Effect of steel fibers on the flexural behavior of RC beams with very low reinforcement ratios

This study aims to investigate the implications of hooked-end steel fibers on the flexural performance of reinforced concrete (RC) beams with very low reinforcement ratios. For this, four different fiber volume fractions, vf, of 0.25%, 0.50%, 0.75%, and 1.00%, were incorporated into the concrete mixture and plain concrete without fibers was considered as a control specimen. Four reinforcement ratios of 0.178%, 0.267%, 0.317%, and 0.406%, which are 44%, 66%, 78%, and 100% of the minimum reinforcement ratio, ρmin, were also adopted to evaluate the steel fiber effect on the flexural behavior of RC beams with various very low reinforcement ratios. The test results indicated that the overall flexural performance of RC beams, in terms of flexural strength, deflection capacity, post-cracking flexural stiffness, and cracking behavior, was improved by increasing the reinforcement ratio up to ρmin. Higher initial cracking and yield loads, post-cracking stiffness, and better cracking performance of RC beams were also obtained by including steel fibers. However, the enhancement of ultimate load carrying capacity by steel fibers was relatively minor, and the ductility index and flexural strength margin, used to guarantee a ductile failure mode, deteriorated with the inclusion of steel fibers. The lower reinforcement ratios and higher fiber volume fractions clearly led to lower ductility indices. Therefore, it was concluded that longitudinal steel rebar could not be replaced with discontinuous steel fibers at moderate volume fractions, vf ≤ 1.0%, in terms of ultimate load carrying capacity, ductility, and flexural strength margin. Lastly, analytical results considering material models for steel fiber-reinforced concrete (SFRC), given by the RILEM recommendation, generally overestimated the flexural capacities of reinforced SFRC beams, and the inaccuracy increased with increasing fiber contents.