptible to damage unless the %TSS exceeds 1.0%; however, if surface dressings are
constructed on saline subgrades, it is recommended that an impermeable fabric be placed
beneath the road base to prevent the upward rise of salt and protect the surface dressing
from eventual salt damage. If the road is well trafficked, the susceptibility to damage is
reduced.
11.4 Organic Soils
These commonly occur in swamp areas and require special investigations to evaluate
ground stability and potential for excessive settlement. Typically, remediation consists of
surcharging the pavement structure for a specified time before removing the surcharge and
constructing the pavement. Other remediation measures comprise removal and replacement
of the organic soil or, in extreme cases, construction of the road ‘floating‘ on the swamp
material.
A high content of organic matter (>2%) is undesirable in pavement materials, particularly in
stabilized layers because it causes increased demand for stabilizer to achieve the required
strength.
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12 Gravel Roads
12.1 Introduction
Approximately 80% of the road network length in Kenya is of earth or gravel type and
therefore their construction justifies some consideration. Earth roads consist of tracks
made from the in situ soil and are normally suitable for light traffic in dry weather. The
provision of a gravel surface is part of the process in making an all-weather road
designed to particular standards of alignment and traffic-carrying capacity.
However, there is no doubt regarding the shortcomings of earth and gravel roads. In dry
weather they can be very dusty. Unless they are well maintained their surfaces can
become corrugated and potholed, and impassable in wet weather. The ideal material for
surfacing gravel roads is clayey gravel or clayey sand which should be found in the local
area. However, in many parts of Kenya suitable gravel sources are in short supply, often
necessitating considerable haulage, and this situation is made worse because the gravel
is progressively lost by the action of traffic, at a rate of roughly 10 to 30mm per 100 ADT,
and has to be replaced. Vehicle operating costs are higher on earth and gravel roads
than on roads with permanent surfacings. Road accidents probably occur more
frequently. Thus, there are powerful incentives in all countries to increase the proportion
of permanently surfaced roads.
The mechanism of deterioration of gravel roads is directly related to the number of
vehicles using the road rather than the number of equivalent standard axles. The traffic
volume is therefore used in the design of unpaved roads, as opposed to paved roads
where traffic volumes are converted into a cumulative number of equivalent standard
axles. For earth and gravel roads maximum traffic volumes up to approximately 300ADT
are typical, after which construction of a permanent surfacing is considered.
12.2 Design Elements of Gravel Roads
The elements of a gravel road are illustrated in Figure 12.1:
Figure 12.1: Elements of a Gravel Road
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Subgrade
Part 3 - Materials and Pavement
Gravel Wearing Course
Capping Layer
(If required)
Roadbed
(Where no
embankment is
used)
Fig. 1.1 Elements of a Gravel Pavement
The deterioration of gravel roads differs from those of permanently-surfaced roads. In
this context, the main purposes of the gravel wearing course are:


to enable all-weather trafficking, and
to protect the sub-grade from undue strain
However, the potential defects of a gravel road require other considerations in their
design.
Typical defects affecting gravel roads include dustiness, potholes, stoniness,
corrugations, ruts, cracks, ravelling (formation of loose material), erosion, slipperiness,
impassibility and loss of wearing course material. Many of these have a direct effect on
the road roughness and safety.
A major problem for unpaved roads built on steep alignments is the efficient removal of
surface water to the side drains. As the gradients increase, the problem becomes more
acute irrespective of any increase in the cross-fall of the road. The problem of gulley
erosion along the centre of unpaved roads will be exacerbated as vertical gradients
increase above the value of the cross-fall.
The cross-fall of the carriageway and shoulders of gravel roads should range between 4
and 6%. Although proper drainage is very important for gravel roads, an excessive
cross-fall could cause erosion of the surface. Erosion is frequently manifested in the
form of longitudinal gullies along the surface of steep roads with gradients higher than
about 5%. Construction of both the carriageway and shoulders of gravel roads should be
identical.
12.3 Design of Gravel Roads
It is recommended that pavement and improved subgrade for major gravel roads
(>50ADT) should be constructed in accordance with Table 12.1:
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Table 12.1: Subgrade design for Gravel Roads
Subgrade < 50 ADT
Class
S3
150mm GW
S2
150mm GW
S1
Dry
150mm GW
150mm S2
Wet
150mm GW
300mm S2
50 to 100 ADT
100 to 300 ADT
150mm GW
150mm GW
100mm S3
Dry
150mm GW
150mm S3
150mm S2
150mm GW
150mm GW
150mm S3
Dry
150mm GW
150mm S3
150mm S2
Wet
150mm GW
200mm S3
200mm S2
Wet
150mm GW
200mm S3
300mm S2
The use of an improved sub-grade has the following advantages:







provides extra protection under heavy axle loads
protects underlying earthworks
provides a running surface for construction traffic
assists compaction of upper pavement layers
provides homogenous sub-grade strength
acts as a drainage filter layer, and
enables more economical use of available gravel materials.
It is assumed that 50% of the ADT will be ‘heavy’ vehicles, defined as having an unladen
weight of > 3 tonnes, or buses with > 40 seats.
Roads with approximately < 50 ADT are normally earth roads built by labour-based
methods. However, for subgrade values of Class S1 and longitudinal gradients of > 6%,
a gravel wearing course is recommended.
12.4 Material Specifications
12.4.1
Gravel wearing course materials (GW)
These materials are required to have the following, somewhat conflicting, requirements:


sufficient cohesion to prevent ravelling and corrugating, especially in dry
conditions, and
a limited amount of fines, particularly plastic fines, to avoid slipperiness in wet
conditions.
Table 12.2 shows the essential characteristics required:
Table 12.2: Material specifications for Gravel Roads
Grading, sieve size, mm % passing
37.5
Class 1
Class 2
-
100
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28
20
14
10
5
2
1
0.425
0.075
Plasticity Index (PI)
Linear Shrinkage
(alternative to PI)
Grading Coefficient
Shrinkage Product
Bearing Strength
CBR at 95% T180
Note:
Part 3 - Materials and Pavement
100
95-100
95-100
85-100
80-100
65-100
65-100
55-100
45-85
35-92
30-68
23-77
25-56
18-62
18-44
14-50
12-32
10-40
Wet Areas: 5 to 20
Dry Areas: 10 to 30
Wet Areas: 3 to 10
Dry Areas: 5 to 15
16 to 34
120 to 400
>20 (soaked for wet areas)
>20 (at OMC for dry areas)
Grading Coefficient = [(%passing 28mm) – (%passing 0.425mm) x (%passing 5mm)]/100
Shrinkage Product = Linear Shrinkage x (%passing 0.425mm )
Local knowledge of performance may enable materials outside of the recommended
specifications to be used. In particular, rejected quarry materials fall into this category. In
Kenya, these usually consist of weathered volcanic rocks, whose grading and physical
properties vary according to the degree of weathering and which may be further
changed by compaction during construction.
However, suitable material is hard, angular and durable and should be free of organic
matter and lumps of clay.
Figure 12.1 illustrates the performance characteristics expected of gravel wearing
course materials.
Figure 12.2: Performance Characteristics of Gravel Wearing Courses
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12.4.2
Part 3 - Materials and Pavement
Subgrade materials (S2, S3)
The subgrade of the gravel road is classified according to its CBR strength as shown in
Table 13.3:
Table 12.3: Gravel road Subgrade Classification
Subgrade CBRdesign %
Class
Wet Zones
4 day soaked
S3
>15
S2
7 to 14
S1
3 to 6
Density for CBR determination
(% of MDD)
Dry Zones
OMC
4 day soaked
>15
>7
95% of AASHTO T180
7 to 14 3 to 14
93% of AASHTO T180
3 to 6
2 to 6
100% of AASHTO T90
Note: BS-Light compaction effort is used on poor in-situ soils and deep in-situ soils rather than BS-Heavy
due to its better correspondence with the actual effect from compaction equipment under conditions with
poor support for compaction.
Depending on the CBRdesign of the sub-grade, improved sub-grade layers may be
required, on which the gravel wearing course is placed. Soils used in improved subgrade
layers shall be non-expansive and free from any deleterious matter. Laboratory test
results shall meet the requirements in Table 13.4:
Table 12.4: Improved Subgrade Specifications for Gravel Roads
Material properties
CBR, %, wet climate zone
CBR, %, dry climate zone
CBR swell, %
PI
Max. particle size
Compacted layer thickness
S3 (Upper Layer)
>15 after 4 days soak
>15 at OMC
>7 after 4 days soak
<1.5
<25
⅔ of layer thickness
250mm max.
S2 (Lower Layer)
>7 after 4 days soak
>7 at OMC
>3 after 4 days soak
<2.0
<30
⅔ of layer thickness
250mm max.
Note: CBR swell is measured at 100% of AASHTO T180
12.5 Deterioration and Maintenance
12.5.1
Gravel Loss and Recharge
According to research carried out in the 1970s in Kenya, the annual loss of gravel on a
gravel road is a function of traffic volume, rainfall, gravel type and geometric variation.
The interaction between traffic and rainfall contributes significantly to the loss of gravel.
Annual gravel loss varies between 10 mm and 30 mm per 100 ADT, depending on
climate and road alignment. Fig 12.2 illustrates how loss rates for laterite gravel may
vary with rainfall.
Figure 12.3: Variation of gravel loss with rainfall
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The wearing course of a new gravel road has a thickness D calculated from:
D = D1 + N. GL
D1 is the minimum thickness calculated from Figure
N is the period between re-gravelling operations in years
GL is the annual gravel loss
Re-gravelling operations should be planned to ensure that the actual gravel thickness
never falls below about 0.5 D1.
12.5.2
Maintenance
Periodic dragging and grading must be used to preserve a reasonably even running
surface on gravel roads.
Dragging is a dry season task using tractor-towed brooms, drags or sledges to redistribute loose material over the road surface and reduce the rate at which corrugations
occur. Grading is a wet season task using motor graders to restore the running surface
and bring back some of the gravel lost to the sides of the road. The road surface is
loosened to the depth of the corrugations, the ‘lost’ material reclaimed and the mixture
spread to the correct camber over the running surface. The material can either be left
loose to be compacted by traffic or watered and rolled, an activity which will significantly
reduce the rate at which corrugations appear.
In dry weather dust raised by traffic can be a serious problem, especially in populated
areas. Some relief, albeit temporary, can be obtained by spraying with water but this
must be kept up. The addition of salts, such as calcium chloride or common salt, to the
water act to retain moisture in the surfacing providing the relative humidity is sufficient
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but, of course, they are removed by rain. Certain organic compounds, for example the
liquor which is a by-product of paper making or waste molasses from sugar cane
production are effective for longer. Waste mineral oil can also be used but the surface
becomes slippery when wet and the benefit is lost after re-grading.
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13 Concrete Roads
13.1 Introduction
This Chapter presents summary guidance and recommendations for engineers
responsible for the design of concrete pavements.
Hitherto there has been little interest in constructing concrete pavements in Kenya,
owing principally to lack of familiarity and expertise to build them compared to asphalt
roads. Although their construction cost may be higher than asphalt pavements, once
properly built, however, their maintenance costs are potentially significantly lower.
Concrete roads are popular in other countries: in the Philippines they comprise over 75%
by length of the road network, including low volume roads, whilst in the Mekong delta in
Vietnam they are the normal means of road construction for all types of roads. In Chile,
concrete road building is firmly established, largely because of the initiative of local
cement manufacturers in promoting the training of engineers and workmen in concrete
technology.
It is a characteristic of concrete pavements that either they are a great success, lasting
many years without much attention, or they are a problem from the start, sometimes
because of design faults but more often because of construction faults. Misaligned dowel
bars can cause early trouble and concrete of inadequate strength can be broken up
quickly under heavy traffic loads, the main justification for their proposed construction in
Kenya.
In recent years there has been concern regarding the premature failure of some of the
major asphalt roads in Kenya, manifested in spectacular rutting. Although this is
probably a result of either inappropriate asphalt mix design or implementation, or both,
and while modern developments have sought to mitigate the problem of asphalt failure,
there has lately been renewed interest in concrete for use on heavily trafficked roads.
Pilot concrete road trials have recently been constructed in Kenya. In August 2006 about
4km of dual carriageway (Mbagathi Way) in Nairobi was reconstructed with a concrete
pavement (the cement being donated by Bamburi Cement Co). In March 2007 the Gilgil
weighbridge facility near Naivasha, 200m long by 22m wide, was reconstructed with a
concrete pavement (with an EU grant). The performance of these two projects is
currently being monitored. The Appendix contains details of the construction of Mbagathi
Way.
There are environmental advantages to be gained from constructing concrete road
pavements compared to asphalt. Firstly, owing to its pale colour, concrete is safer than
asphalt roads because it increases visibility, especially at night. Secondly, a concrete
surface will also reflect heat energy better than asphalt, which will be beneficial to the
passage of vehicles in hot climates. Thirdly, vehicles travelling on concrete surfaces
require in general less energy for propulsion than asphalt. Fuel savings between 10-20%
are indicated. Disadvantages include traffic noise and the relative difficulty of repairing
concrete roads compared to asphalt.
This Volume contains a:

description of the different types of concrete pavements, their components and
functions
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

