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CHAPTER 4
SIGNIFICANT GENERAL FINDINGS
INTRODUCTION
(chanter 2 examined in detail resilient modulus testing of asphalt concrete, and Chapter 3 examined
resilient modulus testing of aggregate base and subgrade materials. This chapter reviews the more
significant of these findings, introduces new concepts and integrates this information together to explore
the overall utilization of resilient modulus testing in pavement design. The broad aspects of resilient
modulus testing are examined in this chapter and put into perspective from a practical viewpoint.
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 am to
minimize the poter~fi~fortestiingan~data reduction errors. Another important advantage of using a
fully automated testing system is that a laboratory technician can be readily trained to reliably perform the
test after the system has been calibrated and made operational. The use of a fully automated testing system
is also desirable for research applications.
A completely automated electro-hydraulic testing system for repeated load biaxial testing is shown
in Figure 123. The loading system has been programmed to automatically perform the complete stress
sequence required for a resilient modulus test. Data is also automatically collected and saved using an
analog to digital data acquisition system. A testing and data acquisition system similar to this one is being
used for routine resilient modulus testing by He Alabama DOT. Excellent success including high
production output has been reported using their fully automated testing system.
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. lhe use of a fully autom~edt testing system including
Ma acquts*i~n and reductilon is considered a key component ire adapting resilient modulus testing for
routine use or extensive research applications.
Data Acquisition
Important advantages of a date acquisition system compared to a strip chart recorder include: (1)
load and displacement are measured more accurately and (2) Me readings are automatically stored in digital
form which can be readily manipulated including transferring We data to another computer, if required,
and automatic data reduction. If a sufficient number of readings are not taken during the application of
He load pulse, however, the true peak response is not obtained since the pulse has a very steep slope with
the peak being reached in only 50 ms (0.05 sec.) for a 100 ms (0. ~ sec.) load pulse. A minimum of 200
data points per sec. are required to give reliable results. The number of readings collected on the response
251
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-
-
- ~
~e
L--3 ~.
I,
.. , , ~ · ~
rear ~ ._
-
. ~
' .%'
i., _
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1
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. ~
' a,,..-.._.
_ ~
. . .
Clamps Positioned on the Granular Material Sped men
__
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at, ~ . ~
_n
=~
~ -, ~
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. Am=
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~: ~ .~.~A~ ~
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~ ~. : ~. .:: ,, . ,: ~,, - .,,.- - .- - - _
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icky F3~ai
~9
Data Acquisition System
Figure 123. Fully automated closed-lop, electro-hydraulic testing system
252
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curve depends upon the (1) capability of the data acquisition software used, (2) time of pulse application,
(3) capacity to store and handle the data, and (4) overall speed and operation of the analog to digital
converter. The data acquisition system should use as a minimum a high quality 12 bit analog to digital
(D/A) board to insure accuracy of the data collected. Also, the data acquisition system should have
provisions for outputting the collected data in digital form to facilitate data reduction. In general, a
sotcware/data acquisition system should be selected that samples each channel of data at the same time so
as to obtain a minimum time lapse between data points collected for each channel. Refer to Appendix E
for detailed specifications for the data acquisition system.
Overall System Reliability. Some indication of the overall accuracy of the rate of data acquisition, analog
to digital conversion and testing system can be determined by examining the coefficient of variation and
magnitude of the resilient modulus obtained which is dependent on both the load and displacement
measurements. Consider, for example,the average coefficient of variation for Me repeated load biaxial
test resilient moduli obtained for five 0. ~ sec. pulses for a granular base. Combined external and clamp
mounted EVDT data are used in this comparison. The coefficient of variation (CV) for 102 data point
readings/sec./channel was CV = I.4%, for 205 readings/sec./channel CV = 0.6% and for 666
readings/sec./channel CV = 1.0% (refer to Table ill, Chapter 3~. For a given method of measurement
(external EVDTs or clamp mounted EVDTs), the resilient moduli values for all three data acquisition rates
were within ~ S; of each other. The average resilient modulus determined for 660 readings/sec., which
should be closer to He true value of load, was consistently the largest by a small amount (0.42% more than
for 205 readings/sec.~.
Conclusion -- The most consistent data acquisition rate for the system used is 205 readings/sec.
Theoretically, better and more consistent values of resilient modulus should be obtained as the sampling
rate is increased. However, as He sampling rate is increased more noise is also obtained in the readings.
Cross coupling between channels may also cause noise. The effect of cross coupling on noise can be
evaluated by reducing He number of channels of output collected to see if peaks change. Filtering He data
to remove noise when measuring peak points is dangerous and should, if possible, be avoided. For the
system considered in this example, either 205 or 666 readingsIsec. give good results as shown in the next
section.
