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. •Higher flexural strength of R–UHPFRC beams is achieved by using MS and TU fibers than SS fibers.•Strain energy density is predicted based on the quantity Vf(lf2/df) with an R2 higher than 0.98.•Strain-hardening characteristic negatively affects the cracking behavior and stress redistribution in R–UHPFRC beams.•Ultimate ductility of R–UHPC beams decreases by 48.2–54.1 % with the inclusion of steel fibers.•Small amounts of SS fibers (Vf of ≤1.0 %) with strain-softening behavior improves the peak ductility of R–UHPC beams.