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CHAPTER 5
SUMMARY, CONCLUSIONS AND GENERAL RECOMMENDATIONS
OPTIMUM RESILIENT MODULUS TESTING SYSTEM
For production resilient modulus testing, a completely automated, modern electro-hydraulic
loading and data acquisition system is a necessity to maximize the number of tests performed and to
minimize the potential for testing and data reduction errors. The loading system should be programmed
to automatically perform the complete stress sequence required for a resilient modulus test. Data should
also be automatically collected and saved using an analog to digital data acquisition system. Excellent
success, including high production output, has been reported using fully automated testing systems of this
type. Ideally, after a test is set up and started, resilient moduli should be printed out at the completion of
the test without the need for operator intervention.
By using an automated testing/data acquisition system, approximately 6 to ~ resilient modulus
tests, under ideal conditions, can be performed and the data reduced in one day. One person performs the
tests while a two person team prepares specimens.
Recommendation. Use a fully automated testing system for routine resilient modulus testing or
extensive research applications. A complete, automated testing/data acquisition/data reduction system is
necessary to achieve reliable resilient modulus test results, especially when conducted as a routine test by
a technician.
TESTING SYSTEM SET-I]P AND CALIBRATION
Accurate, reproducible resilient moduli cannot be measure by sending an inexperienced technician
or engineer into the laboratory and having hither start running resilient modulus tests even using a new
system. An automated testing system is complicated to set up, and the electronic measurement and data
acquisition systems must be thoroughly understood. The entire operation of the testing system must be
verified by quantitative measurements. The effects on resilient modulus of poor or lack of system
calibration and choice of instrumentation far outweigh the influence of most testing details.
System Set Up
Before beginning production resilient modulus testing, a careful shakedown is required of both
(~) the system electronics (including the closed-Ioop testing system, load cell, axial deformation
measurement system and data acquisition system) and also (2) the mechanical aspects of the testing system
such as load alignment and extraneous system deformations. A complete check out of the testing system
is required even if the system is new. Verifying the proper operation of the electronics and mechanical
aspects of the testing system requires supervision by an engineer knowledgeable in these types of systems
as well as the availability of a senior level laboratory technician.
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Recommendation
Laboratories have significant problems in properly setting up and making operational the resilient
modulus test including testing equipment, data acquisition apparatus, specimen preparation methodology,
specimen set up and data reduction.
Synthetic Specimens
System calibration by testing synthetic specimens having known resilient moduli is absolutely
essential to insure reliable resilient modulus test results. If external deformation measurements are made
In the repeated load biaxial test, which is not recommended, system compliance must be accounted for in
reducing the data. Each laboratory should own their own synthetic specimens. These specimens are easy
and inexpensive to make or can be purchase ready to test. Synthetic specimens sent from one laboratory
to another take a tremendous beating and often become unreliable as a laboratory standard. Each
laboratory should have the capability of measuring a reference resilient modulus by making accurate
deformation measurements directly on the synthetic specimen. The importance of thorough testing system
calibration cannot be overemphasized. Proof testing using synthetic specimens also serves to train
laboratory technicians.
Human Related Problems
The laboratory validation study clearly showed several considerations to be important in resilient
modulus testing. Before resilient modulus testing is begun, laboratories need to develop a well-planned
and carefully supervised program which includes using synthetic specimens for calibration. Some test
equipment does not work as advertised, and technicians often have too little experience to identify and
correct the source of problems. Also, not all laboratories carefully follow calibration and/or testing
procedures. A rushed laboratory testing schedule frequently leads to problems. The test of reasonableness
of measured resilient moduli was found to be all to frequently absent from laboratory test procedures.
_, _ - or
.
-
Both ~e electronics and mechanical performance of even a new testing system must be carefully
validated by knowledgeable engineers and senior laboratory technicians. The laboratory should own
synthetic specimens for verifying the overall reliability of the entire resilient modulus measurement
process. Serious consideration should be given to obtaining outside help in setting up, calibrating and
establishing He resilient modulus test as a routine laboratory procedure. Calibration procedures are given
in Appendix C for asphalt concrete and Appendix D for base and subgrade materials.
ASPHALT CONCRETE RESILIENT MODULUS TESTING METHODS AND PROCEDURES
Based on findings from this study presented and discussed in Chapter 2, a new protocol for
resilient modulus testing of hot mix asphalt concrete was developed and presented in Appendix C. The
Protocol has been written by incorporating the findings of this study into the final version of SHRP P07
Protocol (November I, 1992~. It was decided to rewrite SHRP P07 instead of the existing ASTM D4123
procedure, as the SHRP protocol had already made significant improvements to the ASTM standard.
