Nonlinear viscoelastic behavior of asphalt concrete and its implication for fatigue modeling

Mechanistic models for asphalt concrete (AC) can consider many different physical mechanisms. However, as more mechanisms are considered, the complexity increases, and it becomes important to balance accuracy and complexity to create a model that can be used by the engineering community. In some cases, the material response is dominated by the effects of only some key processes, and smearing the effects of the minor ones is acceptable. In other cases, many processes are important and need to be considered. In this paper, the importance of modeling the nonlinear viscoelasticity (NLVE) of AC for fatigue response prediction is assessed. Two mechanistic hypotheses are considered for describing this phenomenon: linear viscoelastic with damage and NLVE with damage. The importance of explicitly considering NLVE effects under fatigue loading is evaluated with laboratory tests, the simplified viscoelastic continuum damage (S-VECD) model, and an NLVE form of the S-VECD model. These two models are characterized and used to simulate and compare AC fatigue response under constant and random controlled stress and strain conditions. It is found that while the NLVE-based formulation better represents the material response in random loading and suggests less overall damage accumulation during fatigue, the two models predict similar amounts of modulus reduction. The primary conclusion from this study is that because the goal for fatigue assessment is to find the change in modulus over a long period of time, fatigue response modeling of AC does not need to explicitly consider NLVE.