University of California Transportation Center

Because pavements are being subjected to increasingly heavy loads, many mix design procedures currently in use may not adequately prevent permanent deformation (rutting) in asphalt concrete. This dissertation describes the development of a nonlinear viscoelastic constitutive model that can be used in mix analyses to design rut-resistant mixes.

Shear deformation is the dominant cause of rutting in asphalt concrete, a nonlinear viscoelastic material. To determine the viscoelastic shear properties simple shear frequency sweep tests at constant height were run. The test results indicated that the mix behavior depends on both strain level and temperature. Master curves of the laboratory data were then shifted horizontally and vertically to create a single mast curve from which to extract the viscoelastic material properties. These horizontal and vertical shift factors were used to develop shift factor functions dependent on temperature and strain.

The constitutive model was formulated from the basis of linear viscoelasticity and extended to incorporate the horizontal and vertical shift factors. The strain in the shift factors was the maximum strain experienced in the time history. The model was implemented in a finite element code and the solution was calculated in discrete time steps. The maximum strain and, hence, the shift factors were updated in each time step.

Simulations of the repeated load simple shear test at constant height (RSST-CH) and of pavement structures were performed to validate the model. The RSST-CH simulations matched the laboratory data fairly well. The average shapes of the rutting profiles in the pavement simulations compared favorably with those observed in the Heavy Vehicle Simulator Rutting Study, including the “humps” apparent at the edge of the wheel tracks. Thus, despite limitations in the model, including its inability to account for densification of the mix, the nonlinear viscoelastic model captures the permanent deformation behavior of asphalt concrete reasonably well.

Simulations with the constitutive model were used to evaluate the relationship between rut depth in the field and shear strain in the RSST-CH utilized in the SHRP A003A mix design procedure. The results also demonstrated the usefulness of the model in quantifying the effect of tire type on rutting.