Bases, Subbases, & Low Cost
Surfaces
Text Book Chapter 20
Dr. TALEB M. AL-ROUSAN
Subgrades

Is usually the natural material located along
the horizontal alignment of the pavement.
 It serves as the foundation of the pavement
structure.
 It may consist of a layer of selected borrow
materials that is well compacted to some
prescribed specification.
 It may be necessary to treat the subgrade
material to achieve certain strength properties
required for the type of pavement being
constructed.
Subbase Course

Located immediately above the subgrade.
 It has better quality than subgrade.
 When the quality of the subgrade materials
meets the requirements of the subbase
material, the subbase layer may be omitted.
 When suitable subbase material is not
available, the available materials can be
treated to achieve necessary properties
(Stabilization).
Base Course

Lies immediately above the subbase
 Consists of granular materials (crushed stone,
crushed slag, gravel and sand).
 Specifications are stricter than those for
subbase regarding (gradation, plasticity, and
strength).
 Materials that do not have the required
properties can be used as base course
material if the are properly stabilized with
Portland cement, asphalt, or lime.
Surface Course

Upper course of the road pavement and is
constructed immediately above the base
course.
 Consist of mixture of mineral aggregate and
asphaltic materials.
 It should be capable of :
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withstanding high tire pressure,
Resisting traffic abrasive forces,
Providing a skid-resistance surface.
Preventing the penetration of surface water into the
underlying layers
Soil Stabilization



Soil stabilization: Combination and manipulation of soils, with or
without admixtures, to produce a firm mass that is capable of
supporting traffic in all weather conditions.
Soil Stabilization: Is the treatment of natural soil to improve its
engineering properties.
Soil Stabilization Methods:


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Mechanical: Blending different grades of soil to obtain a required
grade.
Chemical: is the blending of natural soil with chemical agents (most
used are Portland cement, asphalt binders, and lime).
Stable stabilized soil must have sufficient shearing strength to
withstand traffic loadings in all weather conditions without
excessive deformations.
If stabilized soil is to be used as wearing surface, then it must be
capable of withstanding the abrasive effects of traffic.
Soil Stabilization Cont.
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Purpose: Effective utilization of local materials in order
to decrease construction cost.
Used when natural soil has unfavorable characteristics
and require modification.
Primary use of stabilized soil is in base and subbase
construction.
Stabilized base and subbase support thin wearing
surfaces for light traffic.
Stabilized base and subbase may function as base for
high type pavements with heavy traffic.
See Table 20.1 in Text for soil types and stabilization
methods.
Classes of Stabilization
1.
2.
3.
4.
Soil-Aggregate Roads and Granular
Stabilization.
Bituminous Stabilization.
Cement Stabilization
Stabilization by addition of Salt, lime,
and various other chemicals.
Soil-Aggregate Roads and
Granular Stabilization



1.
2.
3.
Consist of a substantial layer of a properly
proportioned and blended mixture of soil and
aggregates compacted to form a road.
AASHTO : Known as (dense-graded
aggregates) It’s a natural or prepared mixture
consisting predominantly of stone, gravel, or
sand and containing silty clay.
Construction procedure:
Road mix construction
Traveling plant construction
Central plant construction
Soil-Aggregate Roads and
Granular Stabilization Cont.
Calcium Chloride is used some times to
expedite the compaction process by
slowing the rate of evaporation from the
mixture.
 Calcium Chloride might increase density
and maintain moisture in wearing
surfaces for desired max stability which
will prevent raveling of the surface and
reduce dust.

Bituminous Stabilization

1.
2.
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Bituminous materials are used in conjunction with
aggregates and soil-aggregate mixture for two
purposes:
Binding and Supply cohesion to the stabilized soil
mixtures (Like in Sand stabilization).
Waterproofing
Mostly used for base construction.
Used for soils with PI < 10
Used with : well-graded soils, sands, and clays.
Finer soils require more bitumen for waterproofing.
Very fine plastic soils can not be stabilized
economically with bituminous materials because of
difficulties of pulverization and mixing.
Bituminous Stabilization Cont.

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1.
2.
3.
4.
Choice of bitumen depend on local experience and
comparative cost.
Use 4 – 7% bitumen
Types of bitumen:
RC: use for sandy soil or soils with min. clay or silt.
MC : used for soils with higher clay or silt particles
(give more homogenous mixes)
SC: used for soils with 30 -40% silt & clay.
MS & SS emulsified asphalt (Table 18-3 in Ref. 1).
Bituminous Stabilization Cont.
 Oiled
earth surfaces: Liquid bitumen
is applied at properly prepared
natural soil surface.
 Prevent dust and provide a thin
stabilized surface that support
limited number of light vehicles.
 See Construction method in Text.
Cement Stabilization
Incorporating Portland cement (5 – 14%
volume of compacted mixture) with
naturally occurring or artificially created
soils or soil-aggregate mixtures.
 Used for bases in city street
constructions, driveways, shoulders,
parking areas, and some airport
runways.
 Also known as cement-treated base.

Cement Stabilization Cont.
Nearly all subgrade soils may be
stabilized with cement except those soils
that contain high percentage of organic
materials.
 Silt and clay require large percentage of
cement for successful stabilization.
 Sandy and gravelly soils with 10 – 35%
silt and clay have most favorable
characteristics.
 See construction method in Text.

