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 ApproachEmpirical Design ApproachMechanistic-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 CourseAsphalt 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)

δ(S

tres

s)

Typical Creep Stress and strain relationship

Resilient Modulus

Layered System ConceptsAnalytical 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

BISARCHEVRON-XMICHPAVE

Mechanistic based Software

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.

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 gradeBasic 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((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

• 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 coefficientΔ 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 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.

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 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.

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23

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Loop 4Loop 5

Loop 6Loop 3

Frontage Road

Frontage Road

Maintenance Building

AASHO Adm’n

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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