Rut initiation mechanism's in asphalt mixtures as generated under accelerated pavement testing

The Florida Department of Transportation (FDOT) conducted an experiment to addiess the effects of polymer modifiers on the performance of Superpave (R) mixes by using a heavy vehicle simulator. Two fine-graded Superpave mixes were considered. One mix included a virgin binder meeting the requirements of PG 67-22, and the other contained a styrene-butadiene-styrene polymer (SBS)-modified binder meeting those of PG76-22. Both respective mixes contained the same effective binder content, aggregate components, and gradation. The mixes were designed for 16 million to 30 million equivalent single-axle loads with the use of 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, whereas rutting in the SBS-modified mixture was caused mainly by 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.