Evaluation of aggregate segregation in gyratory specimens of porous asphalt mixture (PAM) by image-based analysis

Porous asphalt mixture (PAM) comprises a large content of coarse aggregates, making it more prone to aggregate segregation. The high proportion of coarse aggregates in PAM is designed so that it has sufficient air voids to allow water to soak through the pavement surface. Meanwhile, aggregate segregation not only can degrade the mechanical strength of PAM but also disturb its drainage capability. It is critical to select an appropriate PAM gradation that not only possesses a high voids content but also has well-distributed voids. Previously, aggregate segregation was investigated by methods that require attendant costs and/or require complicated equipment. In this research, easy-to-operate image-based analysis software was utilised to assess aggregate segregation of PAM consisting of different gradations. Aggregate gradation is one of the key factors that determine the final characteristics of PAM; therefore, it is important to find a gradation that generates less segregation. Three PAM gradations were used: PAM-C (the largest content of coarse aggregates), PAM-I (intermediate content of coarse aggregates), and PAM-F (the lowest content of coarse aggregates). The results indicated that all PAM gradations suffered from aggregate segregation, with PAM-I having the least segregation, whilst PAM-F had the greatest segregation in both vertical and radial directions. As PAM-F suffered the greatest segregation, it generated a mixture with the most tortuous voids. Therefore, PAM-F is expected to have the worst drainage performance compared to others as voids tortuosity has an obstructive impact on the permeability of the mixture. The results also showed that higher aggregate segregation negatively affects the durability of PAM as it can reduce its resistance to permanent strain.