The effect of loading duration on damage initiation in high-strength concrete

•High strength concrete (HSC) is subjected to short duration constant stress loading.•HSC specimens support applied stress above their failure stress for short durations.•Throughout the loading process a decrease in stiffness is observed.•A decrease in porosity before the onset of major crack propagation is observed.•The porosity decrease is due to void compaction and continues during fracture. The rate sensitivity of the failure strength of concrete and other brittle materials is well documented. However, the underlying mechanism(s) that gives rise to this phenomenon remains a topic of debate. Experimental results suggest that the time-dependent failure process plays a significant role in the observed strength increase. This study investigates the failure process in high-strength concrete (HSC) specimens under uniaxial compression loading. This is accomplished through the use of a modified Kolsky compression bar to apply controlled and repeatable mechanical loading in combination with the non-destructive observation capability of high-resolution X-ray computed-microtomography (micro-CT). The evolution of specimen damage morphology is observed as a result of intermittent short-duration uniaxial stress loadings. Experimental results show that HSC specimens are capable of supporting the applied stress above their quasi-static failure strength for short durations. This duration is found to be a function of the overload stress level. Additionally, micro-CT results reveal that specimens undergo a brief period of void compaction followed by the coalescence and propagation of cracks before completely losing load bearing capability.