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Conclusions. The following general conclusions are made concerning resilient modulus testing of asphalt
concrete specimens:
I. Resilient modulus decreases when testing is repeated on an axis mutually perpendicular to the axis
initially tested.
2.
3.
6.
7.
8.
9.
The resilient modulus decreases significantly with increase in temperature. Thus, it is important
to run the resilient modulus test at the desired test temperatures.
Poisson's ratio is one of the most important parameters influencing the resilient modulus. The
variation in MR values due to the testing axis dependency and different lengths of rest periods are
almost negligible compared to He magnitude of difference in the MR values from assumed and
calculated Poisson's ratios. Poisson's ratio should be evaluated using the EXSUM deformation
measurement system.
A mountable extensometer device, compared to the stand-alone EVDT measurement device,
provides less variance and hence better repeatability within the five consecutive cycles used for
resilient modulus determination. However, using the SHRP EG device EVDTs gave comparable
performance to the mountable extensometer. Mountable deformation measurement devices are
recommended for resilient modulus testing because of the smaller variability.
The SHRP EG device minimizes rocking of the specimen. The main features of the SHRP EG
device are the use of two guide columns, a counterbalance system, an innovative semi-rigid
connection between the upper plate and the load actuator, and its sturdiness. The disadvantages
are its bulkiness, complication of use, possible inertia from the counter-balance system, friction
in the guide columns, and limitation of the size of the sample that can be tested.
The concept is sound behind the use of EVDTs mounted along a small gage length (l in.) on the
surface of the specimen, as in the Gage-Point-Mounted setup. The main drawbacks for its use in
repetitive testing are its heavy dependence on the alignment and homogeneity of specimens. The
gage length of ~ in. seems to be too small for reasonable results with the asphalt concrete
specimens used in this study.
The proposed measurement system, the EXSUM setup, provides a promising measurement method
for determination of consistent and reasonable Poisson's ratios. At 41°F, however, increase in
variability occurs due to misalignment and rocking. Use of the SHRP EG device, or its
modification, together with the EXSUM setup ensures reasonable values even at low
temperatures. The use of the EXSUM setup requires an increase in testing time compared to
conventional systems because of the significant time required for mounting the EVDT on the
specimen. For research applications, improved reliability can be obtained by mounting an L~VDT
on both the front and back surfaces of the specimens.
A square load pulse produces significant specimen damage and smaller resilient moduli compared
to a haversine pulse. The haversine pulse also better simulates the field loading condition than
a square pulse. As a result, the haversine load pulse is recommended for resilient modulus testing.
The loading time significantly affects the MR values. A loading time of 0.2 sec. considerably
reduces MR' and produces more damage as compared to a shorter loading time of 0.05 sec. A
shorter loading time of 0.05 sec. is representative of high vehicle speeds, but is hard to accurately
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apply and monitor. Also, accurate load control at higher temperatures is difficult using very short
loading times. The usually used loading time of 0. ~ sec., represents slow traffic conditions that
cause significant damage to the pavement and should be continued to be used for resilient modulus
testing.
10. Rest period to loading period ratios of 4, 9, 19, 24, and 29 used in the study did not make a
significant difference in the resilient moduli. Also, a rest period to loading period ratio greater
than ~ has bun shown to generate no significant beneficial effect by past research. A rest period
to loading time ratio of 9 gives a rest period of 0.9 second and a loading frequency of ~ Hz. This
is He loading condition specified by SHRP P07 and a change in it is not justified.
. Three levels of preconditioning were studio. There was no significant difference in the variation
of resilient moduli and Poisson's ratio between five cycles for the selected preconditioning levels
2 and 3. However, MR values did decrease with increasing number of preconditioning cycles.
One-hundred preconditioning cycles are recommended at 41 and 77°F and 50 cycles at 104°F.
12. A significant difference exists between resilient moduli and Poisson's ratio values computed using
the SHRP P07 analysis and the elastic analysis which is similar to the ASTM analysis. The SHRP
approach gives higher values when an assumed Poisson's ratio is used as compared to the elastic
analysis with an assumed Poisson's ratio.
13. A 4 in. diameter specimen Is acceptable for testing medium gradation mixes, but a 6 in. diameter
specimen should be used to test coarse gradation mixes (mixes with more proportion of coarse
aggregate or mixes with large aggregate such as base courses or large-stone mixes). Medium and
coarse gradations are given in Appendix B. Table Bet.
14. SHRP protocol P07 recommended load amplitudes are suitable for testing at 41°F and 77°F, but
at 104°F a smaller load should be used. Load levels corresponding to 30, 15, and 4 percent of the
indirect tensile strength at 77°F are recommended for testing at 41°F, 77°F, and 104°F,
respectively.
