Viscoelastic response of asphalt-aggregate mixes to transient confining conditions

An asphalt-aggregate mix is subjected to creep and recovery sequences under isotropic compression mode in the triaxial apparatus. Duration of creep periods ranged between a few seconds to one minute, separated by half an hour of recovery intervals. Resulting strains are partitioned into viscoelastic (VE) and viscoplastic (VP) components with the study focusing on the nonlinear nature of the VE component. The VP response is represented using the classical strain hardening formulation; the VE response is represented using a linear formulation but with applied pressures replaced by 'effective VE pressures'. The ratio between applied and effective VE pressures at any moment during the test is defined as 'relative VE stiffness' and denoted as C-ve. The evolution of C-ve is exposed through direct analysis of the test data with minimal assumptions. Under the short creep periods it is shown that C-ve decreases in value from the very beginning of pressure application and increases during the following recovery intervals. Under the longer creep periods opposite trends appears, with C-ve raising after the initial drop and decreasing during the following recovery intervals. A conceptual breakdown into underlying causes is proposed for the seemingly erratic behavior of C-ve towards the development of a constitutive model.