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By: Asst. Prof. Imran Hafeez
References:
� Pavement Analysis and Design by Yang H.
Huang
� AASHTO Guide for Design of Pavement
structures
� Principles of Pavement Design by
E.J.Yoder
Contents
�Design of Flexible Pavements
�Mechanistic Design Approach
�Empirical Design Approach
�Mechanistic-Empirical Design
Approach
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.
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.
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.
A method that refer to the ability to translate the
analytical calculations of pavement response to
performance. (Function of Traffic & Environment)
Mechanistic Design
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
1) Aging factor can be accommodated in
analysis
2) Seasonal effects like freezing-thaw
weakening
3) Long-term evaluation
4) Drainage factors
Benefits
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 Course Asphalt Concrete
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.
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.
Stress~Strain
Stress~Strain Linearity
(Linear)
(Non-Linear)
ε(Strain)
Typical Creep Stress and strain relationship
Resilient Modulus
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)
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
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
�BISAR
�CHEVRON-X
�MICHPAVE
Mechanistic based Software
Mechanistic based Software
BISAR
(Bitumen Stress Analysis in Roads)Bitumen Stress Analysis in Roads)Bitumen Stress Analysis in Roads)Bitumen Stress Analysis in Roads)
•BISARBISARBISARBISAR 3333....0000 isisisis capablecapablecapablecapable ofofofof
calculatingcalculatingcalculatingcalculating •ComprehensiveComprehensiveComprehensiveComprehensive stressstressstressstress andandandand strainstrainstrainstrain profilesprofilesprofilesprofiles
•DeflectionsDeflectionsDeflectionsDeflections
•HorizontalHorizontalHorizontalHorizontal forcesforcesforcesforces
•SlipSlipSlipSlip betweenbetweenbetweenbetween thethethethe pavementpavementpavementpavement layerslayerslayerslayers viaviaviavia aaaa
shearshearshearshear springspringspringspring compliancecompliancecompliancecompliance atatatat thethethethe interfaceinterfaceinterfaceinterface
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.
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
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
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.
– 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
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
AASHTO Guide basic design equation for flexible pavements.
Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)-
0.20+(log10((∆PSI)/(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
∆PSI = Loss of Serviceability
Empirical Approach
• 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
∆ 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
ROAD TESTS
Maryland 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.
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).
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
23
23 71
71
US
6
North
US
6 Ottaw
a
Loop 4 Loop 5
Loop 6 Loop 3
Frontage Road
Frontage Road
Maintenance Building
AASHO Adm’n
1 2
Proposed FA 1 Route 80
Army Barracks
AASHO Road Test
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
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
National Cooperative Highway Research Projects
National Cooperative Highway Research Projects