Shear stress versus strain responses of ultra-high-performance fiber-reinforced concretes at high strain rates

Shear stress versus strain response of ultra-high-performance fiber-reinforced concretes (UHPFRCs) at high strain rates up to 248 s(-1) was investigated by installing the shear test set-up in an improved strain energy frame impact machine (I-SEFIM). The tensile strain-hardening UHPFRCs also produced shear-related hardening response, even at high strain rates, accompanied with multiple cracks. The shear resistance was obviously sensitive to the applied strain rates even though the shear strain rate sensitivity was not as high as the tensile strain rate sensitivity: the dynamic increase factor (DIF)-1.5 for the shear strength of UHPFRCs with 1.5 vol.-% fibers was significantly lower than the DIF (3.2) for the tensile strength at the strain rate of 248 s(-1) owing to the different distribution of inertial force of mortar matrix surrounding fiber, resulting from difference of loading direction. A DIF predicting equation was finally proposed for the shear strength of UHPFRCs at high strain rates.