Load and Deformation Variation
For tile same aggregate bases discussed in the previous section, the variation in 5 consecutive load
pulses (CV = 0.67%) was about 3 times the corresponding variation in EVDT readings for both external
EVDTs (CV = 0.21 %) and clamp mounted EVDTs (CV = 0.20%~. Since the coefficient of variation
in load increases with decreasing load levels, a 5 kip load cell having a sensitivity of no less than 3mV/V,
which was the sensitivity of the cell used in the base study, should be the minimum used for unstabilized
6 in. diameter specimen base testing to reduce the overall variation in the test results.
Use of a load cell with a sensitivity greater than 3mV/V would more effectively reduce resilient
modulus variability Can increasing the sensitivity of the displacement transducers. A 5 kip load cell was
used since the capacity equals the maximum load range used of the testing system. Use of a lower
capacity, higher voltage output load cell, although desirable, makes it possible to accidentally overload He
cell and hence damage it. At He very low load levels, the maximum practical sensitivity is needed of bow
He displacement transducers and He load cell. To obtain the most reliable results, He maximum practical
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gain of Me output signal for all of the transducers should be used as well as the maximum possible
transducer excitation voltage.
To obtain higher transducer sensitivity it is sometimes desirable to apply a transducer voltage in
excess of We rated value; check with the manufacturer before doing this. For example, a non-fatigue rated
load cell usually has a higher sensitivity than a fatigue rated one having the same rated capacity. One lab
has used non-fatigue rated load cells for years without problems as long as the applied load is one-half of
the rated capacity.
Limited studies, similar to the ones previously described, would be beneficial to conduct for each
testing system go establish He optimum data acquisition rates, as well as the accuracy and variation of Be
readings obtained from 5 consecutive load pulses. Also, always compare the level of voltage output of
the measurement devices used with the [eve! of background noise in the Ma acquisition system. Me
output voltage must be sup entry large to insure accurate readings. Adequate transducer sensitivity is
often a problem in resilient modulus testing.
Data Reduction
Large quantities of data are obtained from a full resilient modulus test sequence using a data
acquisition system. To minimize the chance for errors, the entire test procedure including load
sequencing, data acquisition and data reduction should~ all be automatically integrated together. Ideally,
abler 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. In practice, some computer files may have to be saved and moved to
another computer and perhaps a few buttons pushed to obtain resilient moduli. A complete, automated
testing/da~a acquisition/ data reduction system is necessary to achieve reliable resilient modulus test
results, especially when conducted as a routine test by a techn~c~n.
TESTING SYSTEM CALIBRATION
Accurate, reproducible resilient moduli can not be measured by sewing an inexperienced
technician or engineer into the laboratory and having hen or her start running tests ever 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 must be verified by quantitative
measurements. The system calibration procedure Mat must be followed to measure reliable moduli consists
of the following steps:
I. Measurement Device Verification. Verify by careful calibration that each individual
component of the testing system gives the correct output. For example, check load by
statically loading He laboratory reference proving ring and comparing its indicated load with
the systems. Individually calibrate displacement transducers by subjecting them to known
displacements.
2. Svelter Alignment and Compliance. Measure the alignment and compliance (extraneous
deformation) of each part of the loading system and test apparatus. Just because a $100,000
system is new does not necessarily mean parts are aligned and compliance is small. To
carefully and thoroughly conduct this step takes several days of concentrated effort.
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Specin~en Proof Tess. To verify the overall accuracy of all components of the testing system,
tests must be performed on synthetic reference specimens having known values of resilient
moduli. Testing system problems must be identified and corrected until the correct moduli
are obtained, without applying empirical adjustments to the data, before routine testing is
begun.
System c~ib~tion by proof testing synthetic specimens is absolutely essential to insure reliable
resilient modulus test rests. 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.
Synthetic specimens are temperature sensitive and hence tests must be performed at the reference
temperature or else corrected for temperature difference. Synthetic specimens may also be stress
level dependent. Each laboratory should own their own synthetic specimens. These specimens
are easy and inexpensive to make or can be purchased ready to test. Synthetic specimens sent
from one laboratory to another take a tremendous beating and often become unreliable as a
laboratory standard.
Wire resistance strain gages about 2 in. In length can be bonded to Me synthetic specimen to obtain
accurate, reference strain measurements. Another good approach is to rigidly attach EVDTs to
the synthetic specimen. The simplest approach is to measure, using an optical extensometer,
deflection over a predetermined gage length on the specimen. Using one of the above
measurement methods allows easy periodic verification of the reference resilient modulus of
synthetic specimens at any temperature.
Me Importance of thorough testing system calibration cannot Be overemphasized. Proof testing
using synthetic specimens also serves to train laboratory technicians. Me resilient modulus test should
be repeated on the same synthetic specimen at least 5 times and the coefficient of vacation of MR
calculated. If the coefficient of variation of the test group is more than about 6% repeat the 5 tests.
Calibration procedures are given in Appendix C for asphalt concrete and Appendix D for base and
subgrade materials.
SET UP AND OPERATION OF NEW TAXIING SYSTEM
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. Several factors including He sophistication of the testing system
electronics and difficulty to visually observe specimen behavior mean a greater level of care is required
to obtain meaningful resilient modulus test results Man for most other tests.
