Implications of warm-mix asphalt on long-term oxidative ageing and fatigue performance of asphalt binders and mixtures

The use of warm mix asphalt (WMA) has been increasing in recent years due to its ability to reduce the production temperatures of asphalt concrete. The long-term implications of reduced production temperatures and, hence, reduced short-term ageing on long-term performance remain largely unknown. This study evaluates the effect of age hardening in WMA binders and mixtures compared with hot mix asphalt (HMA) binders and mixtures with respect to fatigue damage. Two WMA technologies are considered: foaming by water injection and Evotherm modification. For this study, the asphalt mixtures were subjected to laboratory conditioning in a forced air convection oven to simulate long-term field ageing according to AASHTO R30. The asphalt mixtures and extracted binders were subjected to linear viscoelastic and fatigue characterisation following ageing. Because oxidative ageing occurs within the asphalt binder phase of asphalt concrete, this paper focuses on the relative performance of WMA and HMA binders at various ageing levels and compares this binder performance to the respective mixture performance. Cyclic direct tension tests were used to measure the fatigue resistance of the asphalt mixtures, and the linear amplitude sweep (LAS) test was used to measure the fatigue resistance of the binders. Simplified viscoelastic continuum damage (S-VECD) analysis was performed to interpret the fatigue test results and predict the fatigue performance of the binders and mixtures using a pavement structural model. The results demonstrate that after substantial long-term ageing, differences between the fatigue performance of WMA and HMA become insignificant. The results also demonstrate good agreement between the binder and mixture results, indicating that the LAS test coupled with S-VECD analysis is able to capture the binder's contribution to mixture fatigue.