A micromechanical approach to investigate asphalt concrete rutting mechanisms

► We determined the changes in asphalt concrete (AC) microstructure under trafficking. ► We determined the contribution of densification to total accumulated AC rutting. ► Majority of densification is occurring at the bottom of construction lift for the PG64-28PM mix. ► A significant shear flow pattern in the direction of traffic was observed for RHMA-G test sections. ► Earlier failure of RHMA-G is due to the gradation and aggregate size induced excessive shear flow. In this study, a new approach was used to evaluate changes in asphalt concrete microstructure under full-scale accelerated pavement test loading with a Heavy Vehicle Simulator (HVS) of composite pavement. The approach compared X-ray computed tomography (CT) images taken before and after HVS rut testing. Results were used to identify the differences in the movement of aggregate and changes in air-void content and distribution occurring during rutting accumulation of rubberized gap graded and polymer modified dense graded mixes for two overlay thicknesses (64 and 114mm). Although high air void content for the sections constructed with rubberized gap graded mix were expected to cause more densification related rutting and earlier failure related to this densification, the actual reason behind the earlier failure was determined to be primarily greater shear flow to the sides of the wheelpath associated with the gap gradation and small aggregate size. Significant movement of aggregate was observed in the direction of travel as well as to the side under the pushed wheel. Important differences in aggregate movement and air-void changes were also observed between different overlay thicknesses indicating the depth of the rut phenomenon, important information for the design of overlays on aged asphalt concrete as well.