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 Me source
of problems Also, not all laboratories carefully follow calibration and/or testing procedures. A rushed
laboratory testing schedule frequently leads to problems. Resilient modulus values should always be
calculated at one or more specific stress states and compared to reference values to verify Hey are
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reasonable. The laboratory validation studies showed the test of reasonableness to be all too frequently
absent from laboratory test procedures. Typical ranges of resilient moduli, for specific stress conditions
and materials, should be tabulated so that technicians and engineers can quickly and easily verify the
laboratory test results.
R - ommendation. Consideration should be given to obtaining outside help in setting up, calibrating and
establishing the resilient modulus test as a routine laboratory procedure.
RESILIENT MODULUS TESTING METHODS AND PROCEDURES
Resilient modulus testing equipment and procedures are specified to help insure accurate results.
A number of different resilient modulus test procedures are available for both the diametral and repeated
load biaxial tests. Many of the differences in these procedures involve small changes in stress conditions
used and other testing details. The effects on resilient modulus of poor or lack of system calibration and
choice of instrumentation far outweigh the influence of most testing details.
Asphalt Concrete
Based on findings from this study 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 He ASTM standard.
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. The resilient modulus decreases significantly with increase in temperature. Thus, it is important
to run the resilient modulus test at the desired test temperatures.
3. 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 the magnitude of difference in He MR values from assumed and
calculated Poisson's ratios. Poisson 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 He 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 He smaller variability.
256
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6
7.
8.
9.
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 He counter-balance system, friction
in He guide columns, and limitation of the size of He sample Hat can be tested.
The concept behind the use of EVDTs mounted along a small gage length (l in.) on the surface
of the specimens as in the Gaze-Point-Mounted setup. is sound. 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 memos
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 He EXSUM setup could ensure reasonable values even at low
temperatures. The use of the EXSUM setup requires an increase in testing time compared to
conventional measurement 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 EVDT on both He front and back surfaces of He specimens. Corrections for bulging
and non-uniform stress distribution using finite element analyses could make the analysis more
relevant.
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
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 Hat
cause significant damage to He 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
~-
~an ~ has been 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 the loading condition specified by SHRP P07 and a change in it is not justified.
Three levels of preconditioning were studied. 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.
100 preconditioning cycles are recommended at 41 and 77°F and 50 cycles at 104°F.
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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
~ ~ , e ~ ~ ~ , , ~ ~ ,
analysls gives nigher values when an assumed Poisson s ratio is used as compared lo tne elastic
analysis with an assumed Poisson's ratio.
13.
14.
15.
16.
The 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 wig large aggregate such as base courses or large-stone mixes). Grain size
distributions for medium and coarse gradations are given in Appendix B. Table Bet.
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.
The relatively large seating loads recommended by the SHRP P07 protocol may not be necessary
as high seating loads seem to damage He specimen at higher temperatures. Instead, 5, 4, and 4
percent of He total load are recommended at 41 °F, 77°F, and 104°F, respectively. However, at
104°F, a minimum load of 5 Ibs. must be maintained to avoid possibility of separation of the
loading strip from the sample surface. The maximum seating load should not exceed 20 Ibs to
ensure minimum damage to the specimen.
The following configuration of test apparatus is recommended for use in resilient modulus testing:
Load Device: A device comparable to the SHRP EG device, possibly with the 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
3. Capability for the testing of 6 inch diameter specimens.
The MTS deformation measurement device was used for the final phase of the testing program
mainly due to time and budget constraints. Although the control of rocking would be a little
inferior to He recommended device, the 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 is recommended for use. However, a faster curing
glue with non-sagging properties is required to reduce the time required for testing. Also, in-
depth 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,
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extensometers are expensive, and a cheaper mountable measurement system wig comparable
accuracy should be developed.
Base and Subgrade Materials
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. Test procedures are given in Appendix E. 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 outside of the biaxial cell. Therefore, the
recommendation is made to make area deformation measurements inside the ceil. An optical
extensometer, non-contact proximity gages and EVDTs mounted on clamps can all be used as previously
discussed in Chapter 3. The use of an inside defo~ation measurement system neither eliminates or
reduces the new for minding system compliance (i.e., extraneous deformation in the loading and testing
system) or measuring and correcting test apparatus alignment (Appendix D).
Comparison of Test Procedures. A general comparison between the proposed base and subgrade
resilient modulus test procedures and those of AASHTO [! 12] and the AASHTO version of SHRP [! 13]
are given in Table 58 and 59, respectively. The complete proposed test procedures are given in Appendix
lo.
Major Issues - Dee foRowzag major msdient modulus test issues completely overshadow other
test details which usually have relay rely minor influence on the measured resilient modulus: (~) fully
automated loading and Ma acquisition system, (2) accurate measurement of arid deformation, (3)
cohesive specunen aging, (47 envuo1unen~ly 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 led to
errors of 30 to 100% or more in the measured resilient modulus.
