Micromechanical Fracture Modeling of Asphalt Mixture Using the Discrete Element Method

Cracks in asphalt pavements create irreversible structural and functional deficiencies that increase maintenance costs and decrease lifespan. The understanding of fracture behavior in asphalt concrete laboratory specimens is currently an open research area that comprises a vital link in the ability to design asphalt concrete paving mixtures and flexible pavement structures that resist cracking. Towards this end, a micro-fabric distinct element modeling (MDEM) approach was implemented in the two-dimensional particle flow software package (PFC-2D) to study the complex crack behavior observed in asphalt concrete fracture tests. This behavior was simulated with an MDEM model having complex morphological features, but relatively simple constitutive models in both the bulk material regions and at the material interfaces. A double-cantilever beam (DCB) test was used to verify the implementation of bilinear cohesive zone model (CZM) in PFC-2D, which yielded a favorable comparison between MDEM results and the closed-form reference solution. The MDEM approach was then employed to investigate the fracture behavior of asphalt concrete in a new disk-shaped compact tension test (DC(T)). The cohesive model parameters of material strength, critical displacement jump and fracture energy were calibrated in conjunction with the homogeneous DC(T) fracture model for a relatively simple mode-I fracture simulation with homogeneous bulk materials properties and a pre-defined crack path. The calibrated model was then extended to simulate fracture in a DC(T) specimen with a heterogeneous fabric description in an attempt to capture some of the more complex fracture mechanisms in the process zone, such as aggregate bridging, crack turning, and crack branching. Both homogeneous and heterogeneous models were successfully calibrated to experimental data, but further experimental validation is still needed.