A Performance-based Evaluation of Superpave Design Gyrations for High Traffic Surface Mixes

AbstractThe number of design gyrations (Ndes) is an extremely important parameter in the asphalt concrete mix design procedure using the Superpave method. The choice of Ndes is a function of total traffic during the pavement service life, expressed in ESALs. Asphalt concrete mixes for higher traffic levels are compacted to a higher Ndes as a denser mix resists rutting more effectively. However, this leads to a lower design asphalt content, which decreases fatigue performance of the mix. Therefore, a performance-oriented approach to determine Ndes was developed that optimizes mix performance with respect to both rutting and fatigue cracking.In this paper, the research methodology adopted to compare the relative performance of surface mixes with 12.5mm nominal maximum aggregate size is presented, along with recommended Ndes values for C and D-level mixes, designed to handle traffic levels of 3-30 Million and greater than 30 Million ESALs, respectively. Asphalt concrete mixes were designed at Ndes levels of 50, 75, 100 and 125 gyrations to determine asphalt content using Superpave design method. Dynamic modulus (E*) was measured at design asphalt content for different gyration levels using the Asphalt Mixture Performance Tester device. The E* data and corresponding binder properties were used as input in the AASHTO Darwin-ME software to predict rutting and fatigue performance of the mixes by assuming a model pavement section and appropriate traffic levels. Relative performance indicators were developed for both rutting and fatigue, and plotted against the asphalt content to determine optimum Ndes. For both surface mixes, the optimum Ndes value was determined to be 85 gyrations