Part 3 - Materials and Pavement
factors influencing the design process and selection of pavement type, and
design procedure for the pavement type, slab reinforcement and joint details.
13.2 Concrete Pavement Characteristics & Types
13.2.1
Characteristics
The strength of a concrete pavement derives mainly from the concrete itself, unlike
asphalt pavements where successive layers contribute to the overall strength. Concrete
is a rigid material, considerably stronger in compression than in tension so the
fundamental design objective is to ensure that the stresses imposed by traffic and
induced by thermal expansion and contraction can be endured by the concrete without it
fracturing. Concrete can be damaged chemically by deleterious salts, either contained in
the aggregate or entering from outside, but in their absence concrete does not
deteriorate from tropical weathering.
Concrete pavements are stressed by variation in temperature, and to a lesser extent by
moisture content, because of the volume changes that occur. Where concrete is
exposed, the volume changes must be accommodated by expansion and contraction
joints, the spacings of which are determined by the temperature variation range. In
humid tropical regions with only small temperature fluctuations, joints can be quite widely
spaced but in deserts there are large fluctuations of temperature and special attention to
joint design and spacing is required. Probably it is not appropriate to consider concrete
pavements for the more arid parts of northern Kenya for this single reason but, in any
case, traffic volumes are small here and concrete pavements could not be economically
justified.
Skid resistance of concrete pavement surfaces is important and must be adequate both
at construction and maintained at regular intervals thereafter.
Figure 13.1: Effects of Temperature on Concrete
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13.3 Types
Concrete pavements are categorized into three different types:



Jointed Unreinforced (JUCP)
Jointed Reinforced (JRCP)
Continuously Reinforced (CRCP)
In JUCP pavements the concrete is cast in predetermined slabs separated by joints to
control the cracking. The slabs are connected by dowels, to transmit the vertical stresses
more evenly, and tie bars, to connect them together.
In JRCP pavements the concrete is cast in slabs and reinforced with steel bars. The
slabs are placed and separated by joints to control the cracks. The slabs are connected
by dowels and tie bars as for JUCP pavements. JRCP pavements are used where it is
suspected that soil movement below the slab will cause it to crack. Longitudinal
reinforcement is the main characteristic but transverse reinforcement is added to assist
the placing of the former.
CRCP pavements are the highest cost but should incur the least maintenance. They are
normally constructed on the highest traffic roads where a good quality (ie least bumpy)
finish is required. The main reinforcement is either prefabricated steel mesh or
longitudinal steel bars installed at mid-depth in the slab and which is used to control
cracks induced by volume changes.
13.4 Pavement Components and Functions
13.4.1
Subgrade and Sub-base
The load bearing capacity of the subgrade (=foundation) is not so important in concrete
road design. The main requirement is to provide a foundation on which construction
traffic can operate without injuring the shape to which it has been trimmed. Subgrades
with CBR >30 are suitable, except they must be free draining because eventually water
will enter the pavement through the joints; when it does it must be able to drain away,
otherwise ‘mud-pumping’ will occur as heavy vehicles pass from one slab to the next. If
a sub-base is laid, it must also be free-draining and should continue through the road
shoulder. In Kenya it is likely that concrete roads will be constructed on old, probably
reconstituted, asphalt pavements: obviously, in this situation subgrade strength will be >
30% CBR but it is reiterated that attention should be paid to the drainage condition.
Fig 13.2 shows that concrete pavements generally consist of a sub-base and slab
constructed on the subgrade, or foundation. (The foundation can also be an
embankment). The foundation consists of the roadbed and, if the roadbed is weak (CBR
< 15), a capping layer comprising selected fill is required which serves to protect the
subgrade during the construction period. The sub-base performs the following functions:



acts as a free-draining layer and prevent ‘pumping’ of water at joints and edges
of slabs
provides a stable construction platform and uniform slab support, and
moderates any shrink or swell of the subgrade.
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If the roadbed material is strong enough and the design traffic relatively low, a subbase
may not be necessary but it is important that the layer immediately below the slab is
free-draining.
Subbase materials shall be granular and non-plastic and can be cement stabilised to
enhance their properties.
Figure 13.2 Structure of Concrete Pavement
13.4.2
Concrete Slab
The slab consists of Portland cement concrete, reinforcing steel (optionally), load
transfer devices (dowels), tie bars and joint sealants.
13.4.2.1 Portland Cement Concrete
The main influences on the structural performance of concrete in roads are the strength
of the concrete and its coefficient of thermal expansion. For concrete to harden
satisfactorily the cement must be sound, the mixture of cement and aggregate properly
designed, the water: cement ratio carefully controlled, and the concrete well compacted
and kept moist during the curing period. The initial setting of the concrete is accelerated
at high temperatures and this requires that particular care is necessary in tropical
climates to compact the concrete before initial setting has occurred and keep it moist
during curing. In drier climates, special measures are required to protect the concrete for
at least 7 days after placing.
13.4.2.1.1 Cement
The cement should conform to KS EAS 18-1. In addition, unless cement is properly
stored and used in a fresh condition, the concrete quality will be substantially reduced.
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Cement that has lost strength due to hydration before use is characterized by the
formation of lumps.
13.4.2.1.2 Water
The water used for concrete preparation should be potable and should ideally conform to
the requirements of BS EN 1008.
13.4.2.1.3 Aggregate
KS 95 2003 specifies the quality and grading requirements for aggregates suitable for
concrete production. It is clearly an advantage to use aggregates with low coefficient of
thermal expansion and in Table 13.1 the effect of aggregate on this parameter is given.
Table 13.0.1: Coefficient of Expansion of various aggregates
Aggregate used
Quartzite
Granite/gneiss
Basalt
Limestone/marble
Coefficient of Expansion of Concrete, per 0C *10-6
Range
Average
11.7 to 14.6
13.2
8.1 to 10.3
11.4
7.9 to 10.4
9.2
4.3 to 10.3
7.3
From Table 13.1 it is clear that limestone would be the most suitable aggregate but it is
not common in Kenya and has a low Polished Stone Value and would not be suitable for
the surface of concrete roads.
Obviously it is important to use tough and durable aggregate and especially important to
limit the proportion of flaky and elongate particles, an excessive amount of which can
prejudice concrete compaction and strength. It is preferable to use aggregate with
nominal maximum size >25mm for better load transfer across slabs. Higher concrete
strengths and better shrinkage reduction are attained with the larger aggregate sizes but
the maximum aggregate size is a function of the slab thickness. Sometimes the
maximum aggregate size is restricted to 20mm to minimise the risk of segregation. Table
13.2 presents general limits for aggregate sizes and gradings but reference should be
made to KS 95 2003 for details.
Table 13.2: Aggregate Sizes and Gradings for OPC Concrete
Sieve
size
(mm)
Percentage by Mass of Total Aggregate Passing Sieve
Coarse Aggregate
40mm down
50
37.5
20
14
10
5
2.36
100
90-100
35-70
10-40
0-5
Fine
Aggregate
20mm down
100
90-100
40-80
30-60
0-10
All-in Aggregate
40mm down
100
30-100
60-100
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20mm
down
100
95-100
45-80
100
95-100
25-50
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1.18
0.6
0.3
0.15
30-100
15-100
5-7
0-10
Part 3 - Materials and Pavement
8-30
10-35
0-8
0-8
13.4.2.1.4 Concrete itself
Stresses from traffic loads require consideration of the modulus of rupture (or flexural
strength) but, although this parameter has a greater influence on the structural
performance of concrete, it is more difficult to measure than compressive strength. Thus,
typical compressive strengths specified are 30 MPa (N/mm2) at 7 days and 40 MPa at 28
days, with the tests performed on 150mm cubes under standard conditions (test KS 02595-1986). The correlation between modulus of rupture and compressive strength
depends on the aggregate type and shape; the amount of elongate or flaky aggregate
particles adversely affects the modulus of rupture. Normally, concrete made with natural
aggregate is inferior in strength to that made with crushed rock. The mechanical strength
required for aggregate for pavement concrete is similar to that required for bituminous
road bases. Regarding the fine aggregate, it is better if possible to use material with
gradings towards the coarse end of the envelope, in order to improve workability. All-in
aggregate, ie crusher-run stone is also commonly used.2
The (compacted) concrete will normally consist of between 250 to 350 kg/m3 of Ordinary
Portland Cement, coarse and fine aggregate, the precise proportions of which are
determined during design by compressive strength tests.
To control workability the tendency is always to increase the added water but it is crucial
to keep the water: cement ratio below 0.5, otherwise the concrete will have insufficient
strength and durability. For small contracts, the workability is measured using the slump
cone test (KS 02-595-1986), where a standard cone is filled with wet concrete, the cone
lifted and the concrete allowed to subside. The slump is then the difference between the
cone height and the highest point of the slumped concrete. It should be limited to 75mm.
However, with the lean and dry mixtures used in roads, the test is not very discerning in
identifying variations, and the more precise compacting factor test is better for large
contracts. In this test, there are two cones one above the other. The uppermost cone is
filled with concrete and then allowed to fall into and overflow the lower, smaller cone. Its
surface is then leveled and the concrete then allowed to fall into a basal cylinder. The
bulk density of the contents of this cylinder is then measured to measure the compaction
produced by the energy of the falling concrete.
The principles of concrete compaction are similar to that of soils. A mix too dry is difficult
to compact. A mix too wet renders the maximum density impossible to achieve. Once
the mix proportions have been specified and the method of compaction selected there is
normally no need to determine the densities achieved in the compacted concrete.
However, in particular with the thicker slabs, there is a tendency for segregation with the
larger particles falling to the bottom of the slab. This risk is greatest when the concrete is
too wet. This is the reason for limiting the maximum aggregate size to 20mm.
2
The following approximate correlation is useful:
Flexural Strength (MPa) = c√Compressive Strength, where c ≈ 0.75. Values of Flexural Strength
ranging from 3.8 to 4.5 after 28 days are usually acceptable for concrete in roads.
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13.4.2.2 Reinforcing Steel
13.4.2.2.1 General
Cracks in concrete develop by:
 temperature and/or moisture-related contractions and expansions, and
 frictional resistance between the slab base and underlying layer
Tensile stresses result, maximizing at mid-slab, and if they exceed the tensile strength of
the concrete, it cracks transversally and the stress is transferred to the reinforcing steel if
present. The purpose of the (longitudinal) reinforcing steel is to control concrete cracking
and hold the cracks tightly closed, maintaining the pavement as an integral unit. In
general the amount of steel is small, and is insufficient to add to the flexural strength of
the concrete slab and thus the structural strength of the pavement.
Transverse reinforcing steel is used to ensure that the longitudinal reinforcing steel
remains in the correct position during slab construction and also mitigates any
longitudinal cracking that could eventually occur.
The selection of JUCP, JRCP or CRCP is a function of the pavement slab length. For
joint spacing less than 5 meters, transverse cracking is not expected and reinforcement
normally not required, therefore JUCP pavements are appropriate. For joint spacing
between 5 and 15 meters, reinforcement is required, increasing in amount in proportion
to the slab length, although the increasing cost of reinforcement is offset by a decreasing
amount of joint dowels and sealants. The upper limit of 15m also allows slab and joint
movements to be restricted and riding quality optimized. Beyond 15m CRCP pavements
are recommended, with no joints but a considerably greater amount of reinforcing steel
than JRCP.
The choice between the different concrete pavement types is fundamentally an
economic one and a balance of traffic levels, construction cost and maintenance
interventions.
13.4.2.2.2 Reinforcing Steel Requirement
The area of reinforcing steel in JRCP and CRCP pavements are given by complex
equations, to be found in the documents referenced. The same applies to the type and
quality of this steel. Needless to say, much more steel is required for CRCP than for
JRCP pavements. In order to achieve its intended function it is important that the
reinforcement is correctly placed and fixed in position to allow uninterrupted paving
operations. The steel should be free of contaminating substances which will prejudice
the bond with the concrete. Bonding and anchorage properties are not affected by rust
which normally forms on steel after normal exposure but after prolonged storage it will
have to be removed.
13.4.2.3 Dowell Bars
The most common failures of concrete roads occur at the transverse joints and it is
imperative that adequate load transfer support is provided to minimise cracking, spalling
and corner breaks. Load transfer support across the slabs is provided by dowels and
enhanced by:
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



