Nonlinear Viscoelasticity and Viscoplasticity Characteristics of Virgin and Modified Asphalt Binders

Many engineering materials have coupled nonlinear viscoelasticity and viscoplasticity, which are affected by complex thermomechanical loadings. This study addressed the challenge of accurately separating the viscoplasticity and the nonlinear viscoelasticity and formulated the viscoplasticity by considering the effects of temperatures and loading levels. First, the nonlinear viscoelastic constitutive equation was adopted to accurately separate the viscoplasticity and the nonlinear viscoelasticity. Then a kinetics-based viscoplastic model and a new viscoplastic activation energy indicator are proposed to consider the effects of the temperature and loading level on the viscoplasticity. As typical nonlinear viscoelastic viscoplastic materials commonly used in pavement engineering, asphalt binders were selected to demonstrate the principles in this study. It was found that the proportion of the viscoplastic strain is larger than the nonlinear viscoelastic strain for virgin asphalt binders (VBs) and it increases with the temperature, whereas the opposite is true for high-viscosity modified asphalt binders (HVBs) and rubber asphalt binders (RBs). The logarithm of the viscoplastic strain rate increases linearly with the reciprocal of the temperature, and the viscoplastic strain rates at different temperatures are correlated and can be predicted based on the established viscoplastic strain kinetics model. The viscoplastic activation energy indicator can characterize the viscoplastic deformation resistance for nonlinear viscoelastic viscoplastic materials, and the order of the viscoplastic deformation resistances of the three binders was $\mathrm{VB}<\mathrm{HVB}<\mathrm{RB}$, based on this indicator.

This work presents a theoretical framework that clarifies the deformation characteristics of engineering materials. We separated the viscoplasticity and nonlinear viscoelasticity, and formulated a nonlinear viscoelastoplastic kinetics model for engineering materials by considering the effects of temperatures and loading levels. The developed model can reveal the deformation characteristics of asphalt binders at different temperatures and loading levels according to laboratory test results. Some calculated mechanical indicators of asphalt binders are inaccurate; we addressed this problem using the proposed model. Specifically, we employed the separated viscoplastic strain to improve the calculation accuracy of the nonrecoverable creep compliance and percentage recovery of asphalt binders. We adopted the proposed viscoplastic activation energy indicator to evaluate the viscoplastic deformation resistance of asphalt binders. Overall, we expect practitioners to use the framework to calculate relevant mechanical indicators more accurately and better evaluate the resistance to permanent deformation of engineering materials.