The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson's ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC^(3D). The aggregate gradation, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson's ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson's ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson's ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson's ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson's ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.
A user-defined micromechanical model was developed to investigate the fracture mechanism of asphalt concrete(AC)using the discrete element method(DEM).A three-dimensional(3D)AC beam was built using the "Fish" language provided by PFC3D and was employed to simulate the three-point bending beam test at two temperature levels:-10°C and 15°C.The AC beam was modeled with the consideration of the microstructural features of asphalt mixtures.Uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling.The 3D predictions were validated using laboratory experimental measurements of AC beams prepared by the same mixture design.Effects of mastic stiffness,cohesive and adhesive strength on AC fracture behavior were investigated using the DEM model.The results show that the 3D DEM fracture model can accurately predict the fracture patterns of asphalt concrete.The ratio of stress at interfaces to the stress in mastics increases as the mastic stiffness decreases;however,the increase in the cohesive strength or adhesive strength shows no significant influence on the tensile strength.
