Rate sensitivity of asphalt concrete in triaxial compression

There is ample evidence based on uniaxial (unconfined) compression that asphalt concrete exhibits behavior that depends on the loading or straining rate. This paper describes the results of an experimental program aimed at investigating rate sensitivity of asphalt concrete in a more general stress state, namely, in triaxial (axisymmetric) compression. Laboratory compacted specimens were subjected to three loading histories: (1) constant load (static creep); (2) repeated constant load (repeated static creep); and (3) haversine load (dynamic creep), The creep and repeated creep test programs allowed for separating the hydrostatic and the deviatoric response, and the contribution of the elastic, viscous, and plastic properties in the overall response. The haversine tests concentrated on the deviatoric response, and were conducted to investigate the influence of load frequency and number of cycles. The creep results show small deviator-induced dilation that is nearly fully recoverable during the rest period, and which, in some applications, can be neglected or approximated by an elastic response. The deviatoric response, on the other hand, is not recoverable, and can be modeled as a sum of elastic, plastic, viscoelastic, and viscoplastic strains, all being linear functions of the deviator stress, Similar behavior was observed in repeated creep tests. The response to haversine loading can be separated into a response close to that observed in creep tests and, beyond initial transient response, into a linearly viscoelastic harmonic response, It was found that power laws accurately describe the time-dependent deviatoric strains.