Part 3 - Materials and Pavement
stiff sub-bases
large sized coarse aggregate (>25mm)
small joint openings, and
dowels
Dowels are normally 20mm diameter, 400mm long and fitted at about 300mm spacings.
Since they are load transfer devices they must be strong and robust and closely spaced
to resist bending and shear of the concrete. To allow slabs to move horizontally relative
to one another, at least 65% of the dowel must be coated with a bond-breaking
compound, eg bitumen. Dowels must not ‘lock’ the joint where they are placed,
otherwise an uncontrolled crack may occur close to the joint.
End dowels should be at least 200mm distant from the slab edge. It is very important
that dowels are aligned parallel with the pavement direction, otherwise strains will be
generated that will be cracking and early deterioration of the concrete.
Where joint openings are less than 1mm, dowels need not be utilized. For dowelled
joints the joint opening should be 6mm or less. Short-slabbed pavements thus do not
need dowels but it is common practice to use dowels regardless of joint opening.
On roads carrying heavy vehicles it is essential to provide dowels across joints to limit
the vertical movement between slabs as vehicles pass over. It is also desirable to use
dowels in roads over unconsolidated soils to prevent differential settlements between
adjacent slabs. In transverse joints dowels are bonded into the concrete on one side of
the joint. Bonding on the other side is prevented, usually by coating the dowels with
bitumen and, for expansion joints, by providing a loose end-cap. It is particularly
important that they are accurately aligned perpendicular to the face of transverse joints
or parallel to the road if the joints are skewed.
13.4.2.4 Tie Bars
In contrast to dowels, tie bars are not load-transfer devices but fixing devices whose
function is to tie two slabs together. Thus, whereas dowels must be smooth and
lubricated on one end to maintain freedom of movement, dowels must be deformed or
hooked and firmly anchored in the slab to function properly. Typically they are used to
prevent separation at longitudinal joints but at the same time allowing some warping to
occur. They hold the joints together so that load transfer is achieved by aggregate
interlock in the concrete.
Tie bars are generally 12mm diameter, 750mm long and spaced at intervals of 600mm.
When the width of the pavement in construction is greater than 4.5m, hinge joints are
made. The concrete is sawed to a third of its thickness; the joint is sealed and then tied
by inserting tie bars at two thirds slab thickness into the slab.
Construction break joints are made by using long (at least 750mm) tie bars to join the old
and new concrete.
Reference is made to the South African M10 manual which contains details on the
length and width of dowels and tie bars.
Figure 13.3: Definition of Dowell and Tie Bars
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13.4.2.5 Joints
Joints are necessary in concrete pavements in order to relieve stresses that build up in
the slab by temperature and/or moisture changes, friction with the underlying layer, and
those necessary at the end of a working day. In directional terms there are transverse
and longitudinal joints and four joint types are fabricated:




Contraction
Expansion
Warping
Construction
The different types of joints are illustrated in Fig 13.4.
Figure 13.4: Types of Concrete Joints
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13.4.2.5.1 Contraction/Expansion Joints
These joints provide weakened sections between slabs to induce tension cracking in the
slab. If they were not made random cracking would develop on the pavement surface.
They also alleviate the warping strain. The principal requirements are to:



induce a crack at a predetermined location and seal it against water and debris
ingress
permit the joint to open and close
transfer loads across the joint
Expansion joints are installed to provide space for the expansion of the pavement,
thereby preventing the development of compressive stresses, which otherwise would
cause the pavement to buckle. They contain joint filler which performs as a spacer
during construction. The filler consists of a semi-elastic material, either a fibrous material
or soft wood, about 15mm thickness.
In concrete spread and compacted by hand, there is a practical advantage in using
expansion joints with a softwood filler. A wooden lath, 25mm x 25mm can be nailed to
the top of the joint filler and used as a guide to round the joint edges (=arrissing). When
the concrete has hardened, the lath is removed to reveal the slot for the sealing
compound. Removal of the lath is eased by slightly tapering its cross section.
Contraction joints are of two types; 1 which is sawn into the hardened concrete and 2,
which developed with the onset of mechanisation, consist of slots sawn across the
pavement to about ⅓ of the slab thickness to induce cracks. The raggedness of the
crack that develops helps to transfer loads across the joint.
The recommended interval between contraction joints is dependent on the shrinkage
properties of the concrete, the friction with the subbase and the slab thickness. For
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unreinforced concrete the maximum spacing is 5m. In reinforced concrete a spacing of
25m is recommended.
13.4.2.5.2 Warping Joints
The warping joint is placed to provide additional strength against heavy traffic loads in
slabs subject to large diurnal temperature variations. It resembles a dummy joint with
fixed dowels placed in the concrete where the crack will form and acts, in effect, as a
hinge. They are used in situations such as in longitudinal joints or special situations
where manholes occur, or in irregular shaped slabs. They allow a slight rotation of the
slab portions.
Longitudinal joints are always warping joints but they can be found sometimes as
transverse joints. The purpose of longitudinal joints is to control longitudinal cracking,
which can occur soon after construction. They should be spaced at intervals equivalent
to a traffic lane width, i.e about 3.7m, and away from wheel paths to minimize edge
loading. CRCP pavements also have longitudinal joints but no transverse joints.
13.4.2.5.3 Construction Joints
Construction joints are required when there are interruptions to concrete pouring, such
as at the end of a working day. They may be located in the middle third of a slab, in
which case a keyed and tied joint is used; or at a planned contraction joint in which case
a dowelled joint is used.
The new concrete should be jointed to the old concrete by using uncoated tie bars. The
length of these tie bars should be at least 750 mm. The intention for the keyed and tied
joint is that it becomes an integral part of the slab. The key provides load transfer and tie
bars are used to hold the joint tightly closed. It will not normally be necessary to seal this
joint. All construction joints should be constructed normal to the longitudinal axis of the
pavement. For JUCP and JRCP, they shall be coupled with other joints and additional
reinforcement shall be placed when dealing with transverse construction joints for
CRCP.
13.4.2.5.4 Other Types of Joints
Mis-matched Joints: reinforcing of slabs is necessary where the joint patterns of adjacent
pavements do not permit the matching of joints. In such circumstances the mis-matching
of the joints can cause cracking in the adjacent pavement slab. Partial slab
reinforcement is therefore required in all cases except where an expansion joint is
provided between the abutting pavement sections. Where this reinforcement is required,
the slab opposite the mis-matched joint is reinforced with steel fabric in a direction at
right angles to the mis-matched joint.
Acute Angles: at kerb returns, curved edges, or at the perimeter of angle-parking areas
joints may form acute angles in the corners of slabs. In these cases there is potential for
a crack to form across the acute angle in the slab corner. This can be avoided by offsetting the joint at least 300mm from the curved edge or corner, thus removing the acute
angle and reducing the potential for the crack to occur.
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13.4.2.6 Effects of weather on placing of joints
With concrete roads constructed in hot weather, contraction predominates as the
weather cools. In any subsequent expansion it is unlikely that all the joints will be able to
close to their original dimensions. For this reason at least one joint in four should be an
expansion joint. With work done in the cool season, all joints should be expansion joints.
13.4.2.7 Joint Protection
Most joints should be sealed with materials meeting the Standard Specifications.
Transverse joints spaced not more than 4.5 m apart are usually 6 to 8 mm in width and
sealed with prefabricated elastomeric compounds. Sealant is applied at the surface; the
dimensions of the sealant reservoir depend on the slab length, and hence the movement
at the joint, and the sealant properties. Tables 3.3 and 3.4 give details:
The edges of the sawn joint slots usually are rounded (=arrissed). In general the depth
to width ratio of sealant ranges from 1 to 1.5 and the sealant should be placed 3 mm to
13 mm below the surface of the pavement. As the concrete expands and contracts,
these sealing compounds accommodate large strains. The sealant will be compressed
between 20 to 50 percent of its normal width. They are unlikely to remain effective in
preventing water from entering the joint for more than two or three years. Nevertheless,
as long as they remain in place, they fulfill their other important function, which is to
prevent loose stones and other debris on the road surface from being wedged in the
joint. Stones so wedged, can cause spalling of the edges of the joint as the concrete
expands.
Table 13.0.3: Reservoir Dimensions for Field Moulded Sealants
Joint Spacing (m) Sealant Reservoir Shape
Width (mm)
Sealant Depth (mm)
5
6
20
6
10
20
10
12
20
12
15
25
Table 13.0.4: Joint & Sealant Width for Pre-formed Compression Seals
Joint Spacing (m)
6
10
12
15
Sealant Dimensions
Joint Width (mm) Sealant Width (mm)
6
11
10
16
11
20
12
22
Figure 13.4: Types of Joints and Dowell Bars
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13.5 Factors influencing the design process and selection of
pavement type
In general, the preferences for the different pavement types are as follows:
1. JUCP: this is the cheapest form of concrete pavement and is suitable where the
traffic volume, especially the number of commercial vehicles, is low. This
pavement type should be constructed in slab lengths less than 5m to retard the
(inevitable) development of uncontrolled cracking
2. JRCP: this type of construction is preferred for all levels of traffic and especially if
there is an enhanced risk of settlement of the subgrade.
3. CRCP: this type of construction is the most expensive, containing the highest
amount of steel reinforcement, and is employed normally for the highest traffic
levels
The main factors influencing the selection of concrete slab type are as follows:





traffic volume and particularly number of commercial vehicles
climate, particularly diurnal temperature range
subgrade characteristics
environment, whether urban or non-urban
quality of the available materials
Since the initial aim in Kenya would be to construct concrete roads on the more heavily
trafficked routes such as the Northern Corridor route, Mombasa-Nairobi-Nakuru-Eldoret,
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where there have been serious shortcomings with asphalt performance, it would be
advisable to use reinforced concrete to restrain the development of cracks. Moreover, on
these higher altitude routes, the diurnal temperature range will exacerbate the expansion
and contraction of the concrete slab.
Other possibilities would be the tropical coast routes where temperatures are high and
the diurnal range not so extreme. Pilot scale concrete road trials were constructed in the
Mombasa area in the 1960s, it is understood with good results.
13.6 Stress Development and Design Criteria
Stresses in rigid pavements develop from environmental changes; the temperature and
moisture variation throughout the day; and, together with the stress imposed by traffic,
deform the slab. The weight and rigidity of the slab resists these stresses until it yields,
when cracks are formed.
The stresses are defined as of three types:

