Highway Engineering Civil Engineering

2. Introduction to
Pavement Design
Concepts

3. 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

4. PAVEMENTPAVEMENT
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.

5. TYPES OF
PAVEMENT
Flexible
Pavements
Rigid
Pavements

6. FLEXIBLE PAVEMENTSFLEXIBLE 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.

7. Natural Soil (Subgrade)
Aggregate Subbase Course
Aggregate Base CourseAsphalt Concrete

8. Wheel Load
Sub-grade
Bituminous Layer
Typical Load Distribution in Flexible Pavement

9. Vertical stress
Foundation stress
Typical Stress Distribution in Flexible Pavement.

10. RIGID PAVEMENTSRIGID PAVEMENTS
 Thus in contrast with flexible pavements the
depressions which occur beneath the rigid
pavement are not reflected in their running
surfaces.
 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.

11. Rigid PavementRigid Pavement
Concrete Slab
Sub-grade

12. PRINCIPLES OF PAVEMENTPRINCIPLES OF PAVEMENT
DESIGNDESIGN
 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 sub-
grade to an acceptable level, and the upper layers must in
a similar manner protect the layers below.

13. 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.
PRINCIPLES OF PAVEMENTPRINCIPLES OF PAVEMENT
DESIGNDESIGN

14. DESIGN LIFEDESIGN 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

15. • 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.
Philosophy of PavementsPhilosophy of Pavements

16. For roads in Britain the currently
recommended design is 20 years for
flexible pavements.
DESIGN LIFEDESIGN LIFE

17. PERFORMANCE AND FAILUREPERFORMANCE AND FAILURE
CRITERIACRITERIA
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.

18. PERFORMANCE AND FAILUREPERFORMANCE AND FAILURE
CRITERIACRITERIA
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.

19. Failure Mechanism (Fatigue and Rut)Failure Mechanism (Fatigue and Rut)
Bitumen Layer
Unbound Layer
Nearside Wheel Track
Fatigue Crack
Rut Depth

20. Granular base/Sub-base
Sub-grade
Bituminous bound Material
Typical Strains in Three Layered SystemTypical Strains in Three Layered System
Elastic Modulus ’E1’
Poison’s Ratio ‘ v1’
Thickness ‘H1’
Elastic Modulus ’E2’
Poison’s Ratio ‘ v2’
Thickness ’H2’
Maximum Tensile Strain at Bituminous Layer
Maximum Compressive on the top of the sub-grade
Er
Ez
Elastic Modulus ’E3’
Poison’s Ratio ‘ v3’

21. log N = -9.38 - 4.16 logεr (Fatigue, bottom of bituminous layer)
log N = - 7.21 - 3.95 logεz (Deformation, top of the sub-grade)
εr = is the permissible tensile strain at the bottom of the
bituminous layer
εz = is the permissible Compressive strain at the top of the
sub-grade.
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;

22. ASPECTS OF DESIGN
Functional Structural
Safety Riding Quality
Can sustain
Traffic Load

23. Structural PerformanceStructural Performance
Strength
Safety
Comfort
Functional PerformanceFunctional Performance

24. 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.
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
3 Psi
Given In Situ Soil Conditions
Asphalt Concrete Thickness?
Base Course Thickness?
Subbase Course Thickness?

25. PAVEMENT DESIGN PROCESS
Climate/Environment
Load Magnitude
Volume
Traffic
Material
Properties
Asphalt Concrete
Roadbed Soil (Subgrade)
Base
Subase

26. • Pavement Design Life = Selected
• Structural/Functional Performance = Desired
• Design Traffic = Predicted
?
Asphalt Concrete Thickness ?
Base Course Thickness ?
Sub-base Course Thickness ?
Truck

27. WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
SELECTED DESIGN LIFE

28. DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?

29. WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
DESIRED STRUCTURAL AND
FUNCTIONAL PERFORMANCE

30. FUNCTIONAL PERFORMANCE CURVE
STRUCTURAL PERFORMANCE CURVE
Rehabilitation
Unacceptable
limit
RideQuality
Perfect
Traffic/ Age
Structural
Capacity
Perfect Traffic/ Age
Rehabilitation Structural
Failure

31. WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
PREDICTED DESIGN TRAFFIC

32. Traffic Loads Characterization
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars Pickups Buses Trucks Trailers

