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EVALUATION OF INDIRECT TENSILE TEST (IDT) PROCEDURES FOR LOW-TEMPERATURE PERFORMANCE OF HOT MIX ASPHALT SUMMARY The indirect tensile (IDT) creep and strength tests were developed during the Strate- gic Highway Research Program (SHRP) to characterize the resistance of hot mix asphalt concrete (HMA) to low-temperature cracking (1,2). Currently, the IDT creep and strength tests are considered the most promising for predicting the low-temperature performance of asphalt concrete mixtures. Many contractors and state highway agen- cies are interested in using these test procedures but feel that additional information is needed on the test device, procedures, and analysis. Therefore, the objective of Phase III of National Cooperative Highway Research Program (NCHRP) Project 9-29, as stated in the expanded scope of work, was to "evaluate and refine the IDT procedures pro- posed for use as the simple performance test for low-temperature cracking and as the materials characterization test for low-temperature cracking in the pavement design guide developed in NCHRP Project 1-37A." This report documents work completed in Phase III of NCHRP Project 9-29. The requirements of AASHTO T322, Standard Method of Test for Determining the Creep Compliance and Strength of Hot-Mix Asphalt (HMA) Using the Indirect Tensile Test Device, have been reviewed, and a number of relatively minor changes have been sug- gested. These include the capacity of the loading machine, the range of the environmen- tal chamber, the sensitivity of the load and deformation measurements, and the use of neoprene strips in the IDT loading fixture. Recommended requirements for specimen dimensions and uniformity were also developed based on those prepared for the simple performance test specimens. Guidance on load levels to maintain strains within the allow- able limits was also developed. Another important suggestion made in this study is that the temperatures used for low-temperature creep and strength tests should vary accord- ing to the stiffness of the mixture. For asphalt concrete mixtures made using PG XX-22 and PG XX-28 binders, the current test temperatures of -20, -10 and 0C should be retained. For mixtures made using PG XX-16 binders or harder binders, these tempera- tures should all be increased by 10C. Similarly, for mixtures made using PG XX-34 binders or softer binders, test temperatures should be decreased by 10C. Tensile strength tests should always be performed at the middle creep test temperature. This protocol will help ensure good test precision and will also help avoid problems that occur when the maximum relaxation time in the Prony series is exceeded during analysis of creep data. Anderson and McGennis evaluated the precision of the IDT strength test and reported a standard error for n = 3 replicates of about 7 percent (3). A precision study of the IDT creep test was performed as part of Phase III of NCHRP Project 9-29, which

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2 included numerous mixtures from six different laboratories. Evaluation of these data resulted in estimated standard errors for compliance for n = 3 replicates of 8 to 11 per- cent, expressed as a percentage of the mean. This corresponds to a d2s precision of 22 to 32 percent. The precision for the IDT strength test appears to be acceptable. The preci- sion for the IDT creep procedure, on the other hand, needs to be improved as part of the implementation process. Further standardization of the procedure and equipment should help achieve improvements in precision. The equation developed during SHRP (1) for estimating mixture coefficient of ther- mal contraction is not accurate and should be abandoned. However, methods for the laboratory measurement of thermal contraction of asphalt concrete mixtures have not been well developed or widely used and are not highly accurate. A simple, improved procedure for estimating the coefficient of thermal contraction has been developed and provides reasonably accurate results. A laboratory test program was executed in this study to evaluate and compare mea- surement of creep compliance using the IDT, uniaxial compression, and uniaxial ten- sion tests. An experiment comparing strength measurements was also performed to compare uncorrected IDT strengths, IDT strengths corrected using Roque's procedure as outlined in AASHTO T322, and strengths determined in uniaxial tension. Analysis of the resulting data indicates that asphalt concrete at low temperatures is anisotropic-- that is, the properties vary depending upon the axis loaded. The compliance measured using the IDT test was lower than the compliance determined in uniaxial compression, which in turn was lower than that determined in uniaxial tension. It would appear that the modulus of asphalt concrete in the plane perpendicular to compaction is gen- erally higher than that determined in the plane parallel to compaction. Because of this anisotropy, uniaxial creep data cannot be used interchangeably with IDT creep data, and the IDT creep and strength test should be retained for use in characterizing the low- temperature properties of asphalt concrete. It was also found that the uncorrected IDT strength, as determined using the maximum load, is significantly higher than that deter- mined using Roque's procedure, in which the failure load is determined through mon- itoring specimen deformation. However, the uncorrected IDT strength correlates well with the corrected IDT strength. Because of the danger of damaging transducers dur- ing the strength test, it is recommended that the IDT strength be determined by using the maximum load during the test and applying an empirical correction to estimate the corrected IDT strength. Comparison of measured compliance values with those estimated using the Hirsch model (4) showed good agreement, and IDT strength values correlated well with voids filled with asphalt (VFA) values. This suggests that critical temperatures can possibly be easily and accurately estimated from binder test data and mixture volumetric com- position. Such a procedure might be useful in situations where time and/or money is not available for IDT creep and strength tests or for quality control purposes. Addi- tional research is suggested to compare critical temperatures estimated in this fashion with those determined using the IDT creep and strength procedure.