Rut Initiation Mechanisms in Asphalt Mixtures as Generated Under Accelerated Pavement Testing (05-1658)**
Salil Gokhale, Dynatest Consulting, Inc.
Bouzid Choubane, Florida Department of Transportation
Thomas Byron, Florida Department of Transportation
Mang Tia, University of Florida

The Florida Department of Transportation (FDOT) conducted an experiment to address the effects of polymer modifiers on the performance of Superpave mixes using a Heavy Vehicle Simulator (HVS). Two fine-graded Superpave mixes were considered. One mix included a virgin binder meeting the requirements of PG 67-22, while the other contained a SBS polymer-modified binder meeting those of PG 76-22. Both respective mixes contained the same effective binder content, aggregate components and gradation. The mixes were designed for 10-30 million ESALs, using the standard Superpave mix design methodology. During placement of these mixes, all standard FDOT density requirements and acceptance criteria were applicable. The subsequent investigation showed that the sections with SBS-modified mixture significantly outperformed those with the unmodified mixture. It was also determined that rutting in the unmodified mixture was primarily a function of shear flow while rutting in the SBS-modified mixture was due mainly to densification. This paper presents a description of the testing program, the data collection effort and the subsequent analyses and findings focusing primarily on the initiation mechanisms of rutting in asphalt mixtures as generated and observed under accelerated pavement testing.