33. Failure = 10,000 Repetitions
13.6 Tons
Failure = 100,000 Repetitions
11.3 Tons
Failure = 1,000,000 Repetitions
4.5 Tons
Failure = 10,000,000 Repetitions
2.3 Tons
11.3 Tons
Failure = Repetitions ?
13.6 Tons
4.5 Tons
2.3 Tons

34. Equivalent
Standard ESAL
Axle Load
18000 - Ibs
(8.2 tons)
Damage per
Pass = 1
• 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
RELATIVE DAMAGE CONCEPT

35. 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,
8.2 Tons
Axle
16.4 Tons
Axle
=

36. 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.,
Consider two single axles A and B where:
=
4.1 Tons Axle 8.2 Tons Axle

37. AXLE LOAD & RELATIVE DAMAGE
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
0
10
20
30
40
50
60
70
80
DAMAGEPERPASS
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SINGLE AXLE LOAD (Tons)

38. 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.

39. PAVEMENT THICKNESS DESIGN APPROACHES
EMPIRICAL
PROCEDURE
MECHANISTIC-
EMPIRICAL
PROCEDURE

40. EMPIRICAL PROCEDURES
• These procedures are derived from experience (observed
field performance) of in-service pavements and or “Test
Sections”
• 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)
between
Pavement
performance
, traffic
loads &
pavement
thickness
for
A given set of
paving materials
and soils,
geographic
location and
climatic
conditions
• These procedures define the interaction

41. 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.

42. 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
EMPIRICAL PROCEDURES

43. 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.

44. 178
Utica
UticaRoad
23
2371
71
US
6
North
US
6
Ottawa
Loop 4Loop 5
Loop 6Loop 3
Frontage Road
Frontage Road
Maintenance Building
AASHO Adm’n
12
Proposed FA 1 Route 80
Army Barracks
Pre-stressed /
Reinforced Concrete
Typical Loop
XX
X X
XX
X X
Test Tangent
Test Tangent
Rigid
Flexible
Steel I-Beam

45. AASHO ROAD TEST CONDITIONS
ENVIRONMENT
•Climate -4 to 24o
C
•Average Annual Precipitation 34 Inches (864 mm)
•Average Frost Penetration Depth 28 Inches
Soil
•Classification A-6/A-7-6 (Silty-Clayey)
•Drainage Poorly Drained
•Strength 2-4 % CBR (Poor)
Pavement Layer Materials
•Asphalt Concrete AC a1 = 0.44
•Base Course Crushed Stone a2 = 0.14
•Subbase Course Sandy Gravel a3 = 0.11

46. AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST
LOOP LANE
LOAD LOAD
1
FRONT LOAD
2
2
WEIGHT IN TONS
0.9 0.9
FRONT AXLE LOAD AXLE GROSS WEIGHT
1.8
0.9 2.7 3.6
FRONT LOAD
1
FRONT LOAD
LOAD
LOAD
4
FRONT LOAD
1
FRONT LOAD
LOAD
LOAD
3
FRONT LOAD
1
FRONT LOAD
LOAD
LOAD
6
FRONT LOAD
1
FRONT LOAD
LOAD
LOAD
5
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

47. 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
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.
ESALs
Terminal
Initial
RIDE
Asphalt Concrete = ?
Base = ?
Subbase = ?
Soil

48. LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE
AASHTO being an EMPIRICAL
procedure is applicable to the
AASHO Road TEST conditions
under which it was developed.

49. 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)

50. 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

51. Kerkhoven and Dormon (1953) first suggested the use
of vertical compressive strain on the surface of sub-
grade 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

52. 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 Methods

53. 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:
Mechanistic - Empirical Design Approach

54. 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

55. 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

56. 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.
Benefits of Mechanistic - Empirical
Design Approach

57. 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.
Draw Back of Mechanistic - Empirical
Design Procedures

58. 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.

59. SOURCES OF PREMATURE PAVEMENT FAILURE
ThicknessDesign
Construction Practices
&
Quality Control
MaterialDesign
Inadequately Designed Pavements Will Fail Prematurely Inspite
Of Best Quality Control & Construction Practices
ThicknessDesign
MaterialDesign
Construction Practices
&
Quality Control
MaterialDesign
ThicknessDesign
Construction Practices
&
Quality Control

60. 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

61. COMPARISON OF TRUCK DAMAGE
PAKISTAN Vs USA
1
2 8
7
6
5
4
3
14
13
12
11
10
9
18
17
16
15
22
21
20
19

62. Plastic Flow Rutting

63. Rutting in Asphalt Layer

64. Rutting in Sub-grade or Base