Improving torsional – Flexural resistance of concrete beams reinforced by hooked and straight steel fibers

Abstract

Abstract This research studied the behavior of hooked and straight steel fiber reinforced concrete beam under the influence of combined torsional-flexural load. The experimental program included three fixed supported fiber reinforced concrete beams of (1800*250*300)mm dimensions. All beams were of volume fraction (Vf = 1.5%) and same details, except their fibers type (no fiber, hook fiber, and straight fiber). From hardened concrete test results, it was concluded that beam of hooked steel fiber gave the best enhancing where the increase in the compressive and tensile strength reached to 33.37% and 55.08% respectively, while straight fiber’s improving was in the second order. Diagonal crack was observed between the two torque-arms. In all fiber reinforced beams, the initial cracks were arising after the no-fiber beam. It was also concluded that hooked fiber had the best influence in improving beam pre and post cracking behavior. Regarding the ultimate load, H1.5 model was detected