AN ALTERNATIVE METHOD FOR MEASURING PERMANENT DEFORMATION OF HOT MIX ASPHALT MIXTURES

Abstract

More than twenty Department of Transportations are currently using a wheel tracking test to measure rutting resistance of mixtures that already meet volumetric properties (Buchanan, 2016). With the expansion of using recycled materials and warm mix additives, it is clear that using volumetric properties alone to approve asphalt mixture designs is a risky approach. Wheel-Tacking tests are among the most widely used methods for evaluating rutting resistance, and the AASHTO T 324 (HWT) is the most widely accepted and followed procedure used today in the USA (Mohammad, 2015). However, there are challenges using the HWT among which the most difficult are the poor repeatability, time required to complete the test, and the sample preparation details. This study reports on an alternative wheel tracking method called the Rotary Asphalt Wheel Tester (RAWT) that can successfully address the challenges currently faced with using the HWT device. The method requires no cutting of the gyratory samples, significantly reduces time to complete the test and collect results, and appears to offer acceptable repeatability of the results. The method has existed commercially for more than 15 years, and although used only in a few labs, acceptance criteria for the test is already developed by one agency. The study includes evaluating an expanded set of mixtures tested at two temperatures, and two air void contents. The results of the RAWT are compared with the results of the HWT for a subset of all the mixtures tested to show the sensitivity of the RAWT to critical mixture variables. The mixtures included in this study were prepared from two sources, both sources utilizing a performance grade (PG) of a ‘58S’, which is a neat asphalt and a PG ‘58H’ grade, which includes binder modification. The mixtures were tested at 46°C and 52°C, and compacted to three and seven percent air voids. The mixture sources differ in that one was lab produced using aggregates from the northern part of the State and is known to have a high moisture damage potential, whereas the other source was plant produced using aggregates from the southern part of the state and was sampled as loose mixture from a truck and delivered in ~15,000 gram boxes. The mixture produced from the northern aggregates included samples produced with and without the use of anti-strip, resulting in twice as many samples tested than those produced from the plant southern source. The results collected in the study are encouraging in that the RAWT is found sensitive to many mix design factors known to impact rutting resistance, including the type and source of aggregates, binder grade, air void content, and testing temperature. The RAWT may also offer a more practical rutting resistance test due to the device’s configuration. Furthermore, there is no sample preparation needed to configure the sample after determining the air void content. To complete testing of one sample, a significant amount of time required for the HWT is reduced, allowing for multiple samples to be tested per day. Lastly, software is provided with the RAWT to conduct the analysis of tested samples. It is also found that variability of test results is significantly reduced as compared to the HWT. Considering these encouraging findings, this device can be considered a potential alternative to the current HWT device.