Impact of unbound aggregate packing on the dense bituminous mix properties: Experimental and discrete element model investigation

The aggregate gradation and the resulting packing structure significantly affect the performance of the bituminous pavements. Available literature reveals the limitations of past studies such as limited range of aggregate gradations and nominal maximum aggregate sizes (NMAS) considered and the lack of validation of DEM results, emphasizing the need for comprehensive evaluation of aggregate packing characteristics and their effect on mix properties. In this study, the influence of the packing characteristics of dense aggregate gradations of different NMAS, evaluated experimentally and using discrete element modelling (DEM), on bituminous mix properties, was studied. The study considers geometrical packing indicators such as voids in coarse aggregate (VCA), coordination number (CN), and the coefficient of curvature (Cc) of gradation curve to evaluate the packing condition of unbound aggregate blends. VCA was measured experimentally to validate the DE models. The CN was evaluated for different gradations using DEM. Volumetric parameters of bituminous mixes, such as effective design binder content (P_b-eff.) and voids in mineral aggregate (VMA), and mechanical properties, such as indirect tensile strength (ITS), resilient modulus (M_r), and permanent deformation characteristics, were evaluated experimentally. The smaller the NAMS and the finer the gradation, the higher the values of VCA and CN. VCA and CN correlated well with VMA and P_b-eff.. CN and Cc explained rutting resistance, ITS, and M_r. Higher CN resulted in better interlocking and higher load-bearing capacity. Aggregate gradations with VCA > 36.5 %, Cc < 1.2, and CN > 3.2 produced bituminous mixes exhibiting significantly higher ITS, M_r and rutting resistance.