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CHAPTER 1
INTRODUCTION AND RESEARCH APPROACH
BACKGROUND
Pavement design and evaluation for purposes of construction and rehabilitation require the careful
evaluation of a number of factors including material properties, traffic type and volume, the environment,
construction and maintenance variables, and engineering economics. Undoubtedly, material properties are
one of the most significant factors in the structural design and performance of pavements. Design
me~ods for flexible pavements in He past have mainly involved empirical correlation of field performance
wig material properties determined either in He laboratory or He field. Extrapolation of these empirical
me~ods beyond the specific conditions for which they were developed can lead to erroneous results. Also,
empirical design me~ods cannot take into account all possible failure mechanisms. These deficiencies can
result In either pavements that are designed with a large factor of safety or pavements that fad! prematurely
thus resulting in a heavier burden to He taxpayers.
Traffic conditions are changing rapidly with trends toward faster and heavier vehicles, higher
traffic volumes and new types of loadings. These changing trends will encourage, in the future, the
replacement of existing empirical design methods by either updated empirical methods or mechanistic
based approaches. The continued use of empirical methods would require costly statistical research at
frequent intervals as new vehicle configurations emerge and new materials are developed. The
development of mechanistic based methods requires a fundamental scientific study to establish the
correlation between pavement performance and response including He effects of material properties. The
SHRP Longterm Pavement Performance (LTPP) monitoring program will provide a valuable data base for
use in developing new pavement design procedures.
The 1986 AASHTO Guide for the Design of Pavement Structures has incorporated the resilient
modulus of component materials into the design process. Also, considerable attention in the pavement
industry has recently been focused on the development of mechanistic based approaches for design and
evaluation of pavements. Both the 1986 AASHTO Guide method and all mechanistic based design
me~ods use He resilient modulus of each layer in the design process. Mechanistic based design methods
use layered theories to predict pavement response. Good relationships have been found to exist between
flexible pavement performance and the stresses, strains, and displacements calculated by lavered pavement
theories using appropriate resilient moduli for the layers.
_~ -~---~ r
More research effort has to be directed toward the development of practical, yet reliable,
laboratory and field test procedures for the characterization of pavement materials. The additional cost
of developing suitable design mesons and laboratory test procedures and their implementation is justified
by the ability to more efficiently use paving materials and develop cost-effective, reliable pavement
designs. '
1
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BASIC CONCEPTS FOR REPEATED LOAD TESTING
Vehicle Loading
As traffic moves over a pavement surface, large numbers of rapidly applied stress pulses of
varying magnitude are applied to each element of material below and for some distance out to the sides
of the wheel path (Figure I). The presence of pavement surface irregularities causes the vehicle to bounce
which leads to impact loads as great as twice the static value. For moderate vehicle speeds, the stress
pulse lasts between about 0.02 to 0.4 sec., with the pulse time increasing with increasing depth below the
pavement surface and decreasing vehicle speed (Figure 2~. The type and geometry of the pavement has
only a secondary influence on the pulse shape and duration. Near the surface the stress pulse has a
pronounced haversine shape. With depth the pulse duration becomes greater and, although it remains
approximately haversine in shape, a triangular loading gives a reasonably good approximation.
Repeated Load and Cyclic Testing
Since pavement materials are subjected to a series of distinct load pulses, a laboratory test
duplicating this condition is desirable. The repeated load type test has for many years been used to
simulate vehicle loading. In the repeated load test, instead of applying a single slow loading, a series of
load pulses are applied that are separated by a distinct rest period as shown in Figure 3a. The repeated
load test concept can be incorporated into many conventional static types of tests such as the diametral,
biaxial, beam bending and simple shear tests. The resilient modulus of the material tested is then
determined from the results of these dynamic tests.
A continuous, sinuso'4ally varying load (Figure 3b) is also sometimes used to characterize the
dynamic response of pavement materials. This wave form is referred to as cyclic loading.