Test System - An automated, closM loop electro-hydraulic testing system including automated
data acquisition and data reduction is specified in the proposed procedure to make He test practical. This
requirement leads to higher productivity while at the same time minimizing the chances for testing and data
reduction human error. Although the SHRP test memos given in AASHTO TP46 [! 13] requires a closed
loop, electro-hydraulic system, a fully automated test is not specified. The AASHTO T292-911 procedure
does not specify either loading or data acquisition systems. Also, He proposed procedure requires and
describes a detailed equipment calibration procedure which is not true for ache other two procedures which
to varying degrees just mention calibration.
Axial Deformation of Granular Material ~ During actual testing, the most critical feature of the
resilient modulus test itself is the accurate measurement of specimen axial deformation. The proposed test
method requires for granular materials Hat axial deformation be measured directly on the specimen using
any one of Tree methods given in Tables 58 and 59. Displacement measurement on the specimen
minimizes the very serious problem of test system compliance (i.e., extraneous deformations). The SHRP
TP46 procedure [~13] specifies measurement of deformation outside the biaxial cell. Although AASHTO
T292-911 [~121 requires deformation measurement using clamp mounted EVDTs on the specimen, no
provision is given for also allowing He use of either an optical extensometer or non-contact proximity type
gages which are bow permitted in He proposed procedure.
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Table 58 . Comparison of selected AASHTO, SHRP and proposed resilient modulus
test requirements for aggregate base
TEST DETAIL AASHT0 St ~PIX)POS~
T292-911 TR46 (ED.1)
GENERAL
TESTING SYSTEM not "pecitied electro-hydraulic fully-automated,
electro-hydraulic
DATA ACQUISITION strip chart or not specif fed AID Data
computer Acquisition Sy$.
.
TYPE TEST
repeated load biaxial repeated load repeated load (R.L.)
GRAN ULAR triaxia I trioxia I
COHESIVE same (see note 2) same (see note 2) R.L. unconfined
LOAD PULSE
SHAPE haversine, rect., haver$ine haversine
triangular
TIME ~variable (see note 1) 0.1 ~0.1
FREQUENCY (~1 ~0.33-1
LOAD CELL LOCATION insideloutside - Extemal (implied) inside
AGGREGATE BASE
. _
COMPACTION METHOD vib.; impact
kneading, static vib ratory vib ratory
CONDITIONING (psi) cs3=20, od=1~; a3=15; ad=1 3.5 03=15; <5d=15
N=1000 reps. N=500-1000 reps. N=200-1000 reps.
AXIAL DEF. inte m al clamps; 2 external LVDTs intern al clamps, non
MEASUREMENT 2 LYDTs contact, or optical
STRESS STATUS (pai) c,3=20,15,10,5,2; 03=3,5,10,15,20 <73=3,4.5,6,1 4
18 oh's ~ N=50 reps. 15 oh's ~ N=100 15 oh's ~ N=50
PERMANENT rapid shear rapid shear; or
DEFORMATION not considered as approx. repeated load perrn.
t6Gt: ~=5O,COO raps.
COMMENTS
PROPOSED METHOD
Fully automated testy
ate acquisition/ reduction
greatly speeds test, reduces
trances for errors.
Unconfined test for cohesive
soils is much simpler,
saves time; easy to measure
optically asocial strain.
Need to use one pulse approx.
puree shape for consistency.
Use one valve for ease of test,
reduce error; empirically
correct for pulse time,
it desired.
Inside location eliminates
friction on piston.
Use one method for
consistency; vib. slightly
better than impact.
N=200 adequate for
good base
Critical to measure def. on
specimen; optical best.
Stress states not critical
for granular mat.
Perm. def. measure is
critical; repeated load
test best.
NOTES:
1. PULSE TIME DEPENDS UPON VEHICLE SPEED AND DEPTH BELOW SURFACE.
2. SESAMES INDICATES THE SAME AS GIVEN ABOVE IN THE SAME COLUMN.
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Table 59. Comparison of selected ~SHTO, SHRP and proposed resilient modulus
test requirements for subgrade soib: ~ ~ PSI
TEST DETAIL "SHTO S H R P PIX)~ED
_ T292-911 TP" (ED.1)
_
COHESIVE AND GR4M)LAR SUElGRADE
__ _
coMPACnOY hIETHOD
~HESNE |veriable (~e note 2) | static | v~r~bl~ (am note 2) |
C~ANW ~vib; impact vibratory vibratory
knasding;static
CON DIT ONING (PSq
COHESIVE 1000 N ~ 0,-3, 501~ N ~ m=6, 2 - N. a' =0,
~s6 (A ~s3.6 off ~
.
C;RANUL4R 1000 N ~ o3 = 1S "me (~e noto 5) soo-1000 N ~ all-6,
~-1 2 . c';~8
AXIAL DEF. IdEASURIAENT _~_ ~ _ _ ~, .
~Intcrnal optical, non.
COHESIVE internal c'.mp.(~), 2 ·xtcrnal LVOTe conted, cbmpe; top to
2 LVDTe bottom LVDTI non
contact.
