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29 Based upon the literature review, review of AASHTO T322, and the results of a substantial laboratory testing program per- formed during Phase III of NCHRP Project 9-29, the following conclusions and recommendations are made: ⢠The IDT creep and strength test should be retained as the standard method for determining the creep compliance and tensile strength of asphalt concrete mixtures at low temperatures. ⢠Asphalt concrete specimens compacted in the laboratory exhibit substantial anisotropy in their creep compliance at low temperatures. The compliance measured across the diameter of the specimen is greater than that measured in both compression and tension along the length of the specimen. This anisotropy does not appear to be caused by differences in air void content or air void distribution, but is probably the result of preferential aggregate par- ticle orientation that occurs during compaction. This anisotropy is the primary reason for preferring the IDT creep and strength tests over other procedures. ⢠Pavement engineers and researchers should be careful when using compliance and modulus values for asphalt concrete in pavement design and analysis, because these and related properties are potentially anisotropic and their values will depend upon the direction and sense of the applied stress with respect to the orientation relative to the compaction process. ⢠The IDT strength test should be performed without LVDTs. The uncorrected IDT strength should be calcu- lated based on maximum load, and the corrected or âtrueâ strength estimated using the empirical equation devel- oped in this project. ⢠Test temperatures for the IDT creep and strength test should be linked to binder grade: â20, â10, and 0°C for PG XX-28 and PG XX-22 binders; â10, 0, and +10°C for PG XX-16 and harder; and â30, â20, and â10°C for PG XX-34 binder and softer. Reasonable adjustments should be made for testing field cores, which may exhibit sub- stantial age hardening. ⢠A number of relatively minor revisions listed in this report should be made to AASHTO T322. These have been forwarded to the task force responsible for recom- mending revisions to this test method to AASHTO. ⢠The coefficient of variation of creep compliance values measured using the IDT test was found to range from about 8 to 11 percent for tests performed at a number of different laboratories using several different test systems. This variability is probably somewhat high for a standard test method, but improved test procedures and equipment will help to reduce this to an acceptable level. The preci- sion of the IDT strength test is probably acceptable in its current form. ⢠Ruggedness testing for the AASHTO T322 creep test procedure is the next logical step in the implementa- tion of this procedure. An initial ruggedness testing plan and estimated level of effort are provided in this report. ⢠Additional research is suggested in two areas. Further testing and analysis is needed to refine the relationship between uncorrected IDT strength and the actual strength as determined using the procedure currently given in AASHTO T322 (using LVDTs to determine the point of failure). Research should also be undertaken to evaluate the accuracy of critical cracking temperatures determined using the Hirsch model to estimate creep compliance values and empirical methods for determining approxi- mate tensile strength. Such a procedure would be useful for general mixture selection, mixture design guidance, quality control applications, and as a possible replace- ment for the current Level 2 and 3 thermal cracking data input for the pavement design guide developed in NCHRP Project 1-37A. CHAPTER 4 CONCLUSIONS AND RECOMMENDATIONS