Evaluation of the development of aggregate packing in porous asphalt mixture using discrete element method simulation

Packing in coarse aggregates is the main factor for porous asphalt mixture's (PAM) resistance to external loading and deformation. Eight PAM gradations were designed, and aggregate backbone generated in the aggregate blend with particles larger than 2.36 mm was assessed through voids content in the aggregates under dry-rodded condition (VA_DRC) for four types of unbound aggregate blend (i.e. Blend-1∼Blend-4). Corresponding models, namely Model-1∼Model-4, were generated for each gradation design using discrete element method (DEM). Through the effect of sequentially adding increasingly finer particle components on the voids content, aggregates larger than 2.36 mm were classified into three categories: main coarse, interceptor, and quasi-fine. The particle-to-particle contact can well explain the development of packing structure in an assembly of particles via the DEM simulation parameter, mean coordination number in coarser fraction (MCN_cr), and MCN_cr of an intermediate value indicated adequate interaction developed by the coarser and finer fractions in a blend. Especially for Model-4, an attained MCN_cr value at 4.0 or higher indicated that aggregates coarser than 4.75 mm were capable of maintaining a stable framework as finer aggregates were being added, accompanied by Voids in the Coarse aggregate ratio being less than 1.0. On the whole, the DEM simulation is effective in evaluating the packing structure and is found to be useful in guiding gradation design for PAM.