Simulation of micro-crack initiation and propagation under repeated load in asphalt concrete using zero-thickness cohesive elements

The fracture resistance of asphalt materials under repeated traffic loads significantly influences the service life of asphalt pavements. The numerical simulation of asphalt materials based on a form of fracture mechanics is considered as an efficient way to model fracture damage. Unlike traditional fracture mechanics approaches assuming the existence of infinitely sharp crack tips leading to stress singularities preceding the crack tips, an alternative approach, called Cohesive Zone Modelling (CZM), is presented to model crack initiation and propagation. To make the CZM more consistent with reality and thus yield more reliable results, a heterogeneous model was developed to explicitly model the different material phases (aggregate and asphalt mastic) captured by X-ray Computed Tomography (CT) (with a resolution of 0.014 mm(3)). A Two-Dimensional (2D) Finite Element (FE) model was reconstructed; the model embedded zero-thickness cohesive elements both in asphalt mastic and at interfaces between mastic and aggregates, in conjunction with viscoelastic material properties. Crack paths in asphalt mixture samples at different deformation levels in the Indirect Tensile Fatigue Test (ITFT) were compared to simulation results. The 2D model was then applied to predict crack initiation and propagation at 25 degrees C. It is found that high resolution X-ray CT applied on small-size samples is a suitable method to visualize detailed crack geometry of asphalt concrete and reconstruct realistic aggregate morphology for simulations. The 2D FEM developed can approximately model the whole cracking process. Based on the model, it is found that factors such as aggregate size and distance between aggregates can affect crack initiation. At 5 degrees C microcracks tend to initiate predominantly at aggregate-binder interfaces. At 25 degrees C microcracks tend to initiate and propagate within the mastic; as the microcracks propagate. At both temperatures, microcracks were observed develop continuously throughout the modelling fatigue process.