Optimum Number of Superpave Gyrations Based on Project Requirements

The Superpave® mix design method was developed as part of the SHRP Asphalt Research Program, which extended from 1987 to 1993. The concept behind the Superpave method was that of incorporating performance, environmental conditions, load factors, and material characterization into one design and evaluation process to improve the performance of the asphalt mixtures by reducing rutting, thermal cracking, and fatigue cracking. Several years have passed since the first Superpave mixtures were laid down in numerous states, including Texas. With the introduction in the design process of rolling wheel testers, such as the Hamburg wheel tracking device (HWTD), it has often been observed that although the mixtures perform well with regard to rutting resistance, some pavement sections begin to crack relatively early. One possible reason for this occurrence is that the Superpave volumetric mix design method (Level 1) and the performance requirements for tests such as the HWTD promote asphalt mixtures with relatively low asphalt contents. The mixtures have superior rutting resistance but often sacrifice cracking performance. One way to increase the cracking resistance of Superpave asphalt mixes is to reduce the number of design gyrations (N_design) currently used in the Superpave gyratory compactor. This paper introduces a simple method for selecting an optimum number of gyrations based on the performance requirements of the specific project with regard to simultaneous rutting and fatigue. The objective is to produce asphalt mixes that are more resistant to fatigue cracking without compromising their already established superior rutting performance.