Stabilization by Addition of Lime

Incorporating small amounts of hydrated lime
improves certain plastic clay soils.
 Most Commonly used Calcium Hydroxide Ca(OH)2
and dolomite [Ca(OH)2 + MgO]
 To be most effective soils should have PI<10.
 Used to reduce plasticity, shrinkage, and swell of clay
soils and increasing their bearing capacity.
 Improve workability of plastic soil (easy to pulverize).
 Waterproof soil to some extent and allow it to dry
quickly when saturated to expedite construction.
Stabilization by Addition of Lime
Cont.
Use 3 – 10%.
 Depth of treatment around 6 inches.
 Subgrade soil is scarified and pulverized,
then lime is spread mechanically, water
is added (+ 5% above opt.), then allowed
to cure for (1 -7days). Mixing and
compaction is then applied.

Macadam Roads

Road surface or base in which crushed or broken
stone was mechanically keyed or locked by rolling and
cemented together by the application of stone
screening and water.
 Later, bitumen macadam roads were built using
bitumen material as binder.
 Water-bound macadam: broken stone bound together
by stone dust and water applied during construction
(Seldom Constructed).
 Bituminous macadam: crushed stone base or wearing
surface in which fragments are bound together by
bituminous material; the aggregate layer is compacted
and bituminous material is applied to the surface
(Known as penetration macadam).
Bituminous Macadam
Construction
1.
2.
3.
4.
Spreading and rolling of coarse
aggregates.
Initial application of bituminous
material.
Spreading and rolling of key
aggregates.
Application of seal coat
Seal Coat

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
Very thin single surface treatment that
is usually less than (½ in) thick.
Single Surface treatment = single
application of bituminous material that
is covered by a light spreading of fine
aggregate or sand (spread
mechanically) then compacted with
pneumatic tired rollers.
Applied as final step in many types of
bituminous wearing surfaces.
Seal Coat Cont.
 Purpose:
1. Waterproof
or seal the surface.
2. Rejuvenate or revitalize old
bituminous wearing surfaces.
3. To nonskid slippery surfaces.
4. Improve night visibility.
Seal Coats
Are usually single application of asphalt
material that may or may not contain
aggregates.
 Three types of asphalt seal commonly
used in asphalt maintenance:

Fog Seal
 Slurry Seal
 Aggregate Seal

Fog Seal Coat

Thin application of emulsified asphalt, usually
with no aggregates added.
 Slow setting emulsions are normally used.
 After diluted , its usually sprayed at arate of
0.1 to 0.2 gal/yd2
 Fog seals are used mainly to:
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Reduce infiltration of rain and water into the
pavement.
Prevent the progressive separation of aggregate
particles from the surface‫و‬
Bring the surface of the pavement to its original
state.
Slurry Seal Coat
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Mixture of well-graded, fine aggregate, mineral
filler (if needed), slow setting emulsified
asphalt, and water.
Use principally for the resealing of old
bituminous wearing surfaces.
Considered as low cost maintenance material
for light traffic
Very thin (1/16” to 1/8” and up to ¼”).
Cracks may appear shortly due to this surface.
See Figure 18-6 in Ref. 1.
Aggregate seal
Obtained by spraying asphalt ,
immediately covering it with aggregates,
and then rolling the aggregate into the
asphalt.
 Asphalt used are usually the softer
grades of paving asphalts and the
heavier grades of liquid asphalts.
 Used to restore the surface of old
pavements.

Prime Coats

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1.
2.
3.
Application of liquid bituminous material to
previously untreated base or wearing
surface.
Bituminous material penetrate surface and
completely absorbed
Purpose:
Promote adhesion “bond” between base and
wearing surface.
Consolidate the surface on which the new
treatment is to be placed.
It may function as a deterrent to the rise of
capillary moisture into the wearing surface.
Prime Coats Cont.

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Use low viscous asphalt or tar.
Asphalt preferred to have high penetrated
qualities and should leave high viscosity
residue in the void spaces.
MC-30 and MC-70 widely used for dense
tightly bond surfaces, and surface with more
opened texture.
Slow setting emulsified asphalts are
recommended for penetrable surfaces.
Recommended quantity (0.9 – 2.3 L/m^2).
Prime Coats


Obtained by spraying asphaltic Binder materials onto nonasphalt
base courses.
Used to:
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Provide a waterproof surface.
Fill capillary voids in the base.
Facilitate the bonding of loose mineral particles.
Facilitate the adhesion of the surface treatment to the base.
Medium curing are usually used.
The amount of asphalt used should be the maximum that could
be absorbed by the base within 24 hrs of application.
The base course must contain a nominal amount of water to
facilitate the penetration of the asphaltic material into the base.
Tack Coats

Single application of bituminous material to an
existing bituminous, Portland cement concrete,
brick, or block surface or base.
 Purpose: Provide adhesion between the
existing surface and the new bituminous
wearing surface.
 RC-70 through RC-250 are frequently used.
 Quantity required for application is small (0.19
to 0.38 L/m^2).
Tack Coats
Is a thin layer of asphaltic material
spayed over an old pavement to facilitate
the bonding of the old layer and the new
one.
 Asphalt emulsions are usually used.
 Amount spayed is less than in prime coat
since no asphalt will be absorbed by the
beneath layer.
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