Effects of coarse aggregate angularity and asphalt binder on laboratory-measured permanent deformation properties of HMA

Rutting has been identified as one of the primary distresses in asphalt pavements. Rutting in hot-mix asphalt (HMA) mixtures can be attributed to either the lack of interlocking of aggregate structure or insufficient bonding between aggregate and asphalt binder, or both. In the present study, efforts have been made to identify the contributions of aggregate structure and asphalt binder to the rutting characteristics of a dense-graded surface HMA mixture. Coarse gravels at five different angularity levels (100, 85, 70, 50 and 35% of aggregate with two or more fractured surfaces) were used to produce mixtures with similar aggregate gradations. Three different asphalt binders (PG 64-22, PG 76-22 and PG 82-22) were used to make mixtures for laboratory rut evaluations. The aggregate imaging system (AIMS), uncompacted voids in coarse aggregate (VCA) and tri-axial shear tests were conducted to evaluate the coarse aggregate angularity (CAA). The US Army Corps of Engineers' gyratory testing machine (GTM), creep and the asphalt pavement analyser (APA) tests were selected to characterise the rut resistance of asphalt mixtures. The results from this study indicated that coarse aggregate AIMS, VCA and tri-axial tests were related to the CAA and laboratory-measured rutting indices. At temperatures close to the binder's upper grade limit, aggregate structures played a critical role in the rut resistance of HMA mixtures; whereas, at temperatures below the binder's upper grade limit, the stiffness of the asphalt binder played a more important role in the rut resistance of asphalt mixtures evaluated in this study.