Porous asphalt mixture (PAM) is a type of hot mix asphalt that allows water to drain through the pavement surface due to its high air voids content, generally around 20%. Gradation design is one of the crucial factors for PAM that determines the permeability and strength of PAM. Generally, coarse aggregates have the function to create a stone-on-stone framework to support the load, while fines fill the voids created by the coarse aggregates. Initial study indicated that fine aggregates have a roller effect on coarse aggregates during the compaction process. When compaction load is applied, fine aggregates roll over each other and give lubrication effect for coarse aggregates to re-orient more easily. Roller effect is undesirable in PAM because it will create a denser packing framework, therefore the proportion of fine aggregates need to be controlled. However, insufficient amount of the finer fractions may compromise the skeleton strength of PAM. In this study, ten gradations were utilised by varying the proportion 2.36-4.75mm fractions. Aggregates passing sieve size 2.36mm are generally less than 15% to maintain high air voids in PAM, thus 2.36-4.75mm fractions are the main concern. Unbound aggregates and PAM models were built in Particle Flow Code 3 Dimension (PFC3D), which is a software based on Discrete Element Method (DEM), while measurements from laboratory specimens were obtained as verification. The results showed that aggregate packing structure is greatly affected by aggregate gradation, in terms of volumetric properties and mechanical properties. The results also indicated that the existence of roller effect decrease compaction energy, and increasing the proportion of fine aggregates will increase the roller effect and creates denser packing of PAM. The research provides in-depth analysis of PAM packing structure, in terms of volumetric and mechanical properties, by laboratory measurements and DEM simulation.

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