Nonlinear viscoelastic behavior of asphalt concrete in stress relaxation

It is accepted that asphalt concrete for paving roads is a viscoelastic material. For engineering purposes, the stress-strain relationship for asphalt concrete has been assumed to be linear viscoelastic. An earlier study (I) indicated that the material has a linear limit of only 0.1 percent deformation or less, implying that the material is nonlinear in normal service conditions of pavements. The actual nonlinear properties, the practical effects of the nonlinearity, and the best way to characterize asphalt concrete remain obscure. In the development of SUPERPAVE, the viscoelastic nonlinearity of asphalt concrete was considered in the analysis of the intensity of thermal cracking (2, p. 2.44) using fracture mechanics for nonlinear viscoelastic materials as proposed by Schapery (3, 4). However, in the final SUPERPAVE protocols, the material was, nevertheless, assumed to be linear, and linear viscoelastic theory was used in thermal stress prediction (2, pp. 3.51-52) which provides input for the analysis of thermal cracking. Generally, the linear response range of viscoelastic materials is usually small compared with the total range of strain available prior to yielding or fracture (5, 6). When considering the stress-strain relationship in the vicinity of failure, it is deemed inappropriate to assume a material such as asphalt concrete to be linear. This study is to investigate the nonlinear viscoelastic behavior of asphalt concrete in stress relaxation under uniaxial-stress conditions at temperatures and at strains that a pavement might undergo. Priority is given to thermal stress prediction in asphalt pavements, since, in cold regions, such as central Canada, thermal cracking is a most serious problem. In this study, viscoelastic theory is first reviewed. Based on the review, a formulation for nonlinear viscoelastic materials is selected for the characterization of asphalt concrete. Laboratory experimentation follows the theoretical consideration. Various aspects relevant to the tests, such as the materials used in the investigation, the equipment, test setup, instrumentation, and test program are described. In particular, the procedures for direct stress relaxation tests are detailed. Based on the test results, the nonlinear properties of the asphalt concrete are analyzed. The techniques used for the data analyses are demonstrated. The practical effects of the nonlinearity on the stress prediction are then evaluated. Three typical temperature and strain histories, which are of interest to engineers, are considered in the evaluation.