Horizontal tensile

Horizontal compressive

Vertical
Obviously, these stresses cannot be prevented from occurring, and the purpose of
concrete pavement design is to keep them within an acceptable range.
For all slabs transverse cracking occurs at intermediate positions between the joints and
the function of the reinforcing steel is to hold the slab together so that the transfer of the
traffic loads will not disrupt the slab. For JUCP the short slab length assists in ensuring
that intermediate cracks do not occur.
13.6.1
Horizontal Tensile
During the setting period and possibly later depending on the humidity, tensile stresses
are induced in the concrete. Subsequently, the acceleration/retardation of traffic also
induces horizontal stresses and, since the reaction of the lower surface of the concrete
is limited by friction with the subbase, cracks can occur because concrete is
comparatively weak in tension. If uncontrolled cracks become too wide, water infiltration
to the subbase occurs and degradation of the slab follows.
Crack development in JUCP and JRCP is controlled by manufacturing joints at regular
intervals and by placing a separation membrane between the slab and the subgrade. In
CRCP the continuous reinforcement allows cracks to occur at regular intervals, thus
limiting their width to acceptable values.
13.6.2
Horizontal Compressive
The compressive stresses are the reverse of the tensile stresses and if they become too
high, the slab will buckle.
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The placing of expansion joints and the provision of a separation membrane allow the
expansion of concrete and the dissipation of these stresses.
13.6.3
Vertical
Vertical stresses occur by the repetitive action of traffic and the weight of the concrete
slab on the underlying subbase.
13.7 Concrete Pavement Design
13.7.1
Traffic
As with bituminous road pavements, the concept of Equivalent Standard Axles (ESA) is
used as an indicator of design traffic loading, with similar reservations regarding the 4th
power relationship between axle loading and permanent damage, especially with
overloaded vehicles. The damaging effects of heavy vehicles vary according to the time
of day but the concrete pavement is most vulnerable at night when the ends of the slabs
curl up and behave as unsupported cantilevers (see Fig 2.1). Where traffic includes
vehicles with axle loads > 10 tonnes special treatment is advisable to reduce the
imposed stresses, which include:




