1. By: Asst. Prof. Imran Hafeez
Empirical & Mechanistic
Flexible Pavement Design
By: Asst. Prof. Imran Hafeez

2. References: Pavement Analysis and Design by Yang H. Huang
AASHTO Guide for Design of Pavement structures
Principles of Pavement Design by E.J.Yoder

3. Contents Design of Flexible Pavements Mechanistic Design Approach
Empirical Design Approach
Mechanistic-Empirical Design Approach

4. METHODS OF FLEXIBLE PAVEMENT DESIGN
Design methods can be classified into five categories.
Empirical method
Mechanistic method
Limiting shear
failure method
Limit deflection
method
Regression method

5. MECHANISTIC DESIGN

6. Mechanistic Approach
Mechanics is the science of motion and the action of forces on bodies.  Thus, a mechanistic approach seeks to explain phenomena only by reference to physical causes.
In pavement design, the phenomena are the stresses, strains and deflections within a pavement structure, and the physical causes are the loads and material properties of the pavement structure.

7. Mechanistic Design
A method that involve numerical capability to calculate the stress, strain, or deflection in a multi-layered system, such as a pavement, when subjected to external loads, or the effects of temperature or moisture.
Engr. Imran Hafeez

8. (Function of Traffic & Environment)
Mechanistic Design
A method that refer to the ability to translate the analytical calculations of pavement response to performance.
(Function of Traffic & Environment)

9. Benefits Improved reliability for design
Ability to predict specific types of distress
Ability to extrapolate from limited field and laboratory results.
Damaging effects of increased loads, high tire pressure, multiple axles can be modeled.
Better utilization of available materials
Improved method for premature distress analysis

10. Benefits Aging factor can be accommodated in analysis
Seasonal effects like freezing-thaw weakening
Long-term evaluation
Drainage factors

11. Assumption
Mechanistic design procedure are based on the assumption that a pavement can be modeled as multi-layered elastic or visco-elastic structure on an elastic or visco-elastic foundation.
Natural Soil (Subgrade)
Aggregate Subbase Course
Aggregate Base Course
Asphalt Concrete

12. Low Temp. ~Short Loading Time
Asphalt is a visco-elastic material. The strain developed by imposing a particular stress will depend on temperature and the loading time. At low temperature or short loading times, the material approaches elastic behavior. Under these conditions, the stiffness of a mix depends only on that of the binder and VMA of the mix, which is called elastic stiffness.

13. High Temp. ~Long Loading Time
At higher temperature or longer loading time, the stiffness of the mix is influenced by additional parameters associated with the mineral aggregates, which is also known as viscous stiffness and depends on the type of the grading, shape, and the texture of aggregate, the confining conditions and the method of compaction in addition to the stiffness and VMA.

15. Stress~Strain

16. Stress~Strain Linearity
(Non-Linear)
ε(Strain)

17. Typical Creep Stress and strain relationship

18. Resilient Modulus

19. Layered System Concepts
Analytical solutions to the state of stress or strain has several assumptions
The material properties of each layer are homogenous,
Each layer has finite thickness except for the lower layer
All layers are infinite in lateral directions
Each layer is isotropic
Full friction is developed between layers at each interface
Surface shearing forces are not present at the surface
The stress solution are characterized by two material properties for each layer (E &µ)

20. Fundamentals of design procedure
The use of multilayered elastic theory in conjunction with a limiting strain criteria (Dorman and Metcalf in 1965) for design involve the consideration of three factors:
The theory
Material characterization values
The development of failure criterion for each mode of distress

21. Stress Components under Pavements
Foster and Ahlvin (1954) presented charts for determining vertical stress radial stress tangential stress shear stress T, and vertical deflection w. The load is applied over a circular area with a radius a

23. Mechanistic based Software
BISAR
CHEVRON-X
MICHPAVE

24. Mechanistic based Software
BISAR
(Bitumen Stress Analysis in Roads)
BISAR 3.0 is capable of calculating
Comprehensive stress and strain profiles
Deflections
Horizontal forces
Slip between the pavement layers via a shear spring compliance at the interface
The center of the loads and the positions at which stresses, strains and displacement have to be calculated are given as co-ordinates in a fixed Cartesian system.

