Mechanisms of Failure in Uniaxial Repeated Creep Test and the Relationship to Aggregate Packing

Abstract

Rutting performance characterization of asphalt mixtures has attracted lots of attentions after the Strategic Highway Research Program (SHRP). The main reason is the lack of a stability or strength test in the Superpave volumetric criteria. Different test methods and analysis have been proposed with a goal of accurately as well as simply predicting the rutting performance of designed mixtures. A uniaxial repeated creep and recovery test is recommended in NCHRP 456 report (i.e. Flow number test) as a performance characterization test method, and it is currently being used by researchers extensively. This study is focused on comparing the mechanisms and fundamental properties that control the performance of asphalt mixtures throughout the uniaxial flow number testing. It is shown that aggregate packing, as measured using an image analysis method, is the key property affecting deformation characteristics in uniaxial testing. Additionally, it is shown that the main cause of tertiary flow of mixtures in the flow number test is structural instability and bulging (dilation) of aggregate skeleton. Aggregate skeleton discontinuities at outer layers of samples are observed after failure in the tertiary zone. It is observed that mixtures with better aggregate packing showed a better rutting performance due to lower stress level within the binder phase due to aggregate skeleton serving as the main stress path, and the aggregates in proximity or contact showed a supporting structure, which delays the tertiary flow in material, and reduces the rate of permanent deformation. Based on these observations, it is shown that applying confinement has a very significant effect in preventing tertiary flow conditions by maintaining the aggregate packing, thus improving rutting resistance of asphalt mixtures.