Characterization of the permanent deformation in the primary stage of creep of asphalt mixtures

The primary stage of deformation in creep of asphalt concrete involves both densification mechanisms and shear flow. The mechanical behaviour at this stage is generally nonlinear. In this study, the deformation in the primary stage was characterised by conducting creep and recovery tests with the creep time specifically chosen such that it coincides with the transition from the primary to the secondary stage of deformation. In this way, creep and recovery tests were conducted on three different asphalt mixtures at three different temperatures. The results highlight the nonlinearity of the mechanical behaviour. Notably, the extent of increase in strain during the ramp-up of stress was found to be several times higher than the recovery during the ramp-down. A visco-elasto-plastic model was used to describe the experimental results. The results show that permanent deformation mechanisms other than shear flow, such as densification, dominate this stage.