Elastic Constants. For isotropic, linear-elastic behavior, a material is completely characterized
by 2 elastic constants determined from suitable material tests. Usually in pavement design, the modulus
of elasticity E and Poisson's ratio v are He 2 elastic constants evaluated in ache laboratory and used in
layered theory. Once any 2 elastic constants such as E and ~ have been evaluated, all other elastic
constants can be calculated using simple equations derived from the theory of elasticity.
The bulk modulus (K) and shear modulus (G) are also sometimes used in layered pavement
analyses. The constants K and G are more fundamental ones than E and ~ since they are related to volume
change and shear distortion, respectively. As a result, in the future they may gain more use in pavement
analysis and design.
Resilient Modulus. The resilient modulus (MR) is analogous to Be modulus of elasticity (E) with
bow terms having the same basic theory of elasticity definition. The resilient modulus is determined from
a repeated load test. Peak values of stress and recoverable (i.e., resilient) deformation occurring in the
test are use to calculate the resilient elastic constants even though peak stress and recoverable deformation
do not occur at the same time in a dynamic test of this type.
2
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12t
10
8
Creep Speed_
~ /~
CONVENTIONAL
/ /' 1 ~ \~x \ 1\
( 18 KS ) /~; '~/ - \ ~ \\ \
l ~ /A _ At
·'\~X if
40 '`
40 30 20 10 0 10 2 030
LEfT TRANSVERSE DISTANCE, INCHESRIGHT
Figure 1. Measured vertical stress pulse applied to the subgrade due to
a slowly moving vehicle - 23 in. thick pavement (after reference 1 )
1~0
.
1 1
ANGLE ~ MEL MEL Led -
_ _
_ VEllICLE VELOCITY. V _
,DE~H lENEAIN ,AVE~E" S0Rf4CE I1N`N~)
- VERTICAL ~N~ -
tU5£ ran
-- VERTICAL no
HEW
Figure 2. Variation of equivalent vertical stress pulse time
with vehicle velocity and depth (after reference 2)
3
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Although this approach neglects energy loss effects, use of the resilient response appears to give
satisfactory results for calculating pavement response.
Rotation of Principal Stress Axes
When a wheel load moves toward and then past an element of material, this element is subjected
to stress states similar to those shown in Figure 4. Each element of material is subjected to a
simultaneous buildup in both the major principal stress (o,) and the minor principal stress (03~. As those
stresses build up, a rotation of the principal stress axes also takes place Figure 4~. A complete reversal
of shear stress also occurs. Unfortunately, as discussed later, these stress rotation effects are not
duplicated in the commonly used repeated load diametral and biaxial tests.
Initial Stress State
Most pavement design approaches, including the AASHTO methods, presently use in the
thickness selection process a single value of the resilient modulus of each layer. Therefore, to select
design resilient moduli the representative stress state acting upon each layer must be either known or
assumed. The complete stress state consists of the combined effect of the initial residual stresses existing
abler construction and the dynamic stresses caused by traffic loading. Temperature and moisture induced
stresses in stabilized layers are also important, but have received almost no attention.
During construction, heavy compaction equipment is used to density in thin lifts the subgrade,
base, and surfacing. The heavy construction equipment then use each completed lift as a temporary
working surface. Usually the greatest stresses to which a particular layer is ever subjected are applied
during either compaction or else by construction equipment before the pavement is completed. The
application of large vertical stresses during this stage of construction cause lateral stresses to develop
which can become locked Into the layer [4, 5, 61. These locked in stresses are called residual stresses or
residual lateral stresses.
Residual Stress. Uzan [4] has pointed out that residual lateral stresses of 2 psi and 6 psi have
been observed for cohesionless and cohesive soils. Methods of analysis proposed by Uzan [4], Selig [7]
and Duncan and Seed [5] are quite encouraging for predicting residual lateral stresses due to compaction
of both granular and cohesive soils.
