Fatigue Crack Evolution at the Bitumen–Aggregate Interface: From Microscale Morphological Observation to Macroscale Predictive Modeling

Although the bitumen–aggregate interface is critical to the fatigue performance of asphalt mixtures, its evolution under cyclic loading remains insufficiently understood. This study develops a crack evolution model for this interface under cyclic pull-off loading, grounded in continuum damage mechanics and the pseudo J-integral Paris’ law. Systematic pull-off fatigue tests were conducted across three aggregate types (limestone, tuff, basalt), two bitumen binders [#70 and styrene-butadiene-styrene (SBS)], and varying temperatures and film thicknesses. By integrating experimental data with the proposed model, Paris’ law coefficients ( A and n ) were determined. Scanning electron microscopy (SEM) analysis corroborated the model’s predictions, identifying crater-like fatigue morphology as a hallmark of interfacial damage. The results elucidate a distinct damage mechanism governed by viscoelastic fibrillation and ligament fragmentation. Rather than coalescing into macroscopic cracks, the confined bitumen film progressively ruptures into a dense proliferation of microscopic craters under cyclic loading. Statistical analysis reveals that coefficient A is positively correlated with temperature while inversely related to film thickness and alkaline material. Conversely, coefficient n exhibits high sensitivity and a strong correlation with total fatigue life. Crucially, the findings suggest that A and n function as intrinsic material constants, remaining independent of loading frequency and strain levels.