GRANUUR "~ (~e note 5) "n. (~e note 5) Internal optical, non~
contact, clempc
_ _ .
STRESS STATES (PSI)
Repr - entatives CIj; ~-6,~,2; t5 ~'. Unconfined (~0);
Ca~VE ~3,5,~,10,15 ~ t4= 100 "ch ts. = 2,4~6,E, 10
~N=SO each ~ t. = 50
GRANlJLAR c~=15,10,5,2; ~me (~ note S) o~2,3,~,6 .
1J' ~'s ~ Na 50 12 o,.s ~ N = 50 ~
~_ · _ ~not consider" Rap~d Sh~r T - t Rep~d Sh~r Tod .
CO~TS
PROPOSED KT~
hlothod u~d has brge aneet
on coil ·tructure ·nd hIR
vib. alightly teen" than
impact; be consi~ent.
-
,
Yery simpic t~t it u"
optical or top~ottom ~.
wiem; relhLb.
Critkal to measure dd. on
soocimen: optical b - t
NOTES:
1. S~E "~ TA~E F" AG - ~AW 8~ F" C - P"~ ~ GENE~ UST
2. COMPAC~ - ~ - ""N" U~ "PEC~D ~ ~RE C~ - AT n" ~ C - PAC~
ANC LONG~; "~O "LO - STA=, IMPACt, - ~G; ~D ~ - ~ STA~ - D IMPACT.
3. USE <5~1= 3 pei If SNEAR STRENGTI1 S 1000 pef.
4. US£ EXTERtlAL LYDTs ON P~TON FOR SOfT SOILS; CALIBRAtE FOR SYSTE.I CONIPLIANCE.
5. ·S"IE- INDK:ArES THE S - E AS Gn,EN "OVE IN THE SA~E COLU~N.
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40,000 psi. For He design parameters given on the figure, experimental error in measuring the resilient
moduli of each layer causes about a 12 to 14% increase of the total equivalent aggregate pavement
Sickness. Experimental error In measuring the resilient modulus of the subgrade, assuming no other MR
testing error Is present, accounts for only about 20% of the tote] possible effect of resilient modulus testing
error. In determining the effect of error in measuring the resilient modulus of specific layers, the
measurement Of MR for the other layers is assumed to be perfect.
Now consider the behavior of a pavement having a relatively den 6 in. A.C. surfacing with a weak
base. For a weak base modelled by a resilient modulus of 20,000 psi, He pavements exhibits an ~ to ~ ~ %
increase in He equivalent base Sickness compared to Hat required in the absence Of MR testing errors.
Experimental error In measuring the value Of MR for a weak base accounts for 77 to SS% of the possible
combined effect of all MR testing errors. In contrast, variation in the resilient modulus of the strong base
due to experimental error accounts for only 25 to 60% of the combined effect as the subgrade resilient
modulus decreases from 10,000 psi to 2000 psi, respectively.
AASHTO Reliability Analyses
Figure 127 shows the effect on total equivalent base thickness of experimental error in measuring
resilient moduTi using He AASHTO type reliability analysis. Reliability levels of 50, 85 and 98% are
shown for pavements having a moderate strength base (MR = 30,000 psi). This pavement is loaded with
6x106 ESALs. The general trends shown for the AASHTO reliability analyses are similar to those
obtained for the Monte CarIo method. The same coefficients of variation of MR for each layer were used
in all analyses. Change in thickness determined by the AASHTO type reliability analysis due to resilient
modulus testing errors, however, are about one-half of the ~ to 14% increase in total equivalent base
thickness (i.e., no A.C.) typically predicted using the Monte CarIo analysis.
The AASHTO reliability method, which is practical to perform, was developed for a single
set of conditions as Indicated by taking the partial derivative of pavement behavior at a point. In contrast,
the Monte CarIo method uses the statistically developed AASHTO equation which should be reasonably
valid over a wider range of variables, and should show trends better than the AASHTO method.
Relationships Between MR and Structural Coefficients
The relationships used in this analysis to express the structural coefficient as a function of resilient
modulus for the A.C. surfacing and base are shown in Figures 128 and 129. These relationships are
almost the same as given in the 1986 AASHTO Guide [1141. Using the Monte CarIo method results in
the use of a reasonably wide variation in resilient moduli. Therefore, He AASHTO curves were slightly
modified to give more realistic relationships between He structural coefficient and resilient moduli at its
extreme values.
Reliability Design Implications
Table 60 gives the approximate relative importance of obtaining a reliable resilient modulus
measurement for the asphalt concrete surfacing, aggregate base and subgrade. This table was developed
from the results of the Monte CarIo reliability study using the AASHTO 1986 Guide for thickness design.
Generalizations of these results to design are summarized as follows:
267
OCR for page 268
23 ~
me
c,
~2
Z Z
~ Y
I C'
::
, ~
-
, ..!
