Effect of confinement pressure on the nonlinear-viscoelastic response of asphalt concrete at high temperatures

Asphalt concrete materials exhibit nonlinear viscoelastic responses at high stress/strain levels. The traffic loading induces multi-axial stress states within the asphalt concrete pavement structure. Therefore, it is imperative to characterize the nonlinear viscoelastic responses of asphalt concrete under the realistic multi-axial stress states. Available methods in the literature for characterizing the viscoelastic nonlinearity are mostly based on simple uniaxial creep-recovery tests without considering the effect of confinement stress levels. In this paper, the nonlinear viscoelastic properties of asphalt concrete materials are characterized considering the effects of confinement pressure. It is shown that the confinement pressure significantly affects the nonlinear viscoelastic response of asphalt concrete materials. The viscoelastic nonlinearity is characterized as a function of triaxiality ratio in order to capture the combined effects of confinement level and deviatoric stress. Cyclic creep-recovery tests are performed at 55 C and at different confinement levels. An equation is proposed to relate the nonlinear viscoelastic parameters to the triaxiality ratio based on the test results. The analyses showed that the triaxiality ratio has substantial effect on the nonlinear strain response of the asphalt concrete.