Effects of steel fiber content and shape on mechanical properties of ultra high performance concrete

This study investigated the effects of three shaped steel fibers (straight, corrugated, and hooked-end) with different fiber contents by volume (Vf = 0, 1%, 2%, and 3%) on mechanical properties of ultra high performance concrete (UHPC). The involved properties included flowability, compressive strength, and flexural behavior. According to the characteristics of the obtained three-point flexural load–deflection curve and the existing constitutive model of uniaxial compression, a new model for flexural load–deflection based on least square fitting was proposed. The results indicated that increased fiber content and use of deformed fibers could gradually decrease the flowability of UHPC. They also had significant effects on compressive and flexural behavior of UHPC. With incorporation of 3% straight steel fibers, its compressive and flexural strengths reached over 150 and 35 MPa at 28 d. For the concrete with 3% hooked-end and corrugated fibers, the compressive strengths at 28 d increased by 48% and 59% compared to those with the same amount of straight fiber. Steel fiber content had limited effect on the first crack strength and first crack deflection of flexural load–deflection curve of UHPC, but showed considerable effects on the peak load. The proposed model fitted well with the experimental results with correlation coefficient over 0.9.