Cyclic Hardening-relaxation viscoplasticity model for asphalt concrete materials

A cyclic hardening-relaxation model is proposed that significantly enhances the prediction of the viscoplastic (VP) strain of asphalt concrete under cyclic compressive-loading conditions at high temperatures. The hardening-relaxation mechanism is physically tied to the changes in the material's microstructure during the rest period. A memory surface that memorizes the viscoplastic deformation history is defined in the viscoplastic strain space as the general initiation and evolution criteria for the hardening-relaxation mechanism. The proposed model is coupled to the classical Perzyna-type viscoplastic model and Schapery's nonlinear viscoelastic model, and the associated numerical algorithms are implemented in the finite element software ABAQUS through the user-defined material subroutine UMAT. Model predictions show that the proposed model predicts well both the axial and radial viscoplastic responses of asphalt concrete subjected to the cyclic creep tests at various loading times, unloading times, confinement levels, and loading scenarios.