Strong load transfer devices across joints
Closer joint spacing
Use of mesh reinforcement in the concrete
Increased slab thickness
13.7.2
Failure Criteria
The ultimate condition determining the life of a concrete pavement is the deterioration in
riding quality. Before this limit is reached, cracks will have appeared in the concrete
surface and the deterioration is expressed in the gradual spreading of these cracks. On
lightly trafficked roads, say Traffic Classes up to T3, it may be possible to extend the life
of the pavement by applying a bituminous surface to restore riding quality. On more
heavily trafficked roads the development of cracks will probably be associated with the
breaking up of the concrete to the extent that it must either be removed and replaced or
broken down in place to create what is essentially a crushed stone base, the so called
‘crack and seat’ technique .
Methods for measuring surface roughness are of limited value in providing criteria for
deterioration in riding quality since concrete roads do not usually deteriorate uniformly.
Variations in construction quality, often due to varying weather conditions, can produce
wide differences in the performance of adjacent lengths of concrete. On the most
affected lengths, deterioration can be measured by the amount of cracking, recording
the length of narrow and wide cracks per square metre. The rate of development of
cracking can thus indicate the timing and extent of repair work necessary. On well
designed and constructed concrete roads, cracks may not appear for 20 years or more,
progressively intensifying thereafter depending on the traffic loading.
Construction faults, for example misaligned dowell bars in joints may cause earlyappearing local areas of disintegration. It is the joints which are the greatest source of
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weakness in concrete roads so the longest lived roads are those where particular care
has been taken over the design and construction of joints.
13.7.3
Thickness design
The analysis of stresses in concrete pavements by multi-layer elastic theory is simpler
than for ‘flexible’ pavements. Westergard produced the first theoretical analysis of the
stresses developed in concrete pavements in 1926 and, subsequently, finite element
analysis methods have emerged to permit greater precision to the estimation of stresses
developed under different loading conditions, see Acum and Fox (1951) and Zienkewicz
(1967). If these methods were employed to design concrete road slabs with the same
rigour used in structural concrete their application would be relatively simple and would
indicate the slab thickness and reinforcement necessary so that the tensile stresses
developed in the concrete never exceeded a critical level with a proper safety margin.
But such construction would be prohibitively expensive and concrete road slabs are
designed with the assumption that fatigue fractures will eventually occur. Safeguards are
built into the design so that the effects of the fractures are contained with the roads
continuing to give good service for many years after the first cracks have appeared. The
theoretical work is useful in identifying the critical points in the slabs where special
measures are needed to counteract the effects of tensile fracture but in producing a
practical design method there is no substitute better than the observation of performance
of concrete roads in the field.
13.7.3.1 Capping Layer and Subbase
A capping layer is required only if CBR of the subgrade is < 15%. The required thickness
of a capping layer for a subgrade CBR value less than 15% can be obtained from Fig
6.1. The capping layer material shall have a minimum CBR value of 15% at 95% of MDD
and OMC of BS Heavy.
A sub-base layer is required when the subgrade CBR is< 30% or to obtain the surface
levels with the tolerances required. Generally, the thickness of the sub-base provided
will be 150 mm and it can consist of cement-stabilized material. The sub-base shall have
a minimum CBR value of 30% at 95% of MDD and OMC of BS Heavy.
Material for fill should have a CBR swell of less than 2% and a minimum CBR value of
5% at 95% of maximum dry density and optimum moisture content using BS Light
compaction.
For subgrade CBR values less than 2%, the roadbed material needs to be treated either
by replacement or in-situ stabilization.
A separation membrane (such as a polythene sheet) is required between sub-base and
concrete slab, mainly in order to reduce the friction between the slab and the sub-base
in JUCP and JRCP pavements, thus inhibiting the formation of mid-slab cracks. The
minimum thickness of the polythene sheet shall be 2.6 mm. It also reduces the loss of
water from the fresh concrete. For CRCP pavements, a bituminous spray should be
used on the sub-base, instead of polythene, because a degree of restraint is required.
Figure 13.5 Relationship between Subgrade Strength and Capping Layer Thickness
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m
m
13.7.3.2 Concrete Slab Thickness and Reinforcement
The following section presents criteria for determining the thickness and reinforcement
for each of the concrete pavement types, based on the design traffic volume. For
practical reasons it is undesirable to construct concrete slabs with a thickness less than
125 mm.
13.7.3.2.1 Jointed Unreinforced Concrete Pavement (JUCP)
Fig 6.2 presents the design thickness of JUCP concrete slab calculated from the design
traffic ESAs. It assumes the presence of an effective lateral support to the edge of the
most heavily-trafficked lane, such as a shoulder with a pavement structure able to carry
occasional loads. If this shoulder is absent, an additional slab thickness is required, and
this additional thickness can be determined from Fig. 6.3.
JUCP pavements have no reinforcements for crack control. However, the longitudinal
and transverse joints are provided with reinforcements. The joint details are discussed
in a previous section.
13.7.3.2.2 Jointed Reinforced Concrete Pavement (JRCP)
Fig 6.3 presents the thickness of JRCP concrete slab calculated from the design traffic
ESAs. Longitudinal reinforcement steel is used between joints for crack control. The
same Figure can also be used to determine the longitudinal reinforcement in terms of
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mm2/m for a design thickness of concrete slab. Thus, several alternate combinations of
thickness of concrete slab and amount of reinforcement can be compared. In the
absence of an effective lateral support provided by the shoulder adjacent to the most
heavily trafficked lane, an additional slab thickness is required and can be determined
using Figure 6.4.
In addition to the longitudinal reinforcement, JRCP pavements shall be provided with
transverse reinforcement, if required, depending on site conditions. In that case,
reinforcement shall be provided at 600 mm spacing and consist of 12 mm diameter steel
bars.
13.7.3.2.3 Continuously Reinforced Concrete Pavement (CRCP)
CRCP pavements can withstand severe stresses induced by differential movements.
CRCP contains relatively high percentages of steel and no joints except for construction
joints and some expansion joints. Since the pavement contains very few joints it is
generally smooth riding and, if the steel is properly designed, it is potentially a lowmaintenance pavement. The minimum and maximum spacing recommended for
longitudinal steel is 100 mm and 220 mm respectively and the minimum steel cover
recommended is 65 mm. For a traffic volume up to 100M ESAs, the thickness of CRCP
concrete slab shall be 200mm .
Longitudinal reinforcement in CRCP pavements shall consist of 0.6% of the concrete
slab cross-sectional area. The diameter of the bars should not exceed 20 mm and the
center-to-center spacing of the bars should not be greater than 225 mm. If required,
transverse reinforcement shall be provided to control the width of any longitudinal cracks
that may form. The diameter of the bars should not be less than 12 mm and the
maximum center-to-center spacing of the bars should not be greater than 750 mm.
Transverse reinforcement is normally required only for ease of construction. It may be
omitted except where there is a risk of differential settlements.
As with JUCP and JRCP pavements, in the absence of effective shoulder support
adjacent to the most heavily trafficked lane, the additional slab thickness required can be
determined using Fig 6.3.
The minimum thickness of concrete pavement for JUCP and JRCP pavement is 150
mm. For CRCP pavements the minimum thickness is 200 mm. Hence, the designer
should carefully assess the necessity and requirements for such pavements, depending
on the design traffic volume.
Figure 13.6: Concrete Slab Design Thicknesses vs Traffic
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Figure 13.7: Additional Slab Thickness where no lateral support is present
13.7.3.2.4 Concrete Slabs constructed on old Asphalt Pavements: ‘whitetopping’
Reference is made to a publication by the American Concrete Association on
‘whitetopping’, or overlaying asphalt roads with concrete.
The support given by the existing pavement + subgrade must be taken into account in
the thickness design of the concrete slab. Figs are nomograms which can be used to
estimate the Westergard modulus of subgrade reaction, or k-value, on top of the existing
pavement. Fig is for asphalt on a granular base and Fig is for asphalt on a cementtreated base. The asphalt thickness is the residual asphalt remaining after milling of the
old surface.
Figure 13.8: Slab design thickness for old asphalt road on granular base
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Figure 13.9: Slab design thickness for old asphalt road on cement-treated base
For Traffic Classes T5 and upwards concrete slab thicknesses ranging from 200mm to
300mm should be satisfactory. For Traffic Classes T4 and below, concrete slab
thicknesses between 130mm and 180mm would be appropriate.
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13.8 Construction issues
There are three levels of sophistication in the construction of concrete roads. At one
extreme there are labour-intensive methods employing a minimum of mechanical plant.
In the middle comes the use of mechanical spreaders and finishers with the side-forms
being placed to correct line and level. In extreme mechanisation there are slip-form
pavers in which the side forms are carried on the machine with wire guidance, or lasers,
to secure correct line and level. Increasing mechanisation makes high output possible
and good surface finish, but complicates the installation of joints and reinforcement.
13.8.1
Labour Intensive works
The minimum plant required, in addition to transport, is a rotating drum mixer and
vibrating tamper bar with an appropriate power source. Timber side forms can be locally
made and used. Coarse and fine aggregates are apportioned using wooden gaugeboxes, the proportions being set so that the correct cement content can be obtained by
adding cement in bags or half-bags. Water is added by volume, using the slump cone
test to control workability. A 12 man gang can lay up to 500m2 of concrete per day. In
alternate-bay construction, a sequence of operations is established in which alternate
bays are constructed on each successive day, facilitating the installation of joints and
accurate location of dowell bars.
Under a competent foreman, labour intensive works can be very effective and provide
gainful employment in regions where unemployment is a problem.
13.8.2
Medium mechanisation works
The availability of ready-mixed concrete calls for more mechanisation in spreading and
finishing. The ready-mixed concrete should be discharged into a mobile hopper from
which the concrete is drawn off into wheelbarrows and raked into an even profile. An
additional 10 to 15% of the finished slab thickness is required to allow for compaction.
Compaction can be carried out using vibratory equipment, either pokers or vibrating
screens working on the side forms. The additional surcharge is regulated automatically
using mechanical spreaders and finishers.
Even the medium mechanisation works require a substantial investment in plant and
equipment and the construction of substantial works is necessary to justify their use.
13.8.3
High mechanisation works
Slip-form pavers are the ultimate in the ingenuity of machinery manufacturers but require
an enormous effort and investment to mobilise. It is doubtful whether they are
appropriate in countries such as Kenya where concrete roads are needed in a few
places.
13.8.4
Roller-compacted concrete pavements
In this process, mechanical spreaders as used for bituminous materials are used,
adapted by increasing the compactive effort of the vibrating screen. The compaction
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process is completed by using steel-wheeled rollers, and joints are cut into the
completed concrete. The concrete mixtures are of low workability, with low water:
cement ratios. The advantage of this method is that it uses plant which has other uses in
road-making, thereby reducing operation costs.
13.8.5
Surface finish
A rough surface of the finished concrete is required to give adequate skid resistance and
this is achieved by cutting closely spaced (at 2cm spacing) grooves in the hardened
concrete. The grooves may be cut in the longitudinal direction or transversally, the
advantage of the latter being that it assists drainage, although being much noisier. The
noise created by grooved concrete roads is an issue in developed countries and has
resulted in concrete roads losing favor.
13.8.5.1 Concreting in hot climates
High temperatures increase the rate of hydration of cement. The concrete thus begins to
harden rapidly after mixing so that it can become difficult to spread and compact. Also,
the rapid early gain in strength can be accompanied by shrinkage and cracking of the
concrete with the result that the subsequent gain in strength is much lower than with
concrete cured at a lower temperatures. For concrete cured in damp conditions at 200C,
normally a gain of about 40% in compressive strength is obtained between 7 and 28
days, with a slight increase afterwards. In contrast, in concrete cured at 500C there is
likely to be little gain in strength between 7 and 28 days, with the likelihood of weakening
afterwards because of the shrinkage cracks that will have developed. Using more finely
ground Portland cement accentuates these effects.
Workability during concrete laying can be increased by adding more water but to
maintain the concrete strength, more cement is the required. This has disadvantages,
apart from cost because of the risk of shrinkage during hydration. Air entraining agents
can be used to improve workability and chemical compounds are available to retard the
concrete setting, eg sugar, but it should be confirmed that their use is not detrimental to
the concrete hardening.
The most effective measures involve keeping the concrete components as cool as
possible before mixing, protecting the surface of the concrete from the sun if possible
and keeping the concrete damp for the first 7 days.
13.9 Maintenance and repair
Well built concrete roads should require little maintenance. Nevertheless, it is impossible
to keep joints sealed against the entry of water, particularly where there are large daily
or seasonal temperature changes, since the the volume changes in the seal slots are too
large for the sealing compound to absorb without parting from the concrete.
Another problem are loose surface stones. If they become wedged in the top of joints
they can cause spalling of the concrete at the edges of joints. Therefore, the joints on
concrete roads should be inspected yearly and loose stones removed. Fresh sealing
compound may also be required in the joints.
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Joints that have become badly spalled can be repaired by cutting out and replacing the
damaged concrete but this is a specialised skill and may only be worthwhile on new
concrete roads.
Restoring transverse grooves in concrete surfaces worn smooth can be done but an
alternative is to provide a bituminous surface dressing. Use of polymer-modified bitumen
may assist in obtaining a more durable result.
Mud-pumping may occur at the joints of more heavily trafficked roads, indicating
structural inadequacies in that there is no provision for water to drain away beneath
joints. Deterioration can be arrested by drilling holes and placing fresh concrete in the
defective foundation.
13.10 References
Design Manual for Roads and Bridges (DMRB), Volume 7 (Pavement Design); IAN
73/06 (Foundation Design); HD29 (Surveys & Investigations); HD30 (Maintenance
Assessment Procedure); HD32 (Maintenance of Concrete Roads)
Manual of Contract Documents for Highway Works (MCHW), Volume 3.
Also: www.standardsforhighways.co.uk/dmrb/index.htm and
www.standardsforhighways.co.uk/mchw/index.htm
Cement & Concrete Association. Australia, Concrete Roads Manual 1997
AASHTO Pavement Design Guide 1993
Whitetopping-State of the Practice: American Concrete Association, 1998
Road Building in the Tropics: HMSO State of the Art Review 9, Millard, 1993
ROAD DESIGN MANUAL: Vol. 3: Pavement Design, Part II: Rigid Pavements. The
Republic of Uganda, Ministry of Works, Housing and Communications
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Appendix : Construction details for Mbagathi Way,
Nairobi
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14 Materials Sampling and Testing
14.1 Introduction
Road design may be divided into three stages, namely feasibility, preliminary design and
final design. Normally, there is always a feasibility study but sometimes the preliminary and
final design phases are compressed together. This Chapter describes the materials
sampling and testing programmes applicable to each design stage.
14.2 Mass of Samples Required
The total mass of sample required depends on the tests to be carried out, the grading of the
material (its maximum particle size, in particular) and its susceptibility to crushing during
compaction.
Table 13.2.1 shows the minimum mass of sample required for various sequences of tests
and typical materials, including allowance for drying, wastage and rejection of coarse
fragments where necessary.
14.2.1
Soil and Gravel
Tests
required
Fine grained
soil
(max. size 2
mm)
Grading
*
*
*
Atterberg
Limits
*
*
Compaction
*
CBR
(1 point)
*
CBR
(3 points)
*
Coarse grained
gravel
(max. size 40
mm) not
susceptible to
crushing
*
*
*
*
Coarse grained
gravel
(max. size 40
mm)
Susceptible to
crushing
*
*
*
*
*
*
*
*
*
*
14.2.2
Property
*
*
*
*
*
*
*
Treatment
Tests
Minimum
Sample
Mass (kg)
*
*
*
*
*
*
*
*
*
*
*
*
*
*
5
20 35 80 20
*
40
60
150 20
60
80
180
Stone
Tests Required
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Toughness
Cleanliness
Durability
Skid
Resistance
Shape
Notes
Part 3 - Materials and Pavement
Aggregate Crushing Value (ACV)
10% Fines Crushing Value (TFV)*
Aggregate Impact Value (AIV)
Sand Equivalent Value (SEV)
% passing 0.075mm sieve
Water Absorption (WI)**
Magnesium Sulphate Soundness
(MSS)***
Polished Stone Value**** (PSV)
KS 1238-11: 2003
KS 1238-12: 2003
KS 1238-13: 2003
AASHTO T176
KS
KS 1238KS 1238-20: 2003
60
120 (dry + wet)
10
5
5
KS 1238-15: 2003
25
Flakiness Index (FI)
Aggregate Angularity (AA)
KS 1238-6: 2003
10
25
* Can be determined on wet and dry samples, thus indicating durability, and is preferred to the ACV
when testing weaker stone. If AIV is determined first, the approximate correlation TFV = 2800/AIV will
indicate the TFV value.
** A good indicator of durability
*** Sodium Sulphate Soundness test sometimes preferred; limiting values are different to MSS.
14.2.3
Feasibility Study
Sometimes known as ‘pre-feasibility’, by the end of this stage it will have been decided
whether a road construction, improvement or rehabilitation project is justified. To a large
extent it is a political decision but some technical input will be necessary in order to estimate
costs. For example, it may be necessary to estimate AADT, have some knowledge of soil
strengths or have an approximate idea of roughness.
This study will indicate what type of project is necessary and provides the information
required to commission a preliminary design study. To this end, it typically identifies
alternative corridors, the traffic patterns, broad environmental and engineering parameters,
and cost estimates to enable the terms of reference of the preliminary design to be drafted.
14.2.4
Preliminary Design
14.2.4.1 Alignment Soils
14.2.4.1.1 Sampling
At least one sample shall be taken per kilometer of the anticipated alignment. More frequent
samples must be taken where there are major changes in soil type, indicated either from
geological data or visual observation. In the proposed cut sections pits shall be dug if
possible down to at least 0.5 m below the proposed formation level. In the case of a new
alignment, the depth of any pit shall in no case be less than 1.5m, unless rock or other
material impossible to excavate by hand is encountered. The position of each trial pit shall
be accurately determined and recorded. In every trial pit, all layers, including top soil, shall
be accurately described and their thicknesses measured. All layers of more than 300 mm
(except top soil) shall be sampled. The sample shall be taken over the full depth of the layer
by taking a vertical slice of material. The log of each trial pit shall be accurately drawn and
included in the Materials Report.
If deep cuttings are proposed the investigation of the material at formation depth may only
be practical near the time of construction.
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14.2.4.1.2 Testing of soils on new alignments
Sufficient sample shall be taken for the following tests to be carried out:





