A new Hopkinson bar method to investigate the compacting behaviour of fresh concrete

Abstract
A new experimental test, based on the Hopkinson bar measurement technique is used in order to analyze the compaction process of a fresh concrete under successive impacts. The use of this modified Hopkinson bar technique allows for the measurement of the compacting stresses during one impact. In fact, the measurement device allows to measure the stress/time response of the material during each shock independently. In particular, the study is focused on the analysis of the stress/time curve during the first shock on the freshly-mixed concrete specimen. It happens that the stress response exhibits the occurrence of the equivalent of a phase change for the granular material. It is therefore possible to measure at what moment this phenomenon happens. Then, we show that, considering a model of shock front for the propagation of the compacting wave, the phase change occurs at a density that depends on the impact velocity. The results shows that the increase of the impact velocity leads to an increase of the phase change density.