ABSTRACT
The fatigue cracking mechanism of asphalt binders in asphalt mixtures is often described using mathematical modeling and theoretical diagrams but has yet to be observed using optical microscopy techniques. We use dark field optical microscopy to observe the bulk microstructure of asphalt binder subjected to high amplitude cyclic loading. This preliminary observation informs that once cavitation begins in asphalt binder, the cavities serve as nucleation sites for microcracks to form. In the first stage of material failure, the cavities grow until they collapse and form macrocracks connecting individual cavities, which serves as stage two of failure. Finally, the cavities are joined by microcracks to form a macrocrack in the specimen. As this process happens, bulk microstructures are pushed between the existing cracks to form high density areas of structure in the undamaged region.
