Strain-hardening effect on the flexural behavior of ultra-high-performance fiber-reinforced concrete beams with steel rebars

This study evaluated the effects of volume fraction, aspect ratio, and shape of steel fibers on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) and the structural behavior of reinforced (R-) UHPFRC beams. The tensile strength and energy absorption capacity of ultra-high-performance concrete (UHPC) are improved by adding steel fibers and increasing its volume contents by up to 3.0 %. Compared with short straight steel fiber, medium-length straight and twisted fibers at a volume fraction of 2.0 % result in twice higher energy absorption capacity and higher flexural strength of R–UHPFRC beams. The flexural strength of R–UHPC beams increases by increasing the fiber content up to 3.0 %. However, the strain-hardening characteristics of UHPFRC negatively influence the cracking behavior and stress redistribution in structural beams, causing 48.2–54.1 % lower ultimate ductility indices. The small amounts of steel fibers with volume fraction of ≤1.0 % that exhibit strain-softening behavior only improve the peak ductility.