Effects of coarse aggregate angularity on the microstructure of asphalt mixture

The microstructure characteristic of asphalt mixture has substantial effects on its macroscopic mechanical properties. The study on the structural characteristics of asphalt mixtures involving the microscopic scale has rarely been considered. This study investigates the effects of the angularity of coarse aggregate on the microstructure of two commonly practiced asphalt mixture gradations (AC-16 and SMA-16) based on the X-ray Computed Tomography (XCT). Three-dimensional angularity was introduced based on the XCT images. Four indicators including average contact point number (ACN), average contact length (ACL), average vertical contact index (AVCI) and average horizontal contact index (AHCI) were proposed to evaluate the skeleton contact characteristics within the asphalt mixture through iPas. The volume of air voids, equivalent diameter and roundness were used to characterize the void characteristic of the asphalt mixtures with different angular aggregates by the analysis of Image-Pro Plus (IPP). The results indicate that the three-dimensional angularity (3DA) could characterize the angularity of coarse aggregate. The angularity has a significant influence on the skeleton contact characteristics of asphalt mixture, whereas a lower angularity has resulted in weaker skeleton contact characteristics. A skeleton contact characteristic of SMA-16, with a dense coarse aggregate structure, is stronger than AC-16, which belongs to the dense suspension structure. The shape of the voids at the top and bottom of the asphalt mixture specimen is the most complex and irregular, while the shape of air voids in the middle section of the specimen is simple and regular. This study could provide support for the further research and application of microscopic properties of asphalt mixtures.