Effect of fiber content and fiber length on the dynamic compressive properties of strain-hardening ultra-high performance concrete

Strain-hardening ultra-high performance concrete (SH-UHPC) exhibits excellent mechanical properties at the quasi-static state and becomes a promising material for protective structures exposed to impact or blast threats. But its performance under loads with high magnitude and short duration remains unanswered. The focus of this study is to investigate the dynamic compressive properties of SH-UHPC under high strain rates, which are achieved by split Hopkinson pressure bar (SHPB) tests. SH-UHPC with different fiber contents of 1, 1.5, and 2 vol % and different fiber lengths of 6, 12, and 18 mm are tested. The considered strain rate ranges from 180 s-1 to similar to 300 s(-1). The results show that SH-UHPC exhibits good impact resistance in terms of integrity. Samples could remain good integrity at a strain rate of 240 s(-1). The key parameters to measure the dynamic properties, i.e. energy absorption, dynamic compressive strength, and dynamic increase factor (DIF) of SH-UHPC show strain rate sensitivity and all of them increase with strain rate. Increasing fiber content leads to a slight improvement of the impact resistance of SH-UHPC while reducing fiber length impairs the impact resistance. Empirical DIF relations of SH-UHPC are firstly proposed in this study.