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CHAPTER ONE
INTRODUCTION
Introduction and Definitions
Other forms of moduli and their definitions are presented
in Figure 2. The initial slope of the “stress–strain” curve
Historically, flexible pavement design practices were typi- is termed as initial tangent modulus (Emax), and the secant
cally based on empirical procedures, which recommend modulus (E1) is termed as the slope of the line that joins the
certain base, subbase, and surface layer types and their origin and a point on the stress–strain curve that represents
thicknesses based on the strength of the subgrade. Recom-
mendation of layer types and their dimensions were estab-
lished based on AASHO road tests performed during the
1950s. The often-used soil strength parameters in this pave-
ment design practice are California Bearing Ratio (or CBR)
value, Hveem R value, and Soil Support Value (SSV). All
these soil parameters are based on the failures of subgrade
soil specimens in the laboratory conditions. However, flex-
ible pavements seldom fail owing to subgrade strength fail-
ures during their service life (Huang 1993).
Most of the flexible pavements fail owing to either exces-
sive rutting or cracking of pavement layers as a result of
fatigue, temperature changes, and/or softening caused by
the surface layer cracking (Barksdale 1972; Brown 1974,
1996). Because flexible pavements do not fail as a result of
soil strength failure, the 1986 AASHTO interim pavement
design guide and subsequently the 1993 AASHTO pave-
ment design guide recommended the use of a soil parameter
known as the Resilient Modulus (MR) to replace strength-
based parameters such as CBR and SSV (Brickman 1989;
Mohammad et al. 1994; Maher et al. 2000). Several other
FIGURE 1 Definition of resilient modulus.
investigations also refer to this modulus parameter as MR in
their studies.
The resilient modulus is analogous to the elastic modulus
used in elastic theories and is defined as a ratio of deviatoric
stress to resilient or elastic strain experienced by the material
under repeated loading conditions that simulate traffic load-
ing. Figure 1 presents a schematic of the resilient modulus
parameter (MR). Most bases and subgrades are not elastic
and they experience permanent deformation under repeated
loads (Uzan 2004). However, because loads applied in the
laboratory test for resilient modulus are small when com-
pared with ultimate loads at failure and also the result of
the application of a large number of cycles of loading that
reduces the plastic deformation, the deformation measured
during test cycles is considered as completely recoverable
or elastic and hence the recovered deformations are used to
estimate the resilient modulus or elastic modulus (or simply FIGURE 2 Definitions of other elastic moduli parameters
modulus or stiffness). (Nazarian et al. 1996).
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50% of the ultimate deviatoric stress at which sample fails. Subsequent pavement design guides, including the 1993
The resilient modulus (MR) parameter is close to Emax for AASHTO Pavement Design Guide and the current Mech-
stiff materials, and for soft soils the modulus lies in between anistic-Empirical Pavement Design Guide (MEPDG),
E1 and Emax. Also, the laboratory methods using repeated describe design methodologies that use resilient modulus
load triaxial (RLT) tests measure resilient moduli of tested as the primary input parameter for characterizing subgrade,
materials whereas the field nondestructive methods using subbase, and base materials. Resilient modulus values of
backcalculation subroutines and other approaches provide these materials are typically determined by performing lab-
the elastic moduli of subgrades and bases. oratory tests using RLT tests simulating traffic loading con-
ditions. Different resilient modulus test protocols, including
The main reason for using the resilient modulus or modu- AASHTO test methods T-292, T-294, and T-307 as well as
lus or stiffness as the parameter for subgrades and bases is TP-46, have been used by the departments of transporta-
that it represents a basic material property and can be used in tion (DOTs) in the laboratory conditions. All these test
the mechanistic analyses for predicting different distresses procedures have certain differences with respect to test
such as rutting and roughness. This parameter has been used sequences of applying confining and deviatoric stresses,
to directly determine the structural capacity of the subgrade measurement of deformations either inside the triaxial cell
or determine the structural coefficient of the untreated base or outside the triaxial cell, application of seating stresses,
and subbase layer. Based on the structural number value, specimen preparation steps, and conditioning methodol-
pavement layers and their dimensions are designed in the ogy applied before the actual testing. These differences do
AASHTO pavement design guide. contribute to several uncertainties, which are explained in
chapter three. Table 1 presents a chronology of how these
The pavement design approach is termed as mechanistic test methods were developed in the estimation of resilient
because the design is based on the mechanics of materials moduli of subgrades.
that relate traffic characteristics and information to pavement
response output parameters, such as stress or strain of mate- Also, a CD-ROM guide developed by the FHWA as a part
rials. Pavement response parameters are used to estimate or of Long-Term Pavement Products (LTPP) presents several
predict distress using laboratory- and field-measured moduli interactive tutorials on resilient modulus of unbound mate-
values. For example, the estimation of vertical compressive rials. Three videos that depict the MR test procedures were
strains of subgrade and the moduli properties can be used to developed to aid practitioners, laboratory managers, and
understand plastic deformation of subsoil, which contributes technicians (FHWA 2006).
to the overall rutting of the pavement system. Such design
approach is considered as mechanistic empirical because Irwin (1995) presented various limitations in both cur-
statistical empirical relationships are used to correlate pave- rent test methods with respect to applied confining and devi-
ment response parameters and distress magnitudes. atoric pressures and current nonlinear regression models
Table 1
Chronology of AASHTO Test Procedures for MR Measurements
Test Procedure Details
Earliest AASHTO test procedure; No details on the sensitivities of displacement measurement devices
AASHTO T-274-1982 were given; Criticisms on test procedure, test duration (5 hours long test) and probable failures of soil
sample during conditioning phase; testing stresses are too severe.
AASHTO procedure introduced in 1991; Internal measurement systems are recommended; Testing
AASHTO T-292-1991
sequence is criticized owing to the possibility of stiffening effects of cohesive soils.
AASHTO modified the T-292 procedure with different sets of confining and deviatoric stresses and
their sequence; Internal measurement system is followed; 2-parameter regression models (bulk stress
AASHTO T-294-1992
for granular and deviatoric stress model for cohesive soils) to analyze test results; Criticism on the
analyses models.
Procedural steps of P-46 are similar to T-294 procedure of 1992; External measurement system was
Strategic Highway Research
allowed for displacement measurement; Soil specimen preparation methods are different from those
Program P-46-1996
used in T-292.
T-307-1999 was evolved from P-46 procedure; recommends the use of external displacement
AASHTO T-307-1999 measurement system. Different procedures are followed for both cohesive and granular soil specimen
preparation.
This recent method recommends a different set of stresses for testing. Also, a new 3-parameter model
NCHRP 1-28 A: Harmonized
is recommended for analyzing the resilient properties. The use of internal measurement system is
Method-2004 (RRD 285)
recommended in this method.
