Investigating Fatigue Damage Accumulation of Asphalt Binders Considering Amplitude Sequence and Loading Interaction

The obvious nonlinear characteristics of fatigue damage accumulation for asphalt binders can be induced by variable amplitude loading. However, the corresponding nonlinear characteristics are difficult to be characterized due to loading interaction and amplitude sequence. This study aims at characterizing the nonlinear fatigue damage accumulation (NFDA) of asphalt binders by establishing an NFDA model. First, the stress-controlled time sweep tests with variable oscillation amplitude were conducted under the two loading modes of the high-low stress amplitude (σhigh-σlow) and low-high stress amplitude (σlow-σhigh). Then, the damage variable of asphalt binders was proposed using a crack length model, and the effects of loading interaction and amplitude sequence on the corresponding NFDA were analyzed. Furthermore, an NFDA model for asphalt binder considering amplitude sequence and loading interaction was developed based on one continuum damage mechanics model. Finally, the cumulative life fractions of asphalt binders were analyzed, and the established NFDA model was used to determine the optimum first life fraction which maximizes the cumulative life fractions. The results showed that the defined fatigue damage of asphalt binders shows a two-stage evolution trend. The NFDA of asphalt binders is affected by both loading interaction and amplitude sequence, which can be determined by the established NFDA model with reasonable accuracy. The σlow-σhigh loading mode can delay the accumulation process of fatigue damage and extend the fatigue life of asphalt binders leading to the cumulative life fractions greater than one, while the σhigh-σlow mode is the opposite. The optimum first life fractions of the tested styrene-butadiene-styrene-modified and virgin asphalt binders are 0.45 and 0.56, respectively.