Micromechanical investigation of open-graded asphalt friction courses’ rutting mechanisms

• Rutting mechanisms of open-graded asphalt friction courses (OGFC) under full-scale loading is investigated.
• We measured the full-scale load related changes in OGFC microstructure using X-ray CT imaging.
• Thick DGAC layers under OGFC might also induce early rutting failures.
• Densification is the major factor controlling OGFC and DGAC rutting.
• OGFC layer thickness is the major factor affecting the OGFC rutting mechanisms and performance.

This study aims to identify the permanent deformation accumulation mechanisms of open graded friction course (OGFC) mixes under full-scale loading. Changes in OGFC and underlying dense graded asphalt concrete (DGAC) layers’ microstructures under full scale trafficking were determined using X-ray computed tomography (CT) images taken before and after accelerated pavements tests (APT). Image processing and particle tracking methods were used to identify changes in air-void and aggregate distributions to evaluate the rutting mechanisms of multi-layered (OGFC on DGAC) asphalt concrete sections. Densification was observed to be the major factor controlling the rutting on test sections while shear related deformation was just a minor contributor. Based on APT and laboratory test results, OGFC layer thickness was determined to be the major factor affecting the OGFC rutting mechanisms and performance. Displacement vector distributions for the OGFC layers appeared to be larger than the distributions for the DGAC layers due to the high initial air void content for the OGFC layers. However, significant deformation of the DGAC layers suggested that thick underlying DGAC layers might also induce early rutting failures. Results of this study provide experimental data for continued development of multi-scale continuum mechanics and discrete element method analysis approaches for multi-layered flexible pavement structures.