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By: Asst. Prof. Imran Hafeez
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References:
Pavement Analysis and DesignbyYang H.
Huang
AASHTO Guide for Design of Pavement
structures
Principles of Pavement Designby
E.J.Yoder
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Contents
Design of Flexible Pavements
Mechanistic Design ApproachEmpirical Design Approach
Mechanistic-Empirical Design
Approach
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METHODS OF FLEXIBLE
PAVEMENT DESIGN
Empirical method
Mechanistic method
Limiting shearfailure method
Limit deflectionmethod
Regression method
Design methods can be classified into five categories.
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Mechanistic Approach
Mechanics is the science of motion and the actionof forces on
bodies. Thus, a mechanisticapproach seeks to explain phenomena only
byreference to physical causes.
In pavement design, the phenomena are the
stresses, strains and deflections within apavement structure,
and the physical causes arethe loads and material properties of the
pavementstructure.
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Mechanistic Design
A method that involve numerical capability
to calculate the stress, strain, or deflectionin a multi-layered
system, such as a
pavement, when subjected to external
loads, or the effects of temperature ormoisture.
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A method that refer to theability to translate the
analytical calculations ofpavement response to
performance.(Function of Traffic & Environment)
Mechanistic Design
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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
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1) Aging factor can be accommodated in
analysis
2) Seasonal effects like freezing-thaw
weakening
3) Long-term evaluation
4) Drainage factors
Benefits
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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
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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 shortloading times, the
material approaches
elastic behavior. Under these conditions,
the stiffness of a mix depends only on thatof the binder and VMA
of the mix, which
is called elastic stiffness.
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High Temp. ~Long Loading Time
At higher temperature or longer loadingtime, the stiffness of
the mix is influenced
by additional parameters associated withthe mineral aggregates,
which is alsoknown as viscous stiffness and depends onthe type of
the grading, shape, and the
texture of aggregate, the confiningconditions and the method of
compactionin addition to the stiffness and VMA.
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Stress~Strain
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Stress~Strain Linearity
(Linear)(Non-Linear)
(Strain)
(Stress)
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Typical Creep Stress and strain relationship
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Resilient Modulus
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Layered System Concepts
Analytical solutions to the state of stress or strain hasseveral
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 surface7) The
stress solution are characterized by two material
properties for each layer(E &)
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The use of multilayered elastic theory inconjunction with a
limiting strain criteria
(Dorman and Metcalf in 1965) for design involve theconsideration
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
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Foster and Ahlvin (1954)
presented charts for
determining vertical
stress radial stresstangential stress
shear stress T, and
vertical deflection w.
The load is appliedover a circular area
with a radius a
Stress Components under Pavements
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BISAR
CHEVRON-X
MICHPAVE
Mechanistic based Software
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Mechanistic based Software
BISAR
(Bitumen Stress Analysis in Roads)BISAR 3.0 is capable
ofcalculatingComprehensive stress and strain
profilesDeflectionsHorizontal forcesSlip between the pavement
layers via ashear spring compliance at the interface
The c enter of the loads and the pos itions at w hic h s tres s
es , s trains
and displac ement have to be ca lc ulated are given as c
o-ordinates ina fixed C artes ian s ys tem.
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MICHPAVE
MICHPAVE is a user-friendly, non-linear finite elementprogram
for the analysis of flexible pavements. The programcomputes
displacements, stresses and strains within the
pavement due to a single circular wheel load.
Useful design information such as fatigue life and rut depthare
also estimated through empirical equations.
Most of MICHPAVE is written in FORTRAN 77. Graphicsand screen
manipulations are performed using the ORTRANcallable GRAFMATIC
graphics library, marketed byMicrocompatibles
Mechanistic based Software
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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 TableModel A B Allowable StrainShell
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
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Empirical Approach
An empirical approach is one which isbased on the results of
experiments or
experience.
Generally, it requires a number of observations tobe made in
order to ascertain the relationshipsbetween input variables and
outcomes.
It is not necessary to firmly establish the scientific basis for
therelationships between variables and outcomes as long as
thelimitations with such approach are reorganized.
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It uses material properties that relates
better to actual pavement performance
It provides more reliable performancepredictions
It better defines the role of construction
It accommodates environmental and aging
effects on materials
Benefits
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Empirical equations are used to relate
observed or measurable phenomena
(pavement characteristics) with outcomes
(pavement performance). There are manydifferent types of
empirical equations
available today e.g.
1993 AASHTO Guide basic designequation for flexible
pavements.
Group Index method
CBR Method
Empirical Approach
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AASHTO Guide basic design equation for flexiblepavements.
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 ESALS
PSI = Loss of Serviceability
Empirical Approach
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SN=Structural Number (an index that is indicativeof 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
initialdesign serviceability index,
po, and the design terminal
Empirical Approach
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ROAD TESTS
Maryland Road TestMaryland Road Test
The objective of this project was to determine therelative
effects of four different axle loadings on aparticular concrete
pavement (HRB, 1952). The testswere conducted on a 1-1-mile (1.76
km) section ofconcrete pavement constructed in 1941 on US
301approximately 9 mile (1.44 km) south of La Plata,
Maryland
HRB 1940~ 60.
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WASHO Road Test
After the successful completion of MarylandRoad Test sponsored
by the eleven Midwestern
and eastern states, the Western Association ofStates Highway
Officials (WASHO) conducted asimilar test but on sections of
flexiblepavements in Malad. Idaho, with the same
objective in mind (HRB, 1955).
AASHO Road TestAASHO Road Test
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AASHO Road TestAASHO Road Test
The objective of this project was to determine the
significant relationship between the number of repetitionsof
specified axle loads of different magnitudes andarrangements and
the performance of differentthicknesses of flexible and rigid
pavements (HRB. 1962).The test facility was constructed along the
alignment of
Interstate 80 near Ottawa. Illinois, about 80 miles (128km)
south west of Chicago.
178
Utica
Utic
aR
oa
d23
2371
71
US
6
North
US
6Ottaw
a
Loop 4Loop 5
Loop 6Loop 3
Frontage Road
Frontage Road
Maintenance Building
AASHO Admn
12
Proposed FA 1
Route 80
ArmyBarracks
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AASHO Road Test
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Along with this mechanisticapproach, empirical elements are
used when defining what valueof the calculated stresses,
strainsand deflections result in
pavement failure.
Mechanistic-Empirical Approach
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h d d
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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 Itaccommodates
changing load types
It can better characterize materials allowing for:
Better utilization of available materialsAccommodation of new
materials
An improved definition of existing layer properties
M-E Methods Advantages
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National Cooperative Highway Research Projects
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National Cooperative Highway Research Projects
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b t i il
