On bitumen microstructure and the effects of crack healing

When an asphalt pavement is subjected to repeated traffic loads punctuated by rest periods, the acquisition of damage is interrupted by molecular relaxation and healing: the restoration of continuity across fractured interfaces. The healing effect is responsible for improved fatigue performance at high temperatures and dominates the laboratory-to-field shift factor in design. The mechanism of healing is not well understood, however. To describe this process, myriad investigations are collated with healing in high polymers, but neglect microstructural changes due to the damage processes that precipitate fracture. Yet, the remnants of deformation drive healing phenomena. An enhanced knowledge of healing and the effect of fracture could allow for the direct application of laboratory fatigue in pavement performance prediction. This thesis develops an understanding of the interrelation between binder structure and crack healing, using electron microscopy and mechanical analyses. Cryogenic microscopy indicates that the bulk is amorphous: phase separation in the form of bi-continuous or discrete structure is catalysed by surface effects including composition-dependent short-range interactions and thermal gradients. Environmental microscopy shows that the creation of a free surface during fracture perturbs the bulk solubility continuum, which stimulates phase separation in the form of interconnected fibrils. This system is sensitive to molecular scission and precludes healing by spatial interference and by reduced potential interaction. Rheological tests confirm the space-bound character of the microstructure and emphasise the requirement for an efficient method to quantify healing. Vialit pendulum tests validate the use of cohesive energy for this purpose and define the effect of fracture temperature: the capacity for healing is reduced by rupture of glassy fractions. Although susceptible to high variability, the outcome of direct tension testing confirms the involvement of crystallisable materials and the reduced proliferation of interfacial molecular interaction due to main-chain scission