15.
The relatively large seating loads recommended by the SHRP P07 protocol may not be necessary
as high seating loads seem to damage the specimen at higher temperatures. Instead, 5, 4, and 4
percent of the total load are recommends at 41°F, 7T and 104°F, respectively. However at 104°F
a minimum load of 5 Ibs. must be maintained to avoid the possibility of separation of the loading
strip from the sample surface. The maximum seating load should not exceed 20 lbs. to ensure
minimum damage to the specimen.
16. The following configuration of test apparatus is recommended for use in resilient modulus testing:
Load Device: A device comparable to He SHRP EG device, possibly with He following
modifications:
I. Reduction of the upper plate weight using high strength, light-weight materials and thus
elimination of the counterbalance weights,
2.
Reduction of the size of the device so that it can be easily used in commonly available
environmental chambers, and
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3. Capability for He testing of 6 inch diameter specimens.
The MTS diametral testing device was used for the final phase of the testing program mainly
due to time and budget constraints. Although the control of rocking is a little inferior to the
recommended diametral device, the MTS testing device gives comparable results, especially as
extensometers are to be used for measurement of horizontal deformation. Also, the testing device
can be used in a typical environmental chamber.
Measurement System: The EXSUM setup described in Chapter 2 is recommended for use.
However, a faster curing glue with non-sagging properties is required to reduce the time required
for testing. Also, in~epth finite-element analyses might be required to make corrections for
bulging and non-uniform stress distributions. The capability to use two mounted EVDTs, one
each at the front and back face of the specimen, might make results more trustworthy. Although
accurate and convenient, extensometers are expensive, and a cheaper mountable measurement
system wig comparable accuracy should be developed.
BASE AND SUBGRADE RESILIENT MODULUS TESTING METHODS AND PROCEDURES
The repeated load biaxial test is recommended to evaluate the resilient modulus of base, subbase
and granular subgrade materials. A repeated load test performed on an unconfined specimen is
recommended for cohesive subgrade soils. The round-robin tests (Appendix H) show for base and
subgrade materials that very large variations in MR values were observed between labs when axial
deformation is measured bow outside and inside the biaxial cell using current procedures. Testing details
are considered in Chapter 3, and testing details compared with the AASHTO and SHRP procedures in
Chapter 4.
Recommendation
The recommendation is made to make axial deformation measurements inside the cell. An optical
extensometer, non-contact proximal gages and light, sensitive EVDTs mounted on lightweight clamps can
all be used. Use of He optical extensometer, however, is recommended. The use of an inside deformation
measurement system neither eliminates nor reduces the need by careful calibration for minimizing system
compliance (i.e., extraneous deformation in the loading and testing system) or measuring test apparatus
alignment (Appendix D).
Major Issues
The following major resilient modulus test issues completely overshadow other test details which
usually have relatively minor influence on the measured resilient modulus: (~) fully automated loading
and data acquisition system to minimize errors, (2) accurate measurement of axial deformation including
end bedding effects of cohesive soils, (3) cohesive specimen aging, (4) environmentally induced changes
in MR and (5) soil structure of compacted cohesive specimens. Failure to properly account for any of the
above major factors can easily lead to errors of 30 to 100% or more in the measured resilient modulus.
These important issues are discussed in Chapters 3 and 4.
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Optimum Granular Base and Granular Subgrade Test Method
The optical extensometer can be used to measure deformation of a granular specimen subjected
to confining pressure by applying a vacuum pressure to the inside of the specimen. This test eliminates
the need to use a biaxial cell and hence is both simple to set up and requires no special changes in
equipment to use the optical extensometer. The vacuum type repeated load test has been found to be a
practical method for testing granular materials. Application of a vacuum inside the specimen causes
virtually the same effects on the specimen as applying an external confining pressure. Neither the
AASHTO nor SHRP procedures consider or allow this type system. A quasi-static test can, if necessary,
be used to satisfactorily measure the resilient modulus of granular materials. The quasi-static test does not
require either a sophisticated testing system or data acquisition system.
The coefficient of variation of resilient moduli, with careful equipment calibration, is as small as
about 11% if one specimen is used and 8% for two specimens. The popular K-8 resilient modulus model
cannot always distinguish one material from another at the 95% confidence level. The K-8 model does
not work at all with stabilized materials. The more accurate Uzan, U.T-Austin or FHWA models are
recommended to characterize resilient modulus behavior.
