The performance of porous asphalt concrete (PAC) is dependent on its microscopic void features, which may refer to content and distribution of its constituents. In other words, PAC mixtures with identical air voids may have significantly different performances under given loading and environmental conditions. Air voids and their distributions are determined by aggregate gradation, nominal maximum aggregate size (NMAS), and binder content. Therefore, it is necessary to investigate the correlation among material composition, microscopic void features, and material performance. In this study, X-ray computed tomography (CT) images were obtained and analyzed to study microscopic void features of different PAC mixtures. Experimental tests were performed to study permeability, acoustic absorption coefficients, clogging, shear strengths, wheel tracking, and Cantabro loss of those PAC mixtures. The following findings were observed: (1) microscopic void features were significantly affected by gradation and NMAS; (2) for PAC mixtures with the same air content and NMAS, the coarser graded mixtures had the larger equivalent diameter of air voids; and (3) linear relationships were found between equivalent diameter and material performance, including Cantabro loss, dynamic stability, shear strength, anticlogging property, and noise reduction.