-
u cn
m
21
~9
17
15
13
11
7
-
_
DESIGN VARIABLES
6, - 0,000 ESALe
~ IN. A.C. SURFACE
A.C. OR ~ ~'~
BASE FIR -
APS1 ~ 2
R · R£~81L~
- EFFECT OF LAB SIR ,48~BLm -' .
~ V
` -A
7
,
r R ~ 98%
R · 8S%
1
. ~
6 8 t0 12 14 16
R · 50Ye
0 2 4
SUBGRADE RESILIENT MODULUS (KSI)
Figure 127. Influence of lab variability on base thickness for AASHTO
reliability analysis: Base MR = 3D,000 psi
268
OCR for page 269
:c
hi:
J
:' _ 0.5
~,~ 0.4
a:~
Hi:
o
C)
0.6
0.3
0.2
0.1
it.
-
_
_=
-
z
/
r
0.0 ~
200 400 600 800 1000
ASPHALT CONCRETE RESILIENT MODULUS
AT 68°F, MR (KSI)
Figure 128. Variation of asphalt concrete structural coefficient with
resilient modulus used in reliability analysis
269
l
OCR for page 270
In ~
-
:E
0 ~
to
-
J
oO 20000
ce
In
m
/
42 ~ 0-~49 LOGlo(Ess). o.9n 7
./
J
~,-~-
~ -"I; 42 ~ 4.~361~MR n ~R "~ PSI
O- ' 1 - 1
0.0 0.1 0.2
BASE STRUCTURE COEFFIClE~, A2
Figure 129. Variation of base structural coefficient with resilient
modulus used in reliability analysis
Table 60. Approximate relative effect on pavement thickness of
resilient modulus test variabiliny of each layer (l,2)
0.3
LAYEF
Condition Comment AC Base
.
~.
Full Depth AC 6 0
6 in. AC Surface Strong Subgrade 5.5 2.5
and
Strong Deep Base Weak Subgrade 2.5 6
6 in. AC Surface Strong
and or 0.3 8.3
Weak Deep Base Weak Subgrade
Subgrade
4
2
1.5
2
Notes. 1. NOTATION: Maximum Importance = 10
No Importance = 0
2. AASHTO 1986 Design Guide Used for Pavement
Thickness Design
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Subgrade. The reliability of subgrade resilient modulus evaluation has less effect on pavement Sickness
than the reliability of the MR measurement for the asphalt concrete and for the M R of thick aggregate
bases. This finding suggests that as a practical alternative the subgrade resilient modulus could be
evaluated from empirical relations. Such relationships give resilient modulus as a function of easy to
measure variables such as density, water content, Atterberg limits, etc. The practical concept of use of
empirical relationships for resilient modulus is discussed in the next section.
Base. The reliability of the aggregate base resilient modulus evaluation is more important than for Be
subgrade when the base is thick. Base evaluation is also more important than for the subgrade for thin to
moderately thick A.C. surfacing and (~) strong, deep bases on a weak subgrade or (2) weak, deep bases
on either strong or weak subgrades. These results indicate in areas where marginal base materials of
varying characteristics are common, resilient modulus evaluation of base materials should be carried out
on a routine basis.
Asphalt Surfacing. The reliability of the asphalt concrete resilient modulus evaluation is most important
for full depth or deep strength pavements with strong bases and subgrades.
EMPIRICAL RESILIENT MODULUS RELATIONSHIPS
Resilient modulus testing at the level of sophistication needed to obtain satisfactory results is
probably, for most laboratories, more suitable for a research project Can for routine production type
testing. A very attractive approach for obtaining resilient moduli for use in design, for at least most
agencies, is to calculate values using generalized empirical relationships. Such relationships give resilient
modulus as a fimction of statistically relevant, easy to measure physical properties of He material such as
percent compaction, moisture content, etc. These relationships for resilient modulus can be established
Trough carefully designed and conducted research projects for each class of material: asphalt concrete,
aggregate base and subgrade soils. Statistically based equations, graphs or charts would then be
develops for each class of materials for the range of properties routinely used in design within He region
of interest.
Empirical Equation Justification
The environmental pretty cycle causes variations in resilient moduli with time as large as one
order of magnitude as observed in this study. Also, back calculated resilient moduli of the base from
FWD field tests have been found to have a coefficient of variation (CV) of 23% for a carefully controlled
test section [! 151. Depending upon the time of measurement, CVs have been observed varying from 26
to 45% for 3300 ft. sections of roadway [! 161. Also, both Monte CarIo and AASHTO type reliability
analyses, using He 1986 AASHTO Design Guide, indicate modest errors in evaluating resilient moduli
as large as 10 to 15% of He mean MR value have in general a relatively small effect on He overall
required pavement thickness. Additional variability is introduced due to varying materials properties
during construction, traffic variability and variability in the design equations. Because of all these
factors, empirical correlations between resilient modulus and easy to measure properties can be used
in pavement design considering the large expected overall variation in resilient modulus as material
properties and environmental conditions change with both time and location. This finding is in
agreement with the conclusion of Thompson [851.
