Assessment of Packing Structure of Porous Asphalt Mixture (PAM) Based on Image-Based Analysis

Porous asphalt mixture (PAM) is designed to have high air voids content to let water run-off permeates the pavement surface. In this study, image-based analysis was conducted to evaluate the packing structures of three different PAM gradations, namely PAM-1 (largest proportion of coarse aggregates), PAM-2 (intermediate proportion of coarse aggregates), and PAM-3 (least proportion of coarse aggregates). Three packing parameters were calculated, namely number of contact zones, contact length, and tortuosity. Laboratory tests, namely permeability test, Marshall stability test, indirect tensile stiffness modulus and creep test, were performed to evaluate the properties of PAM. The results showed that contact length has close correlation with mechanical properties of PAM, as a longer contact length indicates better aggregate interlocking. Herein, packing structure of PAM-2 had sufficient number of contact zones with the longest contact length and least tortuosity on the average, indicating that aggregate interlocking in PAM-2 was the sturdiest. The study demonstrates that image-based analysis is a useful technique for studying the quality of the packing structure of PAM.