Selig [7] concluded that the residual lateral stress is the most important factor limiting permanent
deformation in the bottom of a granular base. Residual lateral stress is also an important factor
determining the appropriate confining pressure at which to evaluate He resilient modulus. The residual
lateral stress oh, is relatively large and can be expressed as follows [71:
~hr=Ko(7o (~)
where: o0 = vertical overburden stress
4
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LOAD c~a~
LOAD
(~) Repeated L~ding
~n
~:
_
J ·\ ~ '.W
. .' .,.
.~..
J~ · LOAD ~E
B · RECOVERY ll~E
C · CYCL£ ~
1., \ R£P£ATED
/ \ LO" p
~ · .\
~ "s, - -
~nNG~
r~
INPU7:
~ ~tsoSJN( o3T )
(b) Cyclic Loading - Sinusoltal Wa~ve Fora
Figure 3. Repeated load and cyclic loading wave forms (after reference 3 ~
VEH1CI-E aonoe
~nos ~ ~T10N
~llm tIlll1
/~/_ //~% I/~_ /~y
'% 1'
%`O1
q' I q.
`~ 4
V-~
~ /~// ~
STATIOIIARY ELElilEllT ~<
Of I"TERIAL @3 q. ~`
I ~
r llAJOR FRINCIPAL STRESS AXIS,
FOS~10# ~
\~ BLUOR PRINCIPAL STR£SS AXIS,
FOSITION S
Figure 4 . Rotation of principal stress ax~s of an element as a vehicle
moves over the surface
5
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Ko = coefficient of lateral earth pressure considering
residual stress
The coefficient of lateral earn pressure associated with residual stress is greater than unity but less than
the passive coefficient of earth pressure (Kp). For a granular material Me passive coefficient of lateral
earth pressure Kp = tan2 (45-~/2) where ~ is the angle of internal friction of the material.
Residual Stress Experimental Study. A fi~-scale field study was conducted as a part of this study
to evaluate residual compaction stresses In a I.5 in. maximum size crushed granite gneiss base 12 in. thick.
The base was placed in 6 in. lifts and compacted with a 10 ton smooth drum, vibratory roller. Lateral
compaction and residual stresses were measured using a free field pressure gage consisting of an aluminum
cube approximately 2.5 in. on each side. The cubic-shaped pressure gage was designed to be as rigid as
practical to simulate an aggregate. The pressure gage was partially hollow on the inside, and the hollow
space was filled with a mixture of water and anti-freeze. An active diaphragm was placed in one face of
the gage wig fluid between it and a low displacement, miniature pressure cell located on the other side.
Measurements made in the field on two bases, wig the active diaphragm of the gage oriented horizontally,
indicated that average residual lateral pressures of about 0.5 psi exist in the middle of a 6 in. aggregate
base or the bottom 6 in. of a 12 in. base. In a 12 in. base compacted in two 6 in. lifts, a 3 psi residual
stress was observed in the top 6 in. An additional confining pressure of 3 psi can cause an increase in
resilient modulus in a base on the order of 10 to 159S or more compared to neglecting this effect.
OBJECTIVES OF RESEARCH
The resilient modulus of pavement materials is usually evaluated using the repeated load biaxial
test. A large variation in results, however, has been observed to occur between different test methods and
testing laboratories. The ~rimarv objective of this study is to develop and recommend laboratory test
en ~ e' · ~
· ~ ~ , · e es e , ~ ~ e ~ . . e ~ e ~ en ~
procedures for determining resilient moduli of component materials In a flexible pavement structure.
Another objective is to access the applicability and constraints of using He resilient modulus to establish
structural coefficients for He flexible pavement procedure in the 1986 AASHTO Guide. Note Hat this
second objective was accomplished and the results were submitted separately as changes to the 1986
AASHTO Guide.
The specific tasks of the research as given in the REP are as follows:
I. Review the state-of-the-art procedures and equipment for laboratory resilient modulus
testing.
2. Develop detailed Laboratory test procedures for evaluating the resilient modulus of asphalt
concrete, aggregate base/subbase materials, and subgrade soils.
