Evaluation of low-temperature properties of HMA mixtures

The deterioration of flexible pavements due to low-temperature cracking is a significant and costly problem in the state of Nevada. The Nevada Department of Transportation initiated several research efforts aimed at exploring Nevada's problem with this distress. The research evaluated several newly developed low temperature performance tests under Nevada's conditions. The goal of the research was to determine the applicability of the tests for characterizing the low-temperature response of Nevada's asphalt binders and hot-mixed asphalt (HMA) mixtures. This paper summarizes Nevada's experience with the Strategic Highway Research Program low temperature tests and specifications, highlighting the effectiveness of the Superpave Performance Binder Grading System and the thermal stress restrained specimen test (TSRST). The contribution of asphalt aging to Nevada's cracking problem is also included in the paper. An investigation of the Superpave Performance Graded Binder tests has determined that the bending beam rheometer and the direct tension test correlate very well and it may not be necessary to run both tests, as they are set up in the current Superpave specifications. The TSRST appears to provide the greatest value for evaluating low-temperature properties of HMA mixtures. Findings from the research indicate that there are some significant correlations between the low-temperature properties of asphalt binders and HMA mixtures if the mixtures are aged appropriately. This emphasizes the need to implement the appropriate conditioning procedure when low-temperature cracking is used as part of the mix design and evaluation process. On the other hand, the research showed that, when using polymer-modified asphalt binders, the low-temperature grade of the asphalt binder may be conservative enough that testing of the HMA mix may not be necessary.