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procedure of AASHTO T322. It is recommended that the IDT of thermal contraction of asphalt concrete mixtures have
strength test be performed without LVDTs and that the un- not been well developed or widely used and are not highly
corrected strength determined using the maximum load then accurate. A simple improved procedure for estimating the co-
be adjusted to estimate the corrected IDT strength using the efficient of thermal contraction was developed in this project
empirical relationship presented in this report as Equation 8. and provides reasonably accurate results.
Proposed Changes to AASHTO T322 ESTIMATING CREEP COMPLIANCE
AND STRENGTH VALUES
The requirements of AASHTO T322 have been reviewed,
and a number of relatively minor changes have been recom- NCHRP Project 1-37A recommended a three-level hierar-
mended. Specific changes in transducer specifications and chical system for determining inputs for flexible pavement
most other requirements included in this procedure were pre- design and analysis. For thermal cracking, IDT creep and
sented previously in Table 2. Recommended requirements for strength measurements in accordance with AASHTO T322 are
specimen dimensions and uniformity were listed in Table 3. needed for the most reliable, Level 1 determination. Level
A revised loading protocol is given in Table 4. 2 uses reduced IDT testing at a single temperature; Level 3 is
Another important recommendation made in this study based on typical compliance and strength values for mixtures.
is that the temperatures used for low-temperature creep and In Project 1-37A, predicted thermal cracking based on Level
strength tests should vary according to the stiffness of the mix- 3 input data did not correlate well with measured thermal
ture. For asphalt concrete mixtures made using PG XX-22 cracking for 36 Long Term Pavement Performance (LTPP)
and PG XX-28 binders, the current test temperatures of -20, sections used to calibrate the Level 3 analysis.
-10 and 0°C should be retained. For mixtures made using PG Work performed during Phase III of NCHRP Project 9-29
XX-16 binders or harder, these temperatures should all be suggests that better Level 2 and 3 thermal cracking input data
increased by 10°C. Similarly, for mixtures made using PG might be obtained by determining compliance values using the
XX-34 binders or softer, test temperatures should be decreased Hirsch model (4) and estimating tensile strength from VFA
by 10°C. Highly aged mixtures should also be tested at the using Equations 8 and 9. In evaluating the effects of differ-
higher test temperatures. Tensile strength tests should always ences in air void content on creep compliance, compliance
be performed at the middle creep test temperature. This pro- estimates were made for the mixtures tested in this study using
tocol will help ensure good test precision and will also help the Hirsch model; binder compliance values were estimated
avoid problems that occur when the maximum relaxation time from bending beam rheometer (BBR) test data. Mixtures made
in the Prony series is exceeded during analysis of creep data. with the modified asphalt (PG 76-22) were not included in
this analysis, because only one set of BBR data was avail-
able, rather than the two needed to develop reasonable creep
Precision of the IDT Creep And Strength Tests stiffness estimates over a range of temperatures and loading
times. BBR data were empirically adjusted from the Pres-
Anderson and McGennis (3) evaluated the precision of the sure Aging Vessel (PAV) to the Rolling Thin Film Oven
IDT strength test and reported a standard error for n = 3 repli- Test (RFOT) condition, based upon typical test data as reported
cates of about 7 percent. A precision study of the IDT creep by Christensen and Anderson in their study of the SHRP
tests was performed as part of NCHRP Project 9-29 Phase III, asphalt binders (14). The resulting estimated compliance
which included numerous mixtures from six different labora- values were in excellent agreement with those measured
tories. Evaluation of these data resulted in estimated standard with the IDT test, as shown in Figure 23. This figure demon-
errors for compliance for n = 3 replicates of 8 to 11 percent
expressed as a percentage of the mean (coefficient of variation,
or C.V.). This corresponds to a d2s precision of 22 to 32 per-
1.E-04
cent. The laboratory testing executed in this project gave
Compliance, IDT, 1/psi
nearly identical results, with an estimated C.V. of 9 percent. y = 1.04x
The precision for the IDT strength test appears to be accept- 1.E-05
2
R = 88 %
able. The precision for the IDT compliance procedure, on the
other hand, needs to be improved as part of the implemen-
MD DBASE
tation process. Further standardization of the procedure and 1.E-06 PA GRVL
VA GRNT
equipment should help achieve improvements in precision. VA LMSTN
Equality
1.E-07
COEFFICIENT OF THERMAL CONTRACTION 1.E-07 1.E-06 1.E-05 1.E-04
Compliance, Hirsch, 1/psi
The equation developed during SHRP for estimating mix-
ture coefficient of thermal contraction is not accurate and Figure 23. Compliance values as estimated using the
should be abandoned. Methods for the laboratory measurement Hirsch model and as measured using the IDT test.