3. Perform limited multi-lab validation testing to refine the proposed test procedures.
4. Compare and analyze field determined moduli using common nondestructive devices with
moduli determined using validated test procedures.
6
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Review the 1986 AASHTO GUIDE FOR DESIGN OF PAVEMENT STRUCTURES
assessing the applicability and constraints of using resilient moduli values to establish
structural coefficients of base and subbase materials.
RESEARCH APPROACH
The overall organization used in carrying out the resilient modulus research is shown in Figure
5. To approach this problem In a systematic maimer, consideration was given to a large number of factors
potentially affecting the laboratory measured value of the resilient modulus of each layer in the pavement.
The more important factors considered included equipment type, equipment and instrumentation
calibration, specimen preparation, test conditions, test procedure details and presentation of test results.
Careful equipment and instrument calibration are extremely important in obtaining reliable values of
resilient moduli and their importance cannot be overemphasized. Improvements to the proposed test
procedures were made by conducting limited inter-laboratory studies and also from He valuable comments
from the NCHRP review panel.
The evaluation of resilient modulus in the laboratory is a complex problem. Furthermore, the
influence of the environmental conditions, which are hard to predict over He life of He pavement, have
a significant influence on pavement thickness requirements and must not be neglected in design. For
example, the resilient modulus of an aggregate base or subbase can increase by a factor of 5 or more as
the material goes from a wet to a dry state. Environmental effects of base and subgrade materials were
therefore investigated as a part of this study.
In following the intent of He REP, emphasis in this study was placed on resilient modulus
determination. Permanent deformation characteristics of pavement materials, however, are often more
important than for resilient moduli. Therefore, permanent deformation testing of base and subgrade
materials was also investigated since permanent deformation can be determined as an extension of the
resilient modulus test.
The accuracy required for the resilient modulus test is a very important practical question. To
study this important aspect, a Monte CarIo reliability simulation using 10,000 trials for each simulation
was performed to determine He effect of testing variability on required pavement thickness. The effect
of resilient modulus measurement variability on required pavement thickness was compared with both the
thickness required if no variability was present and also considering variability from all sources. The
results of the Monte CarIo reliability simulation were also compared wig those from the simplified
AASHTO type reliability analysis. The reliability study gives valuable insight into the level of
sophistication to which the resilient modulus test should be performed.
REPORT ORGANIZATION
Chapter 2 describes diametral tests performed on asphalt concrete specimens and presents and
discusses the results. Chapter 3 summarizes the findings of repeated load biaxial tests performed on
granular bases, lime stabilized granular materials and subgrade soils. Primary emphasis is placed on
resilient modulus test results although permanent deformation is also included.
7
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Laboratory Resilient Modulus Testing
Resilient Modulus Testing
.
AC | Base | Subgrade
· ~
\ 1 /-
Limited Round
Robin Testing
. ~ ~
~ it'
~ Reliabilit y Analysis |
| Synthesis |
1
~-
Overall Recommendation
| Recommended Testing Procedures |
Comparison With
Field Measured Moduli
Figure 5. Organization of research approach for NCHRP Project
I-28
8
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The broad overall findings of the study are discussed, interpreted and developed into practical
recommendations in Chapter 4, and a general summary and conclusions for all the findings are given in
Chapter 5. The appendices include the recommended detailed resilient modulus test procedures for asphalt
concrete (Appendix C) and for aggregate base and subgrade soils (Appendix E). For asphalt concrete,
specimen preparation methods are given in Appendix B and resilient modulus analysis me~ods in
Appendix A. Appendix G gives procedures for preparing subgrade soils. Equipment calibration
procedures are presented for both the diametral test (Appendix C) and the repeated load test (Appendix
E). The detailed approach used in He reliability analysis is given in Appendix F. Appendix H
summarizes~e multi-Laboratory validation study, and Appendix ~ compares laboratory measured resilient
moduli with moduli back-calculated from FWD tests performed in the field.
9
Representative terms from entire chapter:
repeated load