25. Mechanistic based Software
MICHPAVE
MICHPAVE is a user-friendly, non-linear finite element program for the analysis of flexible pavements. The program computes displacements, stresses and strains within the pavement due to a single circular wheel load.
Useful design information such as fatigue life and rut depth are also estimated through empirical equations.
Most of MICHPAVE is written in FORTRAN 77. Graphics and screen manipulations are performed using the ORTRAN callable GRAFMATIC graphics library, marketed by Microcompatibles

26. Allowable Vertical strain at Top of sub grade Basic Equation: Strain (allowable)-A* (N/10*6) *B Where A and B are coefficients, and N is the number of load repetitions Subgrade Strain Criteria Table
Model A B
Allowable Strain
Shell 1978, 50% probability
0.250
318
Shell 1978,84 % probability
250
Shell 1978,95% probability
0.200
251
Chevron, mean rut 10mm
0.223
193
University of Nottingham, mean rut 13mm
0.280
143
South Africa, Terminal PSI=1.5
0.100
667
South Africa, Terminal PSI= 2.0
483
South Africa, Terminal PSI=2.5
0.088
345
NAASRA, Austraila
0.141
680
Verstraeten, rut less than 15 mm
0.230
179
Kenya
0.245
Giannini & Camomilla Italia
0.202
295

27. EMPIRICAL DESIGN

28. Empirical Approach
“An empirical approach is one which is based on the results of experiments or experience.”
Generally, it requires a number of observations to be made in order to ascertain the relationships between input variables and outcomes.
It is not necessary to firmly establish the scientific basis for the relationships between variables and outcomes as long as the limitations with such approach are reorganized.

29. Benefits
It uses material properties that relates better to actual pavement performance
It provides more reliable performance predictions
It better defines the role of construction
It accommodates environmental and aging effects on materials

30. Empirical Approach
Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance).  There are many different types of empirical equations available today e.g.
1993 AASHTO Guide basic design equation for flexible pavements. 
Group Index method
CBR Method

31. AASHTO Guide basic design equation for flexible pavements.
Empirical Approach
AASHTO Guide basic design equation for flexible pavements. 
Log10(W18)=Zr x So x log10(SN + 1)-0.20+(log10((ΔPSI)/( )) /(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07
where:
W18 =standard 18-kip (80.1-kN)-equivalent single-axle load (ESAL)
ZR = Reliability/probability of service
So = Standard Deviation of ESAL’S
ΔPSI = Loss of Serviceability

32. Empirical Approach ai = ith layer coefficient
SN=Structural Number (an index that is indicative of the total pavement thickness required)
SN =a1D1 + a2D2m2 + a3D3m3+...
ai = ith layer coefficient
di = ith layer thickness (inches)
Mi = ith layer drainage coefficient
Δ PSI= difference between the initial design serviceability index, po, and the design terminal serviceability index, pt
MR= sub-grade resilient modulus (in psi)

34. ROAD TESTS Maryland Road Test HRB 1940~ 60.
The objective of this project was to determine the relative effects of four different axle loadings on a particular concrete pavement (HRB, 1952). The tests were conducted on a 1-1-mile (1.76 km) section of concrete pavement constructed in 1941 on US 301 approximately 9 mile (1.44 km) south of La Plata, Maryland

35. WASHO Road Test
After the successful completion of Maryland Road Test sponsored by the eleven Midwestern and eastern states, the Western Association of States Highway Officials (WASHO) conducted a similar test but on sections of flexible pavements in Malad. Idaho, with the same objective in mind (HRB, 1955).

36. AASHO Road Test
The objective of this project was to determine the significant relationship between the number of repetitions of specified axle loads of different magnitudes and arrangements and the performance of different thicknesses of flexible and rigid pavements (HRB. 1962). The test facility was constructed along the alignment of Interstate 80 near Ottawa. Illinois, about 80 miles (128 km) south west of Chicago.
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

37. AASHO Road Test

45. EMPIRICAL-MECHANISTIC
DESIGN

46. Mechanistic-Empirical Approach
Along with this mechanistic approach, empirical elements are used when defining what value of the calculated stresses, strains and deflections result in pavement failure. 

50. M-E Methods Advantages
The basic advantages of a mechanistic-empirical pavement design method over a purely empirical one are:
It can be used for both existing pavement rehabilitation and new pavement construction It accommodates changing load types
It can better characterize materials allowing for:
Better utilization of available materials
Accommodation of new materials
An improved definition of existing layer properties

51. National Cooperative Highway Research Projects

52. National Cooperative Highway Research Projects