Introduction to
Pavement Design
Concepts
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Pavement
Types of Pavement
Principal of Pavement Design
Failure Criteria
Aspects of Pavement Design
Relative Damage Concept
Pavement Thickness Design approaches
Empirical Method
Mechanistic-Empirical Method
PAVEMENT
The pavement
is the structure which
separates the tyres of vehicles from the
underlying foundation material. The later is
generally the soil but it may be structural
concrete or a steel bridge deck.
TYPES OF
PAVEMENT
Flexible
Pavements
Rigid
Pavements
FLEXIBLE PAVEMENTS
Flexible
Pavements
are
constructed from bituminous or
unbound material and the stress is
transmitted to the sub-grade
through the lateral distribution of
the applied load with depth.
Asphalt Concrete
Aggregate Base Course
Natural Soil (Subgrade)
Aggregate Subbase Course
Typical Load Distribution in Flexible Pavement
Wheel Load
Bituminous Layer
Sub-grade
Typical Stress Distribution in Flexible Pavement.
Vertical stress
Foundation stress
RIGID PAVEMENTS
 In rigid pavements the stress is transmitted
to the sub-grade through beam/slab effect.
Rigid pavements contains sufficient beam
strength to be able to bridge over localized
sub-grade failures and areas of inadequate
support.
 Thus in contrast with flexible pavements the
depressions which occur beneath the rigid
pavement are not reflected in their running
surfaces.
Rigid Pavement
Concrete Slab
Sub-grade
PRINCIPLES OF PAVEMENT DESIGN

The tensile and compressive stresses induced in a
pavement by heavy wheel loads decrease with increasing
depth. This permits the use, particularly in flexible
pavements, of a gradation of materials, relatively strong
and expensive materials being used for the surfacing and
less strong and cheaper ones for base and sub-base.

The pavement as a whole limit the stresses in the subgrade to an acceptable level, and the upper layers must in
a similar manner protect the layers below.
PRINCIPLES OF PAVEMENT DESIGN
Pavement design is the process of developing the
most economical combination of pavement layers
(in relation to both thickness and type of
materials) to suit the soil foundation and the
traffic to be carried during the design life.
DESIGN LIFE
The concept of design life has to be
introduced to ensure that a new road will
carry the volume of traffic associated with
that life without deteriorating to the point
where reconstruction or major structural
repair is necessary
Philosophy of Pavements
•
Pavements are alive structures
•
They are subjected to moving traffic loads that are
repetitive in nature
•
Each traffic load repetition causes a certain amount of
damage to the pavement structure that gradually
accumulates over time and eventually leads to the
pavement failure.
•
Thus, pavements are designed to perform for a certain
life span before reaching an unacceptable degree of
deterioration.
•
In other words, pavements are designed to fail. Hence,
they have a certain design life.
DESIGN LIFE
For roads in Britain the currently
recommended design is 20 years for
flexible pavements.
PERFORMANCE AND FAILURE
CRITERIA
A road should be designed and constructed
to provide a riding quality acceptable for
both private cars and commercial vehicles
and must perform the functions i.e.
functional and structural, during the design
life.
PERFORMANCE AND FAILURE
CRITERIA
If the rut depth increases beyond 10mm or the
beginning of cracking occurs in the wheel paths,
this is considered to be a critical stage and if the
depth reaches 20mm or more or severe cracking
occurs in the wheel paths then the pavement is
considered to have failed, and requires a
substantial overlay or reconstruction in
accordance with LR 833.
Failure Mechanism (Fatigue and Rut)
Nearside Wheel Track
Rut Depth
Bitumen Layer
Fatigue Crack
Unbound Layer
Typical Strains in Three Layered System
Elastic Modulus ’E1’
Poison’s Ratio ‘ v1’
Bituminous bound Material
Er
Thickness ‘H1’
Maximum Tensile Strain at Bituminous Layer
Elastic Modulus ’E2’
Poison’s Ratio ‘ v2’
Granular base/Sub-base
Ez
Thickness ’H2’
Maximum Compressive on the top of the sub-grade
Sub-grade
Elastic Modulus ’E3’
Poison’s Ratio ‘ v3’
The following relationship can be used to calculate
permissible tensile and compressive strains by
limiting strain criterion for 85% probability of
survival to a design life of N repetition of 80 kN
axles and an equivalent pavement temperature of
20C;
log N = -9.38 - 4.16 logr (Fatigue, bottom of bituminous layer)
log N = - 7.21 - 3.95 logz (Deformation, top of the sub-grade)
r
= is the permissible tensile strain at the bottom of the
bituminous layer
z
= is the permissible Compressive strain at the top of the
sub-grade.
