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. 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.

5. TYPES OF PAVEMENT
Flexible Pavements
Rigid Pavements

6. 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.

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

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

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

10. 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.

11. Rigid Pavement
Concrete Slab
Sub-grade

12. 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 sub-grade to an acceptable level, and the upper layers must in a similar manner protect the layers below.

13. 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.

14. 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

15. 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.

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

17. 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.

18. 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.

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

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

21. 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 = logr (Fatigue, bottom of bituminous layer)
log N = 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.

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

23. Structural Performance Functional Performance
Strength
Functional Performance
Safety
Comfort

24. RUDIMENTARY DEFINITION
PAVEMENT THICKNESS DESIGN
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
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. Asphalt Concrete Thickness ?
Truck ?
Base Course Thickness ?
Sub-base Course Thickness ?
Pavement Design Life = Selected
Structural/Functional Performance = Desired
Design Traffic = Predicted

27. WHAT DO WE MEAN BY ?
SELECTED DESIGN LIFE

28. DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?

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

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

31. PREDICTED DESIGN TRAFFIC
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 Failure = 100,000 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. RELATIVE DAMAGE CONCEPT
Equivalent
Standard ESAL
Axle Load
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

35. Consider two single axles A and B where:
EXAMPLE
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

36. Consider two single axles A and B where:
EXAMPLE
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 =
This means that B-Axle causes only 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

37. AXLE LOAD & RELATIVE DAMAGE

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-

40. Pavement performance, traffic loads & pavement thickness
EMPIRICAL PROCEDURES
These procedures are derived from experience (observed field performance) of in-service pavements and or “Test Sections”
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
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)

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. 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

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 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. Layout of the AASHO Road Test.
178
Utica
Utica Road 23 71 US 6
North
Ottawa
Loop 4
Loop 5
Loop 6
Loop 3
Frontage Road
Maintenance Building
AASHO Adm’n 1 2
Proposed FA 1 Route 80
Army Barracks
Pre-stressed /
Reinforced Concrete
Typical Loop X
Test Tangent
Rigid
Flexible
Steel I-Beam

45. AASHO ROAD TEST CONDITIONS
ENVIRONMENT
Climate to 24oC
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 % 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 1
FRONT LOAD 2
WEIGHT IN TONS
0.9
FRONT AXLE
LOAD AXLE
GROSS WEIGHT
1.8
2.7
3.6 4 3 6 5
5.5
12.7
10.9
24.6
8.2
19.1
4.1
14.6
33.2
10.2
23.2
18.2
40.5
13.6
31.4
21.8
49.1

47. AASHO ROAD TEST RIDE QUALITY Initial Terminal Subbase = ? ESALs
“Test Sections” were subjected to 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 QUALITY
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. Mechanistic- Empirical Methods
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)

52. 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.

53. 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:

54. 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.

55. 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.

56. 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.

57. 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.

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
Thickness Design
Construction Practices
&
Quality Control
Material Design
Thickness Design
Material Design
Construction Practices
&
Quality Control
Material Design
Thickness Design
Construction Practices
&
Quality Control
Inadequately Designed Pavements Will Fail Prematurely Inspite
Of Best Quality Control & Construction Practices

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
PROBLEM STATEMENT

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

62. Plastic Flow Rutting

63. Rutting in Asphalt Layer

64. Rutting in Sub-grade or Base

65. THANKS