Nonlinear viscoelastic behaviour of asphalt concrete in stress relaxation
Asphalt concrete for paving roads is a viscoelastic material. Although a previous study revealed that asphalt concrete has an exceptionally low linear limit so that it behaves nonlinearly in the field, the material is nevertheless assumed to be linear in the practice of pavement design. This research investigates the viscoelastic nonlinearity of asphalt concrete, with emphasis on thermal stress of pavements. This research also deals with the variability of the thermal coefficient of asphalt concrete, a necessary property in predicting, the thermal stress. Based on an extensive review, Schapery's theory for nonlinear viscoelastic materials was selected as the way to best characterize asphalt concrete. A series of direct tensile relaxation tests, on a single type of asphalt concrete, were for the first time conducted at temperatures from 40 to $-$40$\sp\circ$C and at strains up to.0.8% to determine the material properties. A constant-strain-rate test, a Thermal Stress Restrained Specimen Test, and a thermal coefficient test were also conducted for evaluation purposes. Results of the relaxation tests showed that asphalt concrete is nonlinear from the very beginning of being strained. The importance of nonlinearity increases with increasing strain at a decreasing rate and increases with decreasing temperature. The Schapery theory proved capable of characterizing asphalt concrete. It was also found from the test that the thermal coefficient of asphalt concrete is obviously variable, resulting from the nonlinear relationship between temperature and thermal deformation. The effects of the mechanical and thermal nonlinearities on stress predictions were evaluated through three typical problems of interest to engineers.