In situ moisture content and dry density
Grading to 0.075 mm sieve
Atterberg Limits
Compaction test, BS Heavy
CBR and Swell on samples prepared at 90% and 95% of MDD and OMC (BS
Heavy). (CBR shall be measured after 4 days soak, except in arid areas, where they
can normally be measured at OMC, depending on the equilibrium moisture contents
predicted under the pavement in the area. The moisture contents after 4 day soaking
shall be measured, both on the whole CBR specimen and on a sample taken from
beneath the plunger, after testing.)
14.2.4.1.3 Testing of subgrade and gravel wearing course on alignments of existing
gravel roads
This applies to existing gravel roads which are to be upgraded, the geometric standards of
which are good enough to maintain the existing alignment.
Where more than 100 mm of existing gravel wearing course is in place on the road and
where the shape is adequate, samples of subgrade are to be submitted to the tests
enumerated in the preceding section.
It must be decided whether or not to re-compact the subgrade so the in-situ moisture content
and dry density must be measured and compared against 90% and 95% of BS Heavy.
Where a gravel road is to be upgraded on the same alignment, the existing gravel wearing
course may provide extra material either for sub-base, or for improved subgrade.
Measurements of thickness and width of gravel wearing course shall then be recorded every
100 m. One sample per kilometer of existing gravel wearing course shall be taken, where the
gravel layer is at least 150 mm thick. Each sample shall be submitted to tests enumerated in
the preceding section.
14.2.4.2 Soil and Gravel Borrow Pits
Borrow pits and quarries should be spaced so as to obtain the most economic use of
materials. The spacing depends on the availability of suitable material, environmental
considerations, land use etc but a distance of 20km is optimal to minimize haulage costs.
The minimum thickness of deposit normally considered workable is of the order of 1 m. The
absolute minimum depends on the area of the deposit and the thickness of overburden. If
there is no overburden as may be the case in arid areas, horizons as thin as 300 mm may
be workable.
14.2.4.2.1 Field investigations and sampling procedure
Trial pits shall normally be dug on a 60 m grid, through the full depth of the layer(s) proposed
for use. A minimum of 5 trial pits is required for each proposed borrow pit. The location of
each proposed borrow pit shall be identified, showing the position of each trial pit, the
characteristic features of the site and the means of access and location. In every trial pit, all
layers, including top soil and overburden, shall be accurately described and their thickness
measured. All layers proposed for use shall be sampled. The sample shall be taken over the
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full depth of the layer proposed, by taking a vertical slice of material. The log of each trial pit
shall be accurately drawn and included in the Materials Report.
14.2.4.2.2 Sample testing
For characterization purposes at least one sample shall be obtained per 4,000 m³ of material
proposed for use. At least one sample shall be taken from each positive trial pit, even if the
volume represented is smaller than 4,000 m³. Each sample shall be submitted to the
following identification tests:


Grading to 0.075 mm sieve
Atterberg Limits
Large samples for Compaction and CBR tests shall be obtained by either of the following:
1.Mix Method: Large samples can be obtained by mixing "small" identification samples. A
mix must be representative of a workable area. All the "small" samples to be incorporated in
a mix must be the same type of material and must have fairly consistent Grading and
Atterberg Limits. Within each borrow pit, the mixes chosen should adequately cover the
range of materials proposed for use.
2. Re-sampling Method: Using the identification results, large samples can be obtained by
re-sampling from existing trial pits representative of the material types found within the
potential borrow pit area.
At least one large sample, whether mixed or re-sampled, is required per 15,000 m³ of
material proposed for use and shall be submitted to the following tests:




Grading to 0.075 mm sieve
Atterberg Limits
Compaction test (BS Heavy)
CBR and Swell after 4 days soaking, on specimens normally prepared at OMC and
normally 95% and 100% of MDD of BS Heavy.
The moisture contents after soaking shall be measured as indicated previously.
For types of gravel susceptible to crushing during compaction, the grading of the specimen
compacted closest to 95% MDD shall be determined after compaction and CBR testing and
compared with the grading before compaction of the specimen prepared for CBR.
14.2.4.2.3 Stabilization testing
If the natural materials do not meet the CBR requirements, stabilization tests shall be carried
out on the relevant large samples, as defined above:
Each sample shall be mixed with cement or lime, or both, whichever from the
characterization tests is expected to give the best results. (If both stabilizers are used, lime is
added first to reduce the plasticity followed by the addition of cement to achieve the required
strength). Three additive amounts (normally at 1% intervals up to a maximum of about 5%)
shall be chosen to span the performance requirements and the following tests carried out :


Compaction test (BS Heavy) on the large sample mixed with the amount of additive
expected to be appropriate (usually the intermediate value of the three), followed by:
CBR and/or UCS tests on specimens prepared at OMC and 95% MDD of BS Heavy
with each of the 3 additive amounts. With lime, samples shall be soaked for 7 days
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and ideally cured for 24 days but at least 7 days: for cement samples shall be soaked
for 7 days and cured for 7 days. UCS tests are carried out for all samples stabilized
with cement: with lime it is possible to achieve viable CBR results at low levels of
lime addition.
At least one sample per 15,000 m3 of material proposed for stabilisation shall be submitted
to the above tests.
14.2.4.3 Stone Quarries
Sources of stone visually considered suitable, both in terms of stone quality and quantity,
should be selected for further investigation. The suitability of stone from existing quarries
should obviously first be investigated since the overburden thickness, degree of weathering
and, possibly, the available reserve will be apparent. (Even if the material from a quarry has
been exhausted or is no longer available it is still worthwhile examining the quarry to gather
information regarding, for instance, the depth of weathering.) The location of each potential
source of stone shall be indicated on a key plan. A site plan of each potential quarry shall be
prepared, showing the characteristic features of the site, such as the orientation and heights
of faces and benches, whether the face is of overburden or stone, the water table level, the
means of access and potential location for a crusher.
14.2.4.3.1 Sampling
Hand sampling from existing faces or outcrops shall be carried out. (If it is an existing quarry
residual stockpiles of crushed stone may still remain and should be sampled.) At least three
hand samples shall be taken from each potential source. The position of each sampling
point, or group of sub-sampling points, shall be accurately located and indicated on the site
plan. Each sample shall be accurately described, from a geological and mineralogical
viewpoint. In particular care shall be taken to ensure that the samples are obtained from
fresh rock and not from weathered or altered rock and, if in doubt, the services of a specialist
should be sought. In Kenya, there are particular problems with identifying fresh rock because
much of the available stone aggregate is basic igneous rock susceptible to weathering.
14.2.4.3.2 Testing
Sufficient sample shall be taken to carry out the following tests:




TFV, dry and wet, and PI on the fines from the TFV dry test, or
ACV
FI
MSS (if doubt still remains on the soundness of the stone)
14.3 Final Design
14.3.1
Earthworks and Subgrade
14.3.1.1 Sampling
At least one sample shall be taken per 500 m along the length of the proposed alignment: if
changes of soil type are evident either from visual observation or from geological data, more
samples are necessary to enable a comprehensive evaluation of the subgrade strength. A
good knowledge of the materials to be cut is also essential, as they will possibly be used as
fill material.
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Pits shall be excavated mostly in proposed cut areas, down to at least 0.5 m below the
anticipated formation level, unless rock is encountered. The position of each pit shall be
accurately determined and reported. In hilly or mountainous terrain, deep holes will be
required to accurately determine the materials to be cut. It is sometimes impossible to dig
trial pits to the depth of the anticipated formation level. It is then recommended to use a hand
or power auger to drill holes to the depth required.
In every pit, all layers, including top soil, shall be accurately described and their thicknesses
measured and recorded. All layers of more than 300 mm (except top soil) shall be sampled.
In every pit in cuts, one sample shall be taken at the approximate level of the formation. The
other samples shall be representative either of the anticipated fill materials or of the
anticipated subgrade in fills.
The sample shall be taken over the full depth of the layer by taking a vertical slice of
material. The log of each test hole shall be accurately drawn and included in the Materials
Report.
It is important to be able to assess the quantities of the various earthwork categories, i.e.
either rock, rippable or diggable material. It will in some cases be necessary to drill
boreholes or use effective indirect means, for example seismic or ground penetrating radar,
to achieve these estimates.
14.3.1.2 Testing of Soils on new alignments
14.3.1.2.1 Basic testing
For each sample, sufficient material shall be obtained to carry out the following tests:




Grading to 0.075 mm sieve
Atterberg Limits
Compaction test (BS Heavy)
CBR and Swell on samples prepared at 90% and 95% of M.D.D and OMC (BS
Heavy).
(CBR shall be measured after 4 days soak, except in arid areas, where they can normally be
measured at OMC, depending on the equilibrium moisture contents predicted under the
pavement in the area. The moisture contents after 4 day soaking shall be measured, both on
the whole CBR specimen and on a sample taken from beneath the plunger, after testing.)
14.3.1.2.2 Subgrade Classification and testing of samples representative of each soil
category
The results from the above basic testing, combined with the relevant field observations, will
enable the subgrade soils to be grouped into homogeneous zones. A zone should include
soils of similar grading, Atterberg Limits, Compaction and CBR. Usually, the number of soil
zones will not exceed 4 or 5 for a given road project.
For each soil zone, one representative large sample shall then be taken and submitted for
the following tests:




Full grading analysis
Atterberg Limits
Compaction test (BS Heavy)
"6 points" CBR test, as summarized below
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
Part 3 - Materials and Pavement
Mineralogical composition determination
For a "6 points" CBR test the material shall be compacted at 3 levels of compaction,
normally around 90, 93 and 95% MDD at BS Heavy. The specimens shall be moulded at the
moisture content expected at the time of in-situ compaction, in general at OMC. At each
level of compaction, one CBR shall be measured immediately and one CBR shall be
measured eventually on one soaked specimen. The time of soaking will depend on the
anticipated subgrade conditions. The amount of water absorbed during soaking and the
eventual swell shall be measured. This method enables an estimate to be made of the
subgrade CBR at different densities and thus assists in determining the relative compaction
to be specified. It also indicates the loss of strength which soaking may cause.
14.3.1.2.3 Treatment tests (when appropriate)
If treatment of the alignment materials is contemplated, for use either as improved subgrade
or as subbase, the treatment tests shall be carried out on the large zone samples in the
manner indicated in the section on Preliminary Design above.
14.3.1.3 Testing of subgrade on existing gravel road alignments
14.3.1.3.1 Basic Testing
This applies to existing gravel roads which are to be upgraded, the geometric standards of
which are good enough to maintain the existing alignment.
Where more than 100 mm of existing gravel wearing course is in place on the road and
where the shape is adequate, samples of subgrade are to be submitted to the tests
enumerated in the preceding section.
It must be decided whether or not to re-compact the subgrade so the insitu moisture content
and dry density must be measured and compared against 90% and 95% of BS Heavy.
Where a gravel road is to be upgraded on the same alignment, the existing gravel wearing
course may provide extra material either for subbase, or for improved subgrade.
Measurements of thickness and width of gravel wearing course shall then be recorded every
100 m. One sample per kilometer of existing gravel wearing course shall be taken, where the
gravel layer is at least 150 mm thick. Each sample shall be submitted to tests enumerated in
the preceding section.
14.3.1.3.2 Subgrade Classification and testing of samples representative of each soil
category
An identical procedure discussed in the relevant Section above should be followed.
14.3.1.4 Existing Gravel Wearing Course (where appropriate)
No further sampling or testing is required at this stage. Indeed, existing gravel wearing
courses are subject to changes both in quantity and quality, under the action of traffic and
weather. They should be considered as possible extra sources of material, to be reevaluated at the construction stage.
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14.3.2
Part 3 - Materials and Pavement
Soil and Gravel Borrow Pits
Information obtained at the Preliminary Design stage will enable the most suitable borrow
areas to be selected, taking into consideration the following factors:



quality of the materials
location of the proposed borrow pits, minimizing haul distance
ease of working, considering, inter alia, land acquisition, clearance and restoration,
access, overburden thickness
14.3.2.1 Field investigations and sampling procedures
Pits shall be dug on a 30 m grid, through the full depth of the layer(s) proposed for use.
The position of each proposed borrow pit shall be indicated on a key plan. A site plan of
each proposed borrow pit shall be prepared, showing the position of each trial pit, the
characteristic features of the site and the means of access and location. In every trial pit, all
layers, including top soil and overburden, shall be accurately described and their thicknesses
measured and recorded. All layers proposed for use shall be sampled.
The sample shall be taken over the full depth of the layer proposed for use by taking a
vertical slice of material.
The log of each trial pit shall be accurately drawn and included in the Materials Report.
14.3.2.2 Frequency of sampling and testing
For characterization purposes at least one sample shall be obtained per 1,000 m³ of material
proposed for use. At least one sample shall be taken from each positive trial pit, even if the
volume represented is smaller than 1,000 m³. Each sample shall be submitted to the
following identification tests:


