The document discusses different approaches to flexible pavement design, including empirical, mechanistic, and mechanistic-empirical approaches. It provides details on each approach, such as the empirical approach using the 1993 AASHTO Guide equation relating pavement characteristics to performance, and the mechanistic approach modeling the pavement as layers and calculating stress/strain. The mechanistic-empirical approach combines both, using mechanics to calculate stresses/strains and empirical data to define failure criteria. Road tests like the AASHO and Maryland tests helped develop the empirical relationships used in design methods.

1. 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
Empirical method
Mechanistic method
Limiting shear
failure method
Limit deflection
method
Regression method
Design methods can be classified into five categories.

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. Engr. Imran Hafeez
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.

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

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. 1) Aging factor can be accommodated in
analysis
2) Seasonal effects like freezing-thaw
weakening
3) Long-term evaluation
4) Drainage factors
Benefits

11. 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.
Assumption
Natural Soil (Subgrade)
Aggregate Subbase Course
Aggregate Base CourseAsphalt 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
(Linear)
(Non-Linear)
ε(Strain)
δ(Stress)

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
1) The material properties of each layer are homogenous,
2) Each layer has finite thickness except for the lower layer
3) All layers are infinite in lateral directions
4) Each layer is isotropic
5) Full friction is developed between layers at each interface
6) Surface shearing forces are not present at the surface
7) The stress solution are characterized by two material
properties for each layer (E &µ)

20. 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:
(a) The theory
(b)Material characterization values
(c) The development of failure criterion for
each mode of distress
Fundamentals of design procedure

21. 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
Stress Components under Pavements

23. BISAR
CHEVRON-X
MICHPAVE
Mechanistic based Software

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 centerof 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. 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
Mechanistic based Software

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 0.000885 0.250 318
Shell 1978,84 % probability 0.000696 0.250 250
Shell 1978,95% probability 0.000569 0.200 251
Chevron, mean rut 10mm 0.000482 0.223 193
University of Nottingham,
mean rut 13mm
0.000451 0.280 143
South Africa, Terminal
PSI=1.5
0.001005 0.100 667
South Africa, Terminal PSI=
2.0
0.000728 0.100 483
South Africa, Terminal
PSI=2.5
0.000495 0.088 345
NAASRA, Austraila 0.001212 0.141 680
Verstraeten, rut less than 15
mm
0.000459 0.230 179
Kenya 0.001318 0.245 483
Giannini & Camomilla Italia 0.000675 0.202 295

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. – 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
Benefits

30. 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
Empirical Approach

31. AASHTO Guide basic design equation for flexible
pavements.
Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)-0.20+(log10((ΔPSI)/
(4.2-1.5)) /(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
Empirical Approach

32. • 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)
Empirical Approach

34. ROAD TESTS
Maryland Road TestMaryland Road Test
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
HRB 1940~ 60.

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 TestAASHO 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
UticaRoad
23
2371
71
US
6
North
US
6Ottaw
a
Loop 4Loop 5
Loop 6Loop 3
Frontage Road
Frontage Road
Maintenance Building
AASHO Adm’n
12
Proposed FA 1
Route 80
Army Barracks

37. AASHO Road Test

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

50. 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
M-E Methods Advantages

51. National Cooperative Highway Research Projects

52. National Cooperative Highway Research Projects