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Asphalt Concrete
Quite useful generalized resilient modulus relationships for asphalt concrete, for example, were
developed by Miller, et al. [151 as discussed in of Chapter 2. The resilient modulus prediction equations
presented by Miller, et al. were determined from cyclic biaxial compression testing. Resilient moduli of
asphalt concrete determined from bending [! 17] and also from dimetrial tests (perhaps to a lessor degrees
are usually smaller Han obtained from biaxial compression testing. The tensile stress caused by the
bending and diametral tests is more severe of a condition than biaxial compression which helps to explain
the difference in resilient moduli observed between tests. The upper portion of the asphalt concrete
pavement layer beneath a wheel loading is subjected to compression and the lower portion to tension. As
a result, probably the correct value of resilient modulus for use in design falls somewhere between the two
extreme types of tests.
The Miller et al. empirical correlation approach, which has been programmed for the P.C., offers
an excellent siting point for developing design relationships for asphalt concrete for a particular region.
Resilient modulus testing of the specific types of mixes (including aggregate typed used in the region is
needed to modify the approach as necessary for local conditions.
Aggregate Base
The resilient modulus of an aggregate base is strongly dependent upon the state of stress to which
an element of material is subjected. The resilient modulus is also affected to a much less degree by Me
following additional factors given in approximately decreasing order of importance: (~) degree of
saturation, (2~' aggregate size, (3) angularity, (4) density, and (5) surface roughness. For best performance
a minimum of 100% of AASHTO TI8O density should always be specified. Agency specifications also
control, within reasonably tight limits, aggregate size and grading. Variation in moisture content from
the optimum value can be considered using moisture sensitivity factors analogous to those discussed in the
next section.
Therefore, for practical design application only a limited number of independent variables
influencing resilient modulus need to be considered by most agencies. As a result, a table of design
resilient moduli can be prepared based on laboratory testing of the base and subbase materials used by a
particular agency. In areas where marginal materials of variable quality commonly are encountered,
caution must be exercised including performing periodically resilient modulus tests. Marginal materials
include materials with more than ~ to 9% fines, materials that contain plastic fines or Hose Hat degrade
significantly upon compaction.
Cohesive Subgrades
The resilient moduli of cohesive subgrades are well suited for developing empirical correlations
with basic physical parameters. The resilient moduli of cohesive subgrades vary from 2000 psi to 30,000
psi or more. Hence, as found in the study of the effect of variability on pavement Slickness, a modest
variation of up to 10 or 15% of the true resilient modulus has, for most conditions, only a small effect
on the overall design thickness.
272
OCR for page 273
I
Em pirical correlations for cohesive subgrades have been developed for use in design by Thompson
[85] In Illinois' Woolsturrn [118] in Nebraska, Li and Selig [57, 110] in Massachusetts, Pezo and Hudson
[119] In Texas, Sandla [120] in Georgia, Drumm et al. [121] in Tennessee, Elliott et al. [122] in Arkansas
and Burczyk et al. [123] in Wyoming. The most general approaches for estimating resilient moduli are
by Li and Selig [57, 110] and the approach developed during this study. Both of these very useful
approaches are summarized In Chapter 3. The other approaches referenced above generally use statistics
to relate the resilient modulus measured in the repeated load biaxial test to simple to measure physical
parameters.
Figure 130 shows the reasonably good correlation found between actual and predicted resilient
moduli(MR) using Me universal type equation of Uzan Table IS, Chapter 3) for selected cohesive soils
found in Georgia [1201. FiDy percent of the estimate values of resilient modulus (MR) were within i7%
of He measured values while 90% of the estimated values were within + 18%.
Moisture Sensitivity
Evaluating the moisture sensitivity of base, subbase and subgrade soils is an important aspect of
design. Moisture sensitivity is the change in resilient modulus (or permanent deformation) caused by a
change in moisture content of the material. Repeated load biaxial tests to evaluate He effects of the
environmental moisture cycle, as previously discussed, can be performed to evaluate moisture sensitivity.
For cohesive subgrade soils, Figure 131 and Table 61 illustrate two practical approaches used for
estimating the relative effects of moisture sensitivity on resilient modulus. The empirical correction, such
as given in Figure 131 and Table 6l, of a reference resilient modulus for varying moisture conditions
offers a practical way of handling moisture sensitivity.
PERMANENT DEFORMATION
The resilient modulus of pavement materials has received considerable publicity in recent years
because of its introduction in the 1986 AASHTO Design Guide. Me evaluation of permanent
deformation charactenstics of the asphalt concrete, base and s~ilgrade materials is just as unporta~ as
the KS ~= am hence saw Bier be fo~offen nor neglected. Tnd~, the evaluation of the
permanent deformation characteristics of He base is usually even more important than the evaluation of
resilient modulus. As previously discussed in of Chapter 3, the permanent deformation characteristics of
the base and subgrade can be readily determined as an extension of the proposed resilient modulus test.
This test is conducted using a repeated load 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. A loaded wheel tester could perhaps also be used to evaluate the permanent deformation
behavior of aggregate base and subgrade materials although this has apparently not been done in He past.