Grading to 0.075 mm sieve
Atterberg Limits
Large samples for Compaction and CBR tests shall be obtained by either of the following:
1.Mix Method: Large samples can be obtained by mixing "small" identification samples. A
mix must be representative of a workable area. All the "small" samples to be incorporated in
a mix must be the same type of material and must have fairly consistent Grading and
Atterberg Limits. Within each borrow pit, the mixes chosen should adequately cover the
range of materials proposed for use.
2. Re-sampling Method: Using the identification results, large samples can be obtained by
re-sampling from existing trial pits representative of the material types found within the
potential borrow pit area.
At least one large sample, whether mixed or re-sampled, is required per 5,000 m³ of material
proposed for use and shall be submitted to the following tests:




Grading to 0.075 mm sieve
Atterberg Limits
Compaction test (BS Heavy)
CBR and Swell after 4 days soaking, on specimens normally prepared at OMC and
normally 95% and 100% of BS Heavy.
The moisture contents after soaking shall be measured as indicated previously.
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2009
Part 3 - Materials and Pavement
For types of gravel susceptible to crushing during compaction, the grading of the specimen
compacted closest to 95% MDD shall be determined after compaction and CBR testing and
compared with the grading before compaction of the specimen prepared for CBR.
In addition to the following tests, the 10% Fines test (TFV) shall be determined on the coarse
particles of at least one typical sample from each gravel site.
14.3.2.3 Stabilization testing (where required)
The results obtained at the Preliminary Design stage combined with the results of the above
tests will enable the design engineer to decide which borrow pit materials require treatment
and the nature of that treatment (i.e. type of additive and approximate percentage needed,
method of mixing). Stabilisation tests shall then be carried out on the relevant large zone
samples, as defined above.
The Initial Consumption of Lime test (BS 1924) should first be carried out to determine if
there are adverse components within the material to be stabilized. In particular, soils with a
significant organic matter content, or high pH, or high sulphate content may require too high
a level of stabilizer for economic use.
Each sample shall be mixed with cement or lime, or both, whichever from the
characterization tests is expected to give the best results. (If both stabilizers are used, lime is
added first to reduce the plasticity followed by the addition of cement to achieve the required
strength). Three additive amounts (normally at 1% intervals up to a maximum of about 5%)
shall be chosen to span the performance requirements and the following tests carried out :


Compaction test (BS Heavy) on the large sample mixed with the amount of additive
expected to be appropriate (usually the intermediate value of the three), followed by:
CBR and/or UCS tests on specimens prepared at OMC and 95% MDD of BS Heavy
with each of the 3 additive amounts. With lime, samples shall be soaked for 7 days
and ideally cured for 21 days but at least 7 days: for cement samples shall be soaked
for 7 days and cured for 7 days. UCS tests are carried out for all samples stabilized
with cement: with lime it is possible to achieve viable CBR results at low levels of
lime addition.
At least one sample per 5,000 m3 of material proposed for stabilisation shall be submitted to
the above tests.
14.3.2.4 Stone Quarries
The most suitable potential quarry sites will have been selected from investigations carried
out at the Preliminary Design stage. It is advisable to obtain expert advice to confirm the
selection of quarry sites, especially new sites. There are subtleties of tropical weathering
and/or geological complexity which may elude the untrained investigator and the
repercussions of poor quarry selection can have potentially serious and costly
consequences.
14.3.2.4.1 Investigations, drilling and sampling:
A comprehensive quarry investigation would normally comprise fieldwork and laboratory
testing phases.
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2009
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14.3.2.4.1.1 Fieldwork
Production of a topographical map of the proposed quarry at scale 1:1000 showing:



terrain relief, means of access and location
rock outcrops and/or existing quarry faces, and
location of surface samples, excavated samples and boreholes.
The boreholes shall be drilled on an approximate 30 m grid to prove overburden thickness
and nature, and quantity and quality of stone. Normally the core diameter should be 76 mm,
in order to recover stone in sufficient quantity for testing. The log of each borehole shall be
accurately recorded, drawn and included in the Materials Report.
A bulldozer or mechanical excavator should be used if necessary to prove the availability of
solid rock. The excavation can be shown to tenderers during a conducted site visit.
Samples of fresh rock shall be obtained by hand, or pneumatic drilling from existing faces
and outcrops. Great care shall be taken to avoid sampling from weathered and/or altered
rock zones and to ensure that the samples are representative of the stone to be used. In
addition, whenever possible, deeper samples shall be obtained by blasting. Depending on
the consistency of the stone and whether it is an existing or a new quarry 5 to 10 samples
are required per quarry.
14.3.2.4.1.2 Testing
Each sample shall contain sufficient material to carry out the following tests:






TFV, dry and wet, and PI on the fines from the TFV dry test, or
ACV
Specific Gravity (oven-dry method), including Water Absorption
Magnesium Sulphate Soundness
Plasticity Index on 10% Fines Value fines & Plasticity Index on Material passing the
425 micron sieve
Bitumen Affinity (for stone proposed for use with bitumen)
In addition, one large sample shall be obtained from each quarry, so as to be representative
of the stone to be used. This large sample shall be crushed with a small crusher (and not
broken by hand), to a maximum size depending on the proposed use of the stone (usually
ranging from 20 to 40 mm). The crushed stone shall be submitted to the above tests and in
addition to the following tests:




Grading to 0.075 mm sieve
Flakiness Index
Sand Equivalent Value
Compaction test (Vibrating Hammer method), if appropriate.
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2009
Part 3 - Materials and Pavement
15 Standard Methods of Testing
In the previous RDM the recommended test methods for soils and gravels were based on
both BS and AASHTO standards. The Kenya Bureau of Standards (KEBS) have now
published a complete set of standards, wholly adopted from BS 1377 (Testing of Soils) and
BS 812 (Testing of Aggregates) but not BS 1924 (Stabilised materials for civil engineering
purposes). As these are locally available it is proposed to adopt them in this Manual.
However, over the past 10 years, the BS standards have been progressively integrated and
replaced by European ‘EN’ standards. This has not yet reached BS 1377 and BS 1924 but
BS 812 has been replaced with only a few of the original tests retained. This presents a
dilemma for this Manual because it will be seen to be promoting the use of standards which
have become or are becoming obsolescent. Changing laboratory equipment to cater for the
new tests will be a costly process which is considered to be unacceptable in the Kenyan
environment.
15.1 Soils
In 2001 KEBS published a set of soil testing standards wholly adopted from BS 1377 1990.
Table 14.1 itemizes these, together with the equivalent AASHTO and ASTM standards:
Table 15.1: Soil Testing Standards
KS 999 2001
Part No.
Subject
1
General Requirements and Sample
preparation
Classification
Moisture content
Liquid & Plastic Limits
Shrinkage Limit
Linear Shrinkage
Mass density
Particle density
Particle Size Distribution
2
3
4
9
Chemical Tests
Organic Matter Content
Total Sulphate Content
Total Dissolved Solids
pH value
Compaction-related tests
Includes BS Heavy and CBR tests
In-Situ Tests
Includes sand replacement and
nuclear density methods
AASHTO
Test No
T89 & T90
T11 & T27
T180
T191
ASTM Test No
(Vols 04.02,
04.03 & 04.08)
D0421
D2216
D4318
D0427
D42.7
C127 & C128
D0422, D1140
D0698
D1157
D1883
D4546
D1556
D2922
Notes:
 The mass in g) of sample required for sieve analysis is around 400 D, D being the
maximum particles size (in mm), and
 Samples containing particles larger than 20 mm shall be prepared for compaction
and CBR tests by sieving an adequate quantity of the representative material over
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2009
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Part 3 - Materials and Pavement
the 50 mm and 20 mm sieve. Weigh the material passing the 50 mm sieve and
retained on the 20 mm sieve and replace it with an equal mass of material passing
the 20 mm sieve and retained on the 5 mm sieve. Take the material for replacement
from the remaining portion of the main sample. For gravel samples the aggregations
of particles shall be broken with a wooden hammer or pestle. Care shall be taken that
no discrete particles are crushed in this operation.
Differences have been recorded when determining Atterberg limits on either ovendried or air-dried soils. For the sake of consistency it is recommended that Atterberg
Limits should be determined on oven-dried soils.
15.2 Aggregates
In 2003 KEBS published a complete set of aggregate testing standards for use in Kenya, KS
1238, wholly adopted from BS 812. Table 14.2 itemizes some of these tests, together with
the equivalent AASHTO and ASTM standards:
Table 15.2: Aggregate Testing Standards
Property
Tests Required
Toughness
Aggregate Crushing Value (ACV)
10% Fines Crushing Value (TFV)
Aggregate Impact Value (AIV)
Cleanliness Sand Equivalent Value (SEV)
% Deleterious Materials
Durability
Los Angeles Abrasion Value
(LAA)
Water Absorption (WI)
Shape
Other
Tests
Magnesium Sulphate Soundness
(MSS)
Polished Stone Value (PSV)
Flakiness Index (FI)
Moisture Content
Water-soluble Chloride Content
Sulphate Content
KS 1238
Test No
11
12
13
21
-
AASHTO
Test No
T176
T112
T96
8
T84 (fine)
T85 (coarse)
T104
20
15
6
10
16
17
ASTM
Test No
C2419
C142
C131 or
C535
C88
T278
D4791
BS 812 was replaced by CEN standards at the turn of the millennium. A list of the relevant
aggregate tests is given in Table 15.3. It is clear that the range of tests is more
comprehensive than the current list.
Table 15.3: CEN Tests
BS EN No.
932-1
932-2
932-3
932-4
932-5
932-6
Tests for General properties of aggregates
Method of Sampling
Methods for reducing laboratory samples
Procedure & terminology for simplified petrographic description
Common equipment and calibration
Definitions of repeatability and reproducibility
Tests for geometric properties of aggregates
933-1 Determination of particle size distribution: sieving method
933-2 Determination of particle size: test sieves and nominal size of apertures
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2009
933-3
933-4
933-5
933-6
933-7
933-8
1097-1
1097-2
1097-3
1097-4
1097-5
1097-6
1097-7
1097-8
Part 3 - Materials and Pavement
Determination of particle shape: flakiness index
Determination of particle shape: shape index
Percentage of crushed & broken surfaces in coarse aggregate
Flow coefficient of coarse aggregate
Determination of shell content: percentage of shells in coarse aggregates
Sand Equivalent test
Tests for mechanical and physical properties of aggregates
Determination of resistance to wear: Micro-Deval test
Methods for the determination of resistance to fragmentation
Determination of loose bulk density and voids
Determination of the voids in dry-compacted filler
Determination of the water content by drying in a ventilated oven
Determination of particle density and water absorption
Determination of the particle density of filler-pyknometer method
Determination of the polished stone value
Tests for thermal and weathering properties
1367-2 Magnesium Sulphate test
1367-3 Boiling test for Sonnenbrand basalt
15.2.1
Determination of Average Least Dimension
Riffle out a representative sample of about 200 aggregate particles of each size fraction
Sieve the sample through a sieve with an aperture size half the nominal size of the
aggregate to be tested and discard the particles passing the sieve
By means of calipers with platens of at least 5 mm in diameter (or square), measure the
smallest dimension of each particle retained on that sieve, accurate to 0.1 mm, and record
the measurements and the number of particles tested.
Calculations
Calculate the average least dimension to the nearest 0.01 mm as follows:
Average least dimension (mm) = A/B, where
A = sum of the smallest dimension of all the particles in mm
B = number of particles
Report the average least dimension to the first decimal place.
15.3 Cement or Lime Stabilised Materials
Stabilized materials should be tested in accordance with BS 1924: 1990, (Stabilised
materials for civil engineering purposes) Parts 1 and 2. These two Parts contain details of
sample preparation, moisture content determination, compaction and strength testing and
durability testing.
Materials to which stabilizer has been added have a different grading. Before testing with
stabilizer, compaction testing should be carried out to determine the OMC and MDD of the
new grading.
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2009
Part 3 - Materials and Pavement
Samples containing larger particles than 20 mm shall be prepared for compaction and CBR
tests as indicated in Section 14.1. (The fraction coarser than 20 mm shall be replaced by an
equal weight of 5/20 mm material).
The determination of Unconfined Compressive Strength shall be carried out according to test
10. The specimens should be compacted to a pre-determined density and cured at a
temperature of 270C ± 20C.
15.4 Cement and Lime Testing
Ordinary Portland cement shall be sampled and tested in accordance with Kenya Standard
KS 1260 2001, ‘Methods of Physical Testing of Cement’.
Lime shall be tested in accordance with Kenya Standard KS 02 97, or BS EN 459-1:2001.
15.5 Bituminous Binders
Tests involving bitumen are traditionally carried out in Kenya mainly to American standards
and it is therefore proposed to retain them.
15.5.1
Sampling procedures
Sampling of straight-run bitumens and cut-backs shall be carried out in accordance with
AASHTO method T40 (ASTM D 140).
Sampling of bitumen emulsion shall be carried out in accordance with BS 434, except that
where a delivery is made in drums or barrels, the number of samples shall be as indicated in
AASHTO Sampling Method T40, paragraph 11.1.
15.5.2
Testing procedures
Tests on straight-run bitumen shall be carried out in accordance with the following test
procedures:

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







Penetration
Softening point (Ring and Ball)
Flash and fire points (Cleveland open cup)
Loss on Heating
Ductility
Water
Thin film Oven Test
Rolling Thin Film Oven Test
Solubility in organic solvents
Specific gravity
AASHTO T49 (ASTM D 5)
AASHTO T53 (ASTM D 2398)
AASHTO T48 (ASTM D 92)
AASHTO T47 (ASTM D 6)
AASHTO T51 (ASTM D 113)
AASHTO T55 (ASTM D 95)
AASHTO T179 (ASTM D 1754)
AASHTO T240
AASHTO T44 (ASTM D 2042)
AASHTO T228 (ASTM D 70)
Tests on cut-back bitumen shall be carried out in accordance with the following test
procedures:





Kinematic viscosity
Flash point (Tag open cup) (RC-MC)
Flash point (Cleveland open cup) (SC)
Distillation
Water
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AASHTO T201 (ASTM D 2170)
AASHTO T79 (ASTM D 1310)
AASHTO T48 (ASTM D 92)
AASHTO T78 (ASTM D 402)
AASHTO T55 (ASTM D 95)
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DESIGN MANUAL for ROADS and BRIDGES
2009
Part 3 - Materials and Pavement



Specific gravity
Asphalt residue from 100 pen (SC)
Tests on residue from distillation
AASHTO T228 (ASTM D 3142)
ASTM D 243
ASTM D 243



Penetration
Ductility
Solubility
AASHTO T49 (ASTM D 5)
AASHTO T51 (ASTM D 113)
AASHTO T44 (ASTM D 2042)

STV viscosity
BS 3235
Tests on bitumen emulsion shall be carried out in accordance with BS 434 test procedures:










Residue on 0.710 mm sieve
Residue on 0.150 mm sieve
Stability to mixing with coarse aggregate
Stability to mixing with cement
Binder content
Engler viscosity
Redwood II viscosity
Storage stability (short period)
Storage stability (long period)
Particle charge
15.6 Bituminous Mixtures
15.6.1
Sampling procedures
Sampling of bituminous mixtures shall be carried out in accordance with AASHTO method
T168 (ASTM D 979).
15.6.2
Testing procedures
Tests on bituminous mixtures shall be carried out in accordance with the following test
procedures:


Moisture and volatile distillates
Quantitative extraction of bitumen

Specific gravity of compacted mixture

Recovery of bitumen from solution




Coating and stripping
Degree of particle coating
Coating and stripping
(with adhesion agent)


Maximum specific gravity
Degree of pavement compaction
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AASHTO T110 (ASTM D 1461)
AASHTO T164 (ASTM D 2172)
or BS 598
AASHTO T166 (ASTM D 1188
and D 2726)
AASHTO T170 (ASTM D 1856)
or BS 598
AASHTO T182 (ASTM D 1664)
AASHTO T195 (ASTM D 2489)
ASTM D 2727
AASHTO T209 (ASTM D 2041)
AASHTO T230
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DESIGN MANUAL for ROADS and BRIDGES
2009


Marshall Stability
Hubbard-Field Stability
15.6.3
Part 3 - Materials and Pavement
AASHTO T245 (ASTM D 1559)
ASTM D 1138
CEN Tests
The following is a list of CEN tests for bituminous mixtures and fillers:
Table 15.4: CEN Tests
CEN Standard
Aggregates: Tests for Fillers
BS EN 1744-4
BS EN 13179-1
BS EN 13179-2
Test Description
Water susceptibility of fillers for bituminous mixtures
Delta Ring and Ball test
Bitumen Number
Bitumen and Bituminous Binders
BS EN 12591
Specification for paving-grade bitumen
prEN 13924
Specification for hard paving-grade bitumen
prEN 14023
Specification for polymer-modified bitumen
BS EN 58
Sampling of bituminous binders
BS EN 1426
Determination of needle penetration
BS EN 1427
Determination of softening point: Ring and Ball method
BS EN ISO 2592
Determination of flash and fire points
BS EN 12592
Determination of solubility
BS EN 12593
Determination of Frauss breaking point
BS EN 12594
Preparation of test samples
BS EN 12595
Determination of kinematic viscosity
BS EN 12596
Determination of dynamic viscosity by vacuum capillary
BS EN 12607-1
Determination of the resistance to hardening under the
influence of heat and air: RTFOT method
BS EN 12607-2
Determination of the resistance to hardening under the
influence of heat and air: TFOT method
BS EN 12607-3
Determination of the resistance to hardening under the
influence of heat and air: RTF method
prEN 13302
Determination of viscosity of bitumen using a rotating spindle
apparatus
Bituminous Mixtures
prEN 13108-1
prEN 13108-2
prEN 13108-4
prEN 13108-20
BS EN 12697-1
prEN 12697-2
BS EN 12697-3
prEN 12697-5
prEN 12697-6
prEN 12697-8
prEN 12697-9
prEN 12697-10
prEN 12697-11
prEN 12697-12
prEN 12697-15
Material specification-Asphalt Concrete
Material specification-Asphalt Concrete for very thin layers
Material specification-Hot Rolled Asphalt
Quality-type testing of asphalt mixes
Test methods-Soluble binder content
Test methods-Particle size distribution
Test methods-Bitumen recovery, rotary evaporator
Test methods-Maximum density
Test methods-Bulk density, measurement
Test methods-Air voids content
Test methods-Reference density
Test methods-Compatibility
Test methods-Affinity between aggregate and binder
Test methods-Moisture sensibility
Test methods-Segregation sensitivity
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prEN 12697-19
prEN 12697-22
prEN 12697-23
prEN 12697-26
prEN 12697-27
prEN 12697-28
Part 3 - Materials and Pavement
Test methods-Permeability of Porous asphalt specimen
Test methods-wheel tracking
Test methods-Indirect tensile test
Test methods-Stiffness
Test methods-Sampling
Test methods-Preparation of samples for determining binder
content, water content and grading
Test methods-Dimensions of a bituminous specimen
Test methods-Specimen preparation by impact compactor
Test methods-Specimen preparation by gyratory compactor
Test methods-Specimen preparation by vibratory compactor
Test methods-Specimen preparation by slab compactor
Test methods-Marshall test
Test methods-Laboratory mixing
Test methods-Common equipment and calibration
prEN 12697-29
prEN 12697-30
prEN 12697-31
prEN 12697-32
prEN 12697-33
prEN 12697-34
prEN 12697-35
prEN 12697-38
Surface characteristics-road and airfield
prEN 13036-1
Test methods-Measurement of pavement macro-texture
using a patch technique
prEN 13036-2
Test methods-Procedure for determination of skid resistance
of a pavement surface
prEN 13036-4
Test methods-Measurement of slip/skid resistance of a
surface: the Pendulum test
prEN 13036-5
Test methods-Determination of longitudinal evenness
parameters or indicators
prEN 13036-6
Test methods-Profilometer-based method for measuring
longitudinal evenness
prEN 13036-7
Test methods-method for measuring surface irregularities:
the Straight-Edge test
prEN 13606-8
Test methods-Determining parameters or indicators for
transverse evenness: Measurement method
15.6.3.1 The Brazilian Test
The Brazilian test comprises the measurement of the force required to crush a cylindrical
sample along one of its diameters between two parallel flat platens. The samples are to be
prepared in accordance with and to the size required by the Marshall test (ASTM D1559)
compacted using 50 blows of the rammer on each, face of the sample. The temperature of
the sample at the time of test is to be 25˚C ±2˚C and the platens during the test are to be
moved together at a content rate of 0.86 mm/sec.
The indirect tensile stress is calculated as:2 x Load at failure/perimeter of sample x thickness of sample
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2009
Part 3 - Materials and Pavement
16 Footpaths
Possibly 50% of all fatalities and serious accidents in Kenya are suffered by pedestrians. A
proportion is certainly caused by careless/dangerous driving, or pedestrian inattention but it
is almost as certain that the numbers would be reduced if footpaths were properly
constructed. That this is manifestly not the case in Kenya is presumably a reflection of
funding.
Footpaths should be constructed on a raised platform on the edge of the road shoulder, if
adjacent to the road, but if there is a drain next to the shoulder then preferably on the outside
of the drain. The edge of the platform should consist of a kerb, manufactured either from
stone or from pre-cast concrete block. This should be a minimum of 100mm height and the
width of the kerb should be at least equal to the height. The kerb should be firmly secured
into the surface on which it is laid.
The footpath itself should be at least one metre wide. The foundation of the footpath should
be thoroughly cleared of vegetation and the subbase of the footpath can consist of either an
unbound mixture, such as non-plastic sand, or a cement-bound mixture, depending on the
footpath surfacing. Most importantly, the surfacing should be inclined to permit drainage,
otherwise water will pond on the footpath and pedestrians will be inclined to use the
carriageway.
The footpath surfacing can consist of either natural stone or pre-cast concrete flagstones, of
maximum size 450mm square, or asphalt concrete (Type II, 0/10). If the surfacing is of
flagstones, the subbase should either consist of a layer of mortar between 10mm and 40mm
thick, or a layer of 0/4mm sand 25mm ±5mm thick. It is important that the sand is well
compacted. The joints between the flagstones should be filled with finer (0/2mm) sand.
There are specifications for this construction.
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Kerbs shall conform to BS EN 1340.
Pre-cast concrete flagstones shall conform to BS EN 1339: natural stone flagstones
shall conform to BS EN 1341.
The method of construction of the footpath should conform to BS 7533-4.
The sand subbase should conform to BS EN 12620.
Figure 16.1: Well constructed footpath and Non-existent footpath, Elegayo Marakwet St,
Kilimani, Nairobi
The Republic of Kenya – Ministry of Roads 176
Draft Document – October 2009