Cohesive Soil Optimum Test Method
An unconfined repeated load test is proposed for both undisturbed and compacted cohesive
subgrade specimens. The considerably more complicated biaxial test is specified by both the AASHTO
and SHRP test procedures. The unconfined compression test is simple to perform and also allows the easy
use of an optical extensometer since a biaxial cell is not required. Axial deformation can be measured
directly on the specimen using either an optical extensometer, cIamp-mounted EVDTs or noncontact
proximity gages. Axial deformation can also be measured between solid end platens if the specimen ends
are either grouted or an empirical end correction is applied to the measured resilient modulus. Neither
He AASHTO nor SHRP test methods consider the important end effects of cohesive specimens.
Measured Subgrade Modulus and Observe Perfor~nance. The measured subgrade resilient modulus has
sometimes been found not to give a correct indication of how the subgrade actually performs. Reasons
for this apparent discrepancy include: (1) Testing problems in measuring the resilient modulus can occur
as discussed throughout this report. (2) The specimen tested may not be representative of the insitu
material due, for example, to sample disturbance, changes in moisture content between the field and the
laboratory, also the specimen may not be representative of the poorer performing material along the route
under consideration. (3) The resilient modulus does not properly depict permanent deformation
characteristics of the subgrade which should be evaluated by the repeated load biaxial test using an
appropriate stress state.
ENVIRONMENTAL MOISTURE CYCLE
The most realistic method of pavement design, as suggested in the 1986 AASHTO Guide, is to
analyze the pavement for a number of different time periods throughout the year. Because of varying
moisture contents throughout the year, the resilient modulus (and also the resistance to permanent
deformation) is continually changing. The difference in resilient modulus for dry and wet conditions can
be as large as 100% or more.
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Recommendation
Develop empirical relations for correcting the resilient modulus measured at a standard reference
moisture/density state to give appropriate resilient moduli for other moisture conditions. Such empirical
corrections make considering realistic variations in resilient moduli with seasons of the year practical in
pavement design.
EMPIRICAL RESILIENT MODULUS RELATIONSHIPS FOR USE IN DESIGN
Resilient modulus testing at the level of sophistication needed to obtain satisfactory results, for
most laboratories, is more suitable for a research project than for routine production type testing. The
resilient modulus test is:
.
Not an easy task unless the test is fully automated
Requires a significant amount of time and is subject to testing errors
Requires equipment that is relatively expensive
May not be production oriented depending upon the available equipment
A very attractive approach for obtaining resilient moduli for use in design, for at least most
agencies, is to determine values of resilient modulus using generalized empirical relationships. Such
relationships give resilient modulus as a function of statistically relevant, easy to measure physical
properties of He material such as percent compaction, water content, etc.
Justification
The resilient modulus has been found to vary as much as 20% to 40% along segments of highways
of limited length. Also, He average variation in resilient modulus likely to occur, under favorable
laboratory testing conditions, is about 10 to 15% for within laboratory tests. Considering the large
variation in resilient moduli due to the environmental moisture cycle as well as the above variations, He
use in design of empirical resilient modulus relationships is considered to be justified. A number of states
have already developed generalized resilient modulus relationships for use in design, particularly for
cohesive subgrade soils.
Recommendation
Give serious consideration to developing generalized resilient modulus relationships to obtain in
a practical manner design values for route use. These relationships should be developed from a well
planned and executed laboratory resilient modulus testing program.
PERMANENT DEFORMATION
The resilient modulus of pavement materials has received considerable publicity in recent years
since its introduction in ache 1986 AASHTO Design Guide. The evaluation of permanent deformation
characteristics of He asphalt concrete, base and subgrade materials is just as important, and usually more
important, as He resilient modulus and hence should neither be forgotten nor neglected. The permanent
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deformation characteristics of the base and subgrade can be readily determined as an extension of the
proposed resilient modulus test using a repeated load biaxial test apparatus. The permanent deformation
behavior of asphalt concrete can be evaluated using a loaded wheel tester or else as an extension of the
repeated load diametral test.
SUGGESTED ADDITIONAL RESEARCH
The following research topics are considered to be important in determ
resilient moduli for design and deserve additional research:
.
fining reliable, practical
I. Develop generalized relationships between resilient modulus and easy to measure material
parameters for asphalt concrete, granular base and subgrade soils and cohesive subgrade
materials.
4.
Study the effect of specimen aging on the resilient modulus of cohesive subgrade soils.
Develop generalized relationships for correcting resilient moduli measured a short time
after compaction to the long-term modulus.
Develop generalized design relationships for relating fiche change in resilient modulus with
change in moisture content for both subgrade and base materials.
Incorporate permanent deformation using realistic material tests into He AASHTO design
method.
285
Representative terms from entire chapter:
resilient moduli
