A viscoplastic model for the confined permanent deformation of asphalt concrete in compression

• Viscoelastic–viscoplastic coupling effect is illustrated using experimental data.
• The model features a convolution integral enclosed in the Macaulay brackets.
• Material hardening and softening is captured via evolution of the internal stress.
• The nonlinear effects of confinement are described by the infinite modulus.
• The model is verified by random loading tests at multiple confinements.

This paper presents a viscoplastic model for the permanent deformation behavior of asphalt concrete in compression. Triaxial repeated load permanent deformation (TRLPD) tests with haversine-shaped load pulses and rest periods were used in the experimental investigations as they are able to simulate real traffic loading patterns. For the first time, the viscoelastic–viscoplastic coupling phenomenon in asphalt concrete is illustrated using experimental data, which motivates the direct collecting and modeling of permanent deformation history in the present work instead of modeling the viscoelastic and viscoplastic responses in a separate and uncoupled fashion as in the traditional scheme. By applying a long rest period (100 s) to allow the viscoelastic deformation to recover sufficiently, the permanent strain data can be collected directly for viscoplastic model development. The proposed viscoplastic model features a convolution integral enclosed in Macaulay brackets. A single viscoplastic relaxation spectrum is assumed as the material's intrinsic property, and the nonlinear stress effects are captured through the variable E∞, which is the infinite modulus expressed as a logarithmic function of the triaxiality ratio. Material hardening (or softening) is described via the increase (or decrease) of the internal stress during loading (or unloading and rest period). The model was characterized using TRLPD test data at three levels of deviatoric stress and confining pressure. Finally, the calibrated model was verified by applying it to random loading tests conducted at other confining levels. The proposed model provides an efficient and convenient approach that is able to determine the material's macroscopic deformation as well as to capture the material's internal hardening/softening mechanisms.