ASPECTS OF DESIGN
Functional
Safety
Riding Quality
Structural
Can sustain
Traffic Load
Structural Performance
Strength
Functional Performance
Safety
Comfort
RUDIMENTARY DEFINITION
Pavement Thickness Design is the determination of required
thickness of various pavement layers to protect a given soil
condition for a given wheel load.
Given Wheel Load
150 Psi
Asphalt Concrete Thickness?
Base Course Thickness?
Subbase Course Thickness?
3 Psi
Given In Situ Soil Conditions
PAVEMENT DESIGN PROCESS
Climate/Environment
Load Magnitude
Traffic
Volume
Asphalt Concrete
Material
Properties
Base
Subase
Roadbed Soil (Subgrade)
Truck
Asphalt Concrete Thickness ?
Base Course
? Thickness ?
Sub-base Course Thickness ?
• Pavement Design Life
= Selected
• Structural/Functional Performance
= Desired
• Design Traffic
= Predicted
WHAT DO WE MEAN BY ?
SELECTED DESIGN LIFE
DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?
WHAT DO WE MEAN BY ?
DESIRED STRUCTURAL AND
FUNCTIONAL PERFORMANCE
FUNCTIONAL PERFORMANCE CURVE
Rehabilitation
Perfect
Ride Quality
Unacceptable
limit
Traffic/ Age
STRUCTURAL PERFORMANCE CURVE
Structural
Capacity
Rehabilitation
Perfect
Traffic/ Age
Structural
Failure
WHAT DO WE MEAN BY ?
PREDICTED DESIGN TRAFFIC
Traffic Loads Characterization
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars
Pickups
Buses
Trucks
Trailers
13.6 Tons
Failure = 10,000 Repetitions
11.3 Tons
Failure = 100,000 Repetitions
4.5 Tons
Failure = 1,000,000 Repetitions
2.3 Tons
Failure = 10,000,000 Repetitions
13.6 Tons
4.5 Tons
Failure = Repetitions ?
11.3 Tons
2.3 Tons
RELATIVE DAMAGE CONCEPT
Equivalent
Standard
18000 - Ibs
ESAL
(8.2 tons)
Damage per
Pass = 1
Axle Load
• Axle loads bigger than 8.2 tons cause damage greater
than one per pass
• Axle loads smaller than 8.2 tons cause damage less than
one per pass
• Load Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4
Consider two single axles A and B where:
A-Axle = 16.4 tons
 Damage caused per pass by A -Axle = (16.4/8.2)4 = 16
 This means that A-Axle causes same amount of damage per
pass as caused by 16 passes of standard 8.2 tons axle i.e,
=
16.4 Tons
Axle
8.2 Tons
Axle
Consider two single axles A and B where:
B-Axle = 4.1 tons
 Damage caused per pass by B-Axle = (4.1/8.2)4 = 0.0625
 This means that B-Axle causes only 0.0625 times damage per
pass as caused by 1 pass of standard 8.2 tons axle.
 In other works, 16 passes (1/0.625) of B-Axle cause same amount
of damage as caused by 1 pass of standard 8.2 tons axle i.e.,
=
4.1 Tons Axle
8.2 Tons Axle
DAMAGE PER PASS
80
70
60
50
40
30
20
10
0
1.0
1.1
2.3
3.3
4.7
6.5
8.7
11.5
14.9
18.9
23.8
29.5
36.3
44.1
53.1
63.4
75.2
AXLE LOAD & RELATIVE DAMAGE
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SINGLE AXLE LOAD (Tons)
PAVEMENT THICKNESS DESIGN
Comprehensive Definition
Pavement Thickness Design is the determination of
thickness of various pavement layers (various
paving materials) for a given soil condition and the
predicted design traffic in terms of equivalent
standard axle load that will provide the desired
structural and functional performance over the
selected pavement design life.
