Modelling of Fatigue Damage Behaviour of Asphalt Binder Under Variable Amplitude Loading

One of the primary failure mechanisms of asphalt pavement is fatigue damage during its service life. Usually, Miner’s damage rule is considered for predicting the cumulation of damage (CFD) in the asphalt materials. However, pavement is subjected to variable amplitude loading (VAL) and thus Miner’s rule which considers fatigue damage to be built up gradually and linearly may not be valid. Considering this, the study focuses on the laboratory evaluation of VAL to predict the fatigue behaviour of asphalt binders, important constituents of asphalt mixture, where binders are subjected to more than one strain level to model the fatigue behaviour. Five binders (viscosity grade: VG30, VG40, polymer modified binder: PMB, crumb rubber modified binder: CRMB, and sasobit-modified binder: SMB) were considered in order to accomplish the goal. Dynamic shear rheometer was employed to perform time sweep test under constant amplitude loading and VAL. For VAL time sweep test, two strain levels (2.5 and 3.5%) were considered and applied at two different strain sequences low to high (L–H) and high to low (H–L). The test results revealed that binders followed a nonlinear trend and thereby did not follow Miner’s rule. The nonlinear behaviour of asphalt binder under VAL was modelled by adopting Zhang and Oeser model. In addition, the expression for damage exponent (α) of Zhang and Oeser model was developed as a function of loading order and material property to approximate the actual process of fatigue build-up. The proposed CFD prediction model matches more closely with experimental data than Miner’s rule.