Dynamic compressive behavior of concrete-filled steel tubes under single and multiple impacts

In this study, the dynamic compression behavior of concrete-filled steel tubes (CFSTs) under single and multiple impacts was investigated using the split Hopkinson pressure bar (SHPB) apparatus. Analyses of the failure patterns, dynamic stress-strain relationship and dynamic compressive strength of CFST members were conducted, and the results for CFSTs were compared with those for plain concrete samples. Moreover, the confinement mechanism of the CFST specimens during the axial impact was investigated, and the influences of strain rate, steel tube thickness, concrete strength and height-to-diameter ratio on the dynamic increase factor of the CFST (DIFCFST) were explored. The experimental results indicate that the CFST specimens had excellent impact resistance under single and multiple impacts, and the dynamic compressive behaviors of CFSTs exhibited significant strain-rate dependence. Larger loading gas pressure can strengthen the confinement from steel tube to concrete, leading to an increase in the dynamic compressive strength of the CFST. Dynamic compressive strength reduction generally occurred during multiple impacts, while the strength enhancement could be observed at lower impact level due to insufficient single impact energy. The DIFCFST decreased with increasing steel tube thickness and concrete strength, while the height-to-diameter ratio had less effect on the DIFCFST. Based on the experimental test data, an empirical formula was proposed for the DIFCFST considering the strain rate, the confinement factor and concrete strength to predict the dynamic compressive strength of CFST members under axial impact.