RESILIENT MODULI FOR DESIGN
Introduction
. . · . . . . .
The AASHTO design approach. as well as presently used methods based on linear elastic theory,
employ a single resilient modulus value for each layer. I-he values of resilient moduli selected for each
layer of a pavement system can have a strong influence on the required pavement thickness. The single
273
OCR for page 274
- - -
I IEE~ F~ lilE P~ ~ ~L ~vs. E~DI~ ~ A ~ 1 ~S, B ~ 2 O3S, El<:.
1 S=:L tfiED 1N ~: ~ E~IY IS ~1
~L 1 1
~1 1
25400 + '* +
1 / 1
srn + ,' i
1 ~1
2~0 ~A'
I A ,~MA I
17500 + AA'' AB Ak +
I A A ~A I
15000 + B AA CYY ~A
I A B Aj3
OCR for page 275
4
~ 3
A
(3 2
·~
1
a
, , , . - . 1 .,
~ F ~
. \
1 a\
. ~
. , . 1 , ~ . ~ .
10 15 20
Moisture Content, w (%)
25 30
Figure 131. Moisture sensitivity corrections for resilient modulus
for cohesive Texas soils (after reference 119)
Table 61 . Moisture sensitivity correction factors for selected Illinois
soils (after reference 85 and 111)
USDA Textural MR Decrease / 1%
Classification Moisture Increase
Nisi / %)
clay, silty clay, silty clay learn 0.7
silt loam I.5
10am 2.!
275
OCR for page 276
resilient modulus required for design has to be evaluated from laboratory test results at one reresentative
stress state. In laboratory testing, however, the use of a sequence of stress states is quite desirable to
insure reliable results.
The resilient modulus for all pavement layers is a sensitive function of stress state to which the
material is subjected. Little, if any, guidance, however, is usually given in design methods concerning
He stress state to use in determining the resilient modulus for a particular pavement geometry and wheel
loading. This very important aspect needs to be given more consideration in the future. For empirical
approaches, such as He present AASHTO memos, the stress states to use~should be clearly defined by the
memos if correct results are to be obtained; this is not done in the AASHTO memos. Use of He correct
stress state in selecting the resilient modulus, even if it is representative of the material, does not insure
this resilient modulus is He appropriate one to use in empirical approaches such as the AASHTO memos.
Vertical stress and confining pressure acting on a representative element of material, as discussed
previously in Chapter I, are the sum of the effects of the (~) wheel loading, (2) weight of material above
the element and (3) locked in residual stresses. . The representative stress state in the subgrade, base and
subbase varies wig several factors including the thickness of He layers and magnitude of wheel loadings.
Layered theory should, therefore, be used to develop generalized tables or charts for estimating the stress
state to use in selecting design resilient moduli for each layer. Development of these tables or charts must
be carried out as an integral part of He development of He design procedure.
Base and Subgrade
Proposed stress states at which the resilient modulus of an aggregate base can be evaluated for
are Riven in Chapter 3. The stress states given take into
relative comparisons of different materials
consideration nonlinear material behavior and, at least approximately, the effects of residual horizontal
stresses. The total vertical stress on He substrate in general decreases with increasing depth until material
~.
~' -- -A
weight becomes more important than the applied wheel loading. The vertical stress on He top of He
subgrade typically varies from about 3 or 4 psi for thick flexible pavement sections to about ~ to 10 psi
for Win sections. The confining pressure generally increases wig depth due to the weight of material
above. A representative confining pressure typically varies from about 2 to 4 psi for He subgrade.
Asphalt Concrete
An asphalt concrete surfacing or base mix suitable for use in a pavement should have a resilient
modules of at least 450,000 psi when measured in the diametral test at 68°F. Using He relationship
between resilient modulus and structural coefficient given in He 1986 AASHTO Guide (refer to Figure
128), a mix wig a 450,000 psi resilient modules has a spectral coefficient of 0.45. Frequently, however,
agencies use a maximum structural coefficient of about 0.45 in design. T/u~refore, if a limiting value of
0.45 is used in design, hide justification emus for performing a diametral resilient modulus test on high
qualify minces following the 1986 AASHTO design procedure. These findings indicate the design
procedure needs to be carefully reexamined. This problem hopefully will be corrected by the year 2002
lo. .. __ r A · ~ ~ · · ~ ~ ~ · ~
after the tHWA provides a mechanistic based pavement design procedure.
276
OCR for page 277
AASHTO Design Guide
Incorporation of reliability considerations and resilient modules concepts into the 1986 AASHTO
Design Guide [l 14] is certainly an important step forward. One of He weakest links in He design process,
however, is how He AASHTO approach utilizes the resilient modulus in flexible pavement design. The
resilient modules concept was added during He 1986 revision, and it does not appear to mesh very well
with the overall design process. Caution should be exercised when it is used. Also permanent deformation
considerations need to be incorporated into the method.
.
277
Representative terms from entire chapter:
resilient moduli
