The Superpave design method requires that asphalt concrete mixtures satisfy various volumetric requirements at specific levels of compactive effort in the Superpave gyratory compactor. These levels are a function of climate and total traffic during the pavement service life, expressed in Equivalent Single Axle Loads (ESALs). Asphalt concrete mixtures for higher traffic levels are compacted to a higher number of design gyrations (Ndesign) as a mix that resists further compaction also resists rutting more effectively. This often negatively impacts the fatigue performance due to lower asphalt content in the mix. Therefore, a performance-oriented approach to determine Ndesign was developed that optimizes mixture performance with respect to both rutting and fatigue cracking.
Surface mixes in North Carolina are designated on the basis of nominal aggregate size (S9.5mm or S12.5mm) and traffic level (A, B, C and D). Asphalt concrete mixes were designed at Ndesign levels of 50, 75, 100 and 125 gyrations for six different surface mixes. Asphalt content was determined for each mix using the Superpave design method. Dynamic modulus specimens were prepared at the determined optimum asphalt contents and dynamic modulus (E*) was measured using the Asphalt Mixture Performance Tester (AMPT) device. The E* data and corresponding binder properties were used as input in the AASHTO Darwin-ME software to predict the rutting and fatigue performance of the mixtures by assuming a model pavement section and an appropriate traffic level.
The concept of relative performance was used to determine the optimum Ndesign such that performance of the mix is optimized with respect to both fatigue cracking and rutting. The recommended value of Ndesign for each type of surface mix was determined after comparing the calculated optimum gyrations with the existing North Carolina DOT specifications, as well as analyzing the relative performance characteristics of the mix.
