Viscoelastic tension-compression nonlinearity in asphalt concrete

A new one-dimensional (1D) viscoelastic creep formulation is proposed for modeling the recoverable deformation component in asphalt concrete (AC). The formulation is designed to accommodate different tensile and compressive properties in analogy with bimodular elasticity. It is structured as a Volterra equation of the second kind with a kernel that alternates between tensile and compressive compliances based on the sign of the viscoelastic (VE) strain. If the response is only in one direction, or if the viscoelastic properties are identical in both directions, the model degenerates to the linear Boltzmann superposition. As a demonstrative application, the theory was utilized to match the response of an asphalt specimen exposed to several load-unload-rest sequences in uniaxial tension-compression. It was found that two dissimilar creep functions were needed to achieve excellent reproducibility. At short times the two were almost overlapping while for longer times they departed such that the compliance associated with compression was lower than the tension-related compliance; this finding is in tune with previously reported experimental evidence. The new theory is deemed capable of correctly simulating viscoelastic tension-compression nonlinear behavior.