PAVEMENT THICKNESS DESIGN APPROACHES
EMPIRICAL
PROCEDURE
MECHANISTICEMPIRICAL
PROCEDURE
EMPIRICAL PROCEDURES
• These procedures are derived from experience (observed field
performance) of in-service pavements and or “Test Sections”
• These procedures define the interaction
A given set of
paving materials
Pavement
and soils,
performance, traffic for
between
geographic
loads & pavement
location and
thickness
climatic
conditions
• These procedures are only accurate for the exact conditions
for which they were developed and may be invalid outside the
range of variables used in their development.
• EXAMPLE
•AASHTO Procedure (USA)
•Road Note Procedure (UK)
EMPIRICAL PROCEDURES
These methods or models are generally used to
determine the required pavement thickness, the
umber of load applications required to cause
failure, or the occurrence of distress due to
pavement material properties, sub-grade type,
climate, and traffic conditions.
EMPIRICAL PROCEDURES
One advantage in using empirical models is that
they tend to be simple and easy to use.
Unfortunately they are usually only accurate for
the exact conditions for which they have been
developed. They may be invalid outside of the
range of variables used in the development of the
method
AASHTO PROCEDURE
 Empirical Procedure developed through statistical
analysis of the observed performance of AASHTO
Road Test Sections.
 AASHTO Road Test was conducted from 1958 to 1960
near Ottawa, Illinois, USA.
 234 “Test Sections” (160 feet long), each
incorporating a different combination of
thicknesses of Asphalt Concrete, Base Course and
Subbase Course were constructed and trafficked
to investigate the effect of pavement layer
thickness on pavement performance.
North
Frontage Road
Maintenance Building
Proposed FA 1 Route 80
Loop 4
Loop 5
Loop 6
Loop 3
2
US
178
1
Army Barracks
6
AASHO Adm’n
Frontage Road
Test Tangent
Flexible
Steel I-Beam
Test Tangent
Typical Loop
71
6
71
23
Pre-stressed /
Reinforced Concrete
X X
X X
Rigid
US
Ottawa
Utica
X X
X X
23
AASHO ROAD TEST CONDITIONS
ENVIRONMENT
•Climate
-4 to 24oC
•Average Annual Precipitation
34 Inches (864 mm)
•Average Frost Penetration Depth 28 Inches
Soil
•Classification
•Drainage
•Strength
Pavement Layer Materials
•Asphalt Concrete
•Base Course
•Subbase Course
A-6/A-7-6 (Silty-Clayey)
Poorly Drained
2-4 % CBR (Poor)
AC
Crushed Stone
Sandy Gravel
a1 = 0.44
a2 = 0.14
a3 = 0.11
AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST
LOOP LANE
WEIGHT IN TONS
1
FRONT AXLE
LOAD LOAD
2
2
FRONT
LOAD
1
3
FRONT
LOAD
FRONT
LOAD
LOAD
LOAD
LOAD AXLE
GROSS WEIGHT
0.9
0.9
0.9
2.7
1.8
3.6
1.8
5.5
12.7
2.7
10.9
24.6
2.7
8.2
19.1
4.1
14.6
33.2
2.7
10.2
23.2
4.1
18.2
40.5
4.1
13.6
31.4
5.5
21.8
49.1
1
4
FRONT
LOAD
FRONT
LOAD
LOAD
LOAD
1
5
FRONT
LOAD
FRONT
LOAD
LOAD
LOAD
1
6
FRONT
FRONT
LOAD
LOAD
LOAD
LOAD
AASHO ROAD TEST
• “Test Sections” were subjected to 1.114 million applications of load.
• Performance measurements (roughness, rutting, cracking etc.) were
taken at regular intervals and were used to develop statistical
performance prediction models that eventually became the basis for the
current AASHTO Design procedure.
• AASHTO performance model/procedure determines for a given soil
RIDE QUALITY
condition, the thickness of Asphalt Concrete, Base Course and Subbase
Course needed to sustain the predicted amount of traffic (in terms of 8.2
tons ESALs) before deteriorating to some selected level of ride quality.
Initial
Asphalt Concrete = ?
Base = ?
Terminal
Subbase = ?
ESALs
Soil
LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE
AASHTO being an EMPIRICAL
procedure is applicable to the
AASHO Road TEST conditions
under which it was developed.
MECHANISTIC-EMPIRICAL PROCEDURES
 These procedures, as the name implies, have two parts:
=>
A mechanistic part in which a structural model
(theory) is used to calculate stresses, strains and
deflections induced by traffic and environmental
loading.
=>
An empirical part in which distress models are used
to predict the future performance of the pavement
structure.
 The distress models are typically developed from the
laboratory data and calibrated with the field data.
 EXAMPLES
• Asphalt Institute Procedure (USA)
• SHRP Procedure (USA)
Mechanistic- Empirical Methods
The mechanistic –empirical method of design is
based on the mechanics of materials that relates
an input, such as a wheel load, to an out put or
pavement response, such as stress or strain. The
response values are used to predict distress based
on laboratory test and field performance data.
Dependence on observed performance is
necessary because theory alone has not proven
sufficient to design pavements realistically
Mechanistic- Empirical Methods
Kerkhoven and Dormon (1953) first suggested the use
of vertical compressive strain on the surface of subgrade as a failure criterion to reduce permanent
deformation, while Saal and Pell(1960) recommended
the use of horizontal tensile strain at the bottom of
asphalt layer to minimize fatigue cracking. The use of
above concepts for pavement design was first presented
in the United States by Dormon and Metcalf (1965)
Mechanistic- Empirical Methods
By limiting the elastic strains on the sub-grade, the
elastic strains in other components above the sub-grade
will also be controlled; hence, the magnitude of
permanent deformation on the pavement surface will
be controlled as well. These two criteria have since
been adopted by Shell Petroleum International
(Claussen et al., 1977) and the Asphalt Institute (Shook
et al., 1982) in their mechanistic-empirical methods of
design, the ability to predict the types of distress, and
the feasibility to extrapolate from limited field and
laboratory data.
Mechanistic - Empirical Design Approach
Researchers assumes that mechanistic empirical design procedures will model a
pavement more accurately than empirical
equations. The primary benefits that could
result from the successful application of
mechanistic empirical procedures include:
Benefits of Mechanistic - Empirical Design
Approach
The ability to predict the occurrence of
specific types of distress.
 Stress dependency of both the subgrade and
base course.
 The time and temperature dependency of the
asphaltic layers.

Benefits of Mechanistic - Empirical Design
Approach
 Estimates of the consequences of new loading conditions
can be evaluated. For example, the damaging effects of
increased loads, high tire pressures, and multiple axles,
can be modeled by using mechanistic processes.
 Better utilization of available materials can be
accomplished by simulating the effects of varying the
thickness and location of layers of stabilized local
materials.
 Seasonal effects can be included in performance
estimates.
Benefits of Mechanistic - Empirical
Design Approach
 One of the most significant benefits of these methods is
the ability to structurally analyze and extrapolate the
predicted performance of virtually any flexible pavement
design from limited amounts of field or laboratory data
prior to full scale construction applications. This offers
the potential to save time and money by initially
eliminating from consideration those concepts that have
been analyzed and are judged to have little merit.
Draw Back of Mechanistic - Empirical
Design Procedures
One of the biggest drawbacks to the use of
mechanistic design methods is that these methods
require more comprehensive and sophisticated data
than typical empirical design techniques. The modulus
of resilience, creep compliance, dynamic modulus,
Poisson's ratio, etc., have replaced arbitrary terms for
sub-grade and material strength used in earlier
empirical techniques.
However, the potential benefits are believed
to far outweigh the drawbacks. In summary,
mechanistic-empirical design procedures offer
the best opportunity to improved pavement
design technology for the next several decades.
SOURCES OF PREMATURE PAVEMENT FAILURE
Construction Practices
&
Quality Control
Construction Practices
&
Quality Control
Construction Practices
&
Quality Control
Inadequately Designed Pavements Will Fail Prematurely Inspite
Of Best Quality Control & Construction Practices
Causes of Premature Failure in Pakistan
 Causes of premature failure of pavements in
Pakistan
 Rutting due to high variations in ambient
temperature
 Uncontrolled heavy axle loads
 Limitations of pavement design procedures
to meet local environmental conditions
COMPARISON OF TRUCK DAMAGE
PAKISTAN Vs USA
1
7
13
2
8
14
19
20
3
9
15
4
10
16
5
11
17
6
12
18
21
22
Plastic Flow Rutting
Rutting in Asphalt Layer
Rutting in Sub-grade or Base