Mesoscale Fracture Simulation Of Asphalt Mixture Considering Material Heterogeneity

Asphalt mixture is widely used in high-grade highway and airport pavement construction due to its high strength, lower noise, easy construction and maintenance and smooth and comfortable for driving. With rapid increase of traffic volume and heavy vehicle amount in recent years, asphalt pavements often fail before reaching its design life due to cracking. As one of the most common distresses, cracking directly reduces the road service capacity and deteriorates the road performance. Crack phenomena pose a severe challenge to application and design of asphalt pavement structures. Understanding the fundamental mechanisms behind crack initiation and propagation is an important issue in development of accurate, mechanistic design procedures based upon pavement performance and life-cycle costs.In the paper, random aggregate generation and packing algorithm is developed to create two-and three-dimensional heterogeneous models of asphalt mixture and a heterogeneous fracture modeling approach by combining the mesostructural modeling technique and the bilinear cohesive zone model is proposed to predict fracture evolution behavior in asphalt mixture. Then, an integrated frame by coupling experiments and heterogeneous fracture simulation is presented to study the crack behavior in asphalt mixture under different load modes. The main creative achievements are shown as follows.(1) The three-point bending tests are performed based on the single-edged notch beams, and mode I and the mixed mode load conditions are designed by offsetting the notch location. The experimental results show that a crack propagates approximately along the initial crack direction under mode I load condition, but tends to propagate towards the top midspan point of beam under mixed mode load condition.(2) Using the self-manufactured direct shear test setup, a series of direct shear test are performed on the cuboid specimens to evaluate the influence of temperature on shear cracking behavior of asphalt mixture. It is revealed from the test results that the temperature significantly influences the mechanical properties of asphalt mixture including initial stiffness, shear strength and post-peak softening behavior.(3) The random aggregate generation and packing algorithm is developed to create a two-or three-dimensional heterogeneous model of asphalt mixture. Asphalt mixture is treated as a composite consisting of randomly distributed coarse aggregates and asphalt mastic, and some mesoscale factors including aggregate content, gradation, shape and distribution are considered as variable design parameters. The techniques of aggregate generation, packing, perturbation and conversion involved in the modeling algorithm are studied.(4) A multiscale modeling frame combining macroscale and mesoscale is proposed to achieve the balance between the computation efficiency and the accuracy. In this frame, the mesostructural models are created in some concerned local regions and the homogeneous models are done in the other regions. As examples, a series of multiscale models are created to investigate the cracking behavior of pre-cracked asphalt mixture under three-point bend loading. It is proved that the mutiscale modeling frame is feasible and reasonable by comparing the numerical simulation results with the experimental results. Then the effects of crack location and aggregate distribution on cracking behaviors are evaluated.(5) A mesoscale heterogeneous fracture modeling approach is proposed by combining the mesostructural model and the cohesive zone model to predict complex cracking behavior in asphalt mixture. The mesostructural model is firstly discretized by finite elements, then, the cohesive elements are inserted into the boundaries of initial elements to be as potential microcrack sources by a self-compiled computer program. The two-dimensional heterogeneous fracture models are created to study the cracking behaviors under three point bending and the comparisons between the numerical simulation results and experimental results prove that the heterogeneous fracture model is feasible and reliable. Then the heterogeneous fracture models are used to study the microcrack evolution characteristics and mechanisms under uniaxial tension and direct shear, respectively. Additionally, the effects of some mesoscale factors such as aggregate gradation, distribution and content on cracking behaviors are evaluated.(6) The heterogeneous fracture simulation is extended from a two dimensional case to a three dimensional one and the complex three dimensional crack propagation in asphalt mixture is simulated. The simulation results show that the three dimensional fracture model predicts more realistic crack path, but its computational efficiency is lower. The aggregate distribution, aggregate amount and the cohesive fracture parameters are important factors influencing the crack performance of asphalt mixture.