A novel method for characterizing the random distribution of mortar thickness in asphalt mixtures

Mortar thickness plays a crucial role in the adhesion/cohesion properties of mortar, and its distribution further affects the stress response, mechanical, durability, and fatigue performance of asphalt mixtures. However, the random distribution of mortar thickness (RDMT) in asphalt mixtures remains inadequately characterized. X-ray computed tomography (CT), digital imaging processing, and sampling mask with different orientations are employed to determine the mortar thickness in seven asphalt mixture designs. These mixtures include open-graded, dense-graded, and gap-graded asphalt mixtures with nominal maximum aggregate size (NMAS) of 13.2 mm, 16 mm, and 19 mm. Three segmentation methods (parallel line, sector, and ring segmentation) with different segmentation numbers are applied on CT images to investigate the RDMT. The variance of mortar thickness, coefficient of variation, and range of mortar thickness are used to evaluate the RDMT. The orientations of the sampling mask have a negligible impact on the distribution of mortar thickness. In contrast, the segmentation methods and segmentation numbers significantly affect the evaluation results of RDMT. Parallel line segmentation with 36 segments demonstrates the best potential and is recommended for characterizing RDMT. The average mortar thickness (T_average) and RDMT are increased with the increasing NMAS, with a particularly significant increase observed when NMAS changes from 13.2 mm to 16 mm. The gap-graded asphalt mixtures exhibit the greatest T_average and RDMT when comparing to the dense-graded and open-graded asphalt mixtures. Furthermore, the T_average and RDMT are smaller at the top and bottom of the specimen compared to the middle section. At the middle section, the T_average and RDMT exhibit fluctuations with multiple peaks, and the distance between two adjacent peaks is approximately equal to the NMAS. This study provides a simplified method to characterized the complicated RDMT and lays a foundation for investigating the role of RDMT in the meso-mechanics of asphalt mixtures.