Impact of Rest Period and Stress Paths on Asphalt Concrete Permanent Deformation Behavior

The traditional repeated-load permanent deformation tests for asphalt concrete (AC) considers only the triaxial compression stress state as the critical loading case. These types of experiments may not represent the most critical stress states or they may neglect the effect of the history of the loading path due to the passing of vehicles. The main goal of the study is to assess the impact of various stress paths to represent moving load stress on permanent deformations. The effect of stress paths on permanent deformation characteristics of a polymer modified and unmodified AC mixture was presented. The stress paths included conventional triaxial compression, reduced triaxial compression, and triaxial simple shear stress paths. The rest period was added to the stress paths as a variable. The mixes were tested using the stress states constituting the three different stress paths and temperature. In addition, the viscoelastic recovery and viscoplastic hardening-relaxation phenomenon to explain the effect of the changing rest period was explored by conducting tests at intermediate temperature and lower stress levels and focusing on the beginning stages of the experiment. It was shown that viscoelastic recovery and viscoplastic hardening-relaxation may have competing mechanisms on the total and permanent strains when the rest period changes. However, viscoplastic effects overwhelmed the viscoplastic deformation and recovery. The increase in the rest period from 0.18 to 2.5 s consistently increased the permanent deformations by almost 2-3 times higher under three stress paths. The significance of the rest period as a testing variable along with stress state and temperature in repeated-load permanent deformation experiments is once again underlined. The results show that the changing stress paths had a significant effect on permanent deformation resistance when compared to conventional repeated-load experiments. The reduced triaxial compression stress path was found to be a more critical stress state in terms of permanent strains.