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25 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATIONS GENERAL RECOMMENDATIONS FOR mined using the IDT appear to agree more closely with com- LOW-TEMPERATURE CREEP AND STRENGTH pliance values determined in uniaxial compression. Because TESTING OF ASPHALT CONCRETE of the extensive work done on the IDT test and analysis-- especially the calibration of the Superpave thermal cracking The IDT geometry was originally selected during SHRP model to field studies--it is recommended at this time that for use in low-temperature characterization of asphalt concrete the IDT creep and strength test be retained as the primary mixtures primarily because the specimen preparation methods method of evaluating the low-temperature properties of available at that time did not include ways of making speci- asphalt concrete mixtures. mens suited for uniaxial measurement of creep compliance, relaxation modulus, or strength. The simple performance tests developed as part of NCHRP Project 9-19 and the dynamic Compliance Measurements modulus master curve characterization methods for struc- tural design recommended in NCHRP Project 1-37A require Although determining compliance in uniaxial compression specimens 100 mm in diameter and 150 mm high to be used is potentially simpler, quicker, and more economical than in uniaxial testing. Therefore, this obstacle to uniaxial testing using the IDT test, these procedures do not provide inter- no longer exists. Uniaxial testing would also potentially allow changeable data. The compliance determined using the IDT the use of relaxation modulus tests, rather than creep tests, test is generally somewhat lower than that determined in uni- which would eliminate the need to calculate the relaxation axial compression and much lower than that determined in modulus from the creep compliance. However, relaxation tests uniaxial tension. This is most likely the result of anisotropy have not been widely performed on asphalt concrete mixtures; in asphalt concrete specimens prepared using the gyratory and, for practical purposes, the creep test should probably be compactor. The compliance in the diametral plane appears to retained regardless of test geometry. A review of the equip- be significantly lower than that in perpendicular planes (e.g., ment required to perform dynamic modulus master curve test- along the length of the specimen). Although the gyratory ing indicated that, with only minor modifications, it could be compactor may not always replicate the conditions of field used to perform low-temperature uniaxial creep tests. This compaction, it seems likely that similar anisotropy exists in would have several advantages: situ and that the IDT creep and strength test is probably the best approach to providing estimates of the properties of Cost savings on purchase of test equipment; asphalt concrete in place. Uniaxial compression is suitable Cost savings on purchase of specimen preparation equip- for determining creep compliance for research purposes, but ment and test accessories; it must be realized that the resulting data may not accurately Cost savings on training engineers and technicians to reflect in situ properties or the results of the IDT or other pro- prepare specimens and perform tests; cedures. In general, pavement engineers and researchers Greater reliability of data due to greater experience with should recognize the anisotropic nature of asphalt concrete a single test geometry and test device; and and make certain that the properties they are using for spec- Greater flexibility in scheduling testing, if more than ification and design purposes are determined using appropri- one device is needed in a lab. ate and uniform methods. For these reasons, significant effort was expended in the labo- Strength Measurements ratory testing of Phase III of NCHRP Project 9-29 to evaluate uniaxial tensile creep testing as the standard low-temperature The IDT strength procedure as currently described in test for asphalt concrete. AASHTO T322 involves using LVDTs to determine the true Unfortunately, the laboratory testing and analysis indicated point of failure and associated tensile strength. This procedure that compliance values determined in uniaxial tension were often results in damaged or destroyed LVDTs and is not prac- significantly higher than those determined using the IDT test. tical. Phase III of Project 9-29 found that a reasonably good Furthermore, the correlation between the two sets of data was relationship exists between uncorrected IDT strength and IDT not extremely strong. In fact, the compliance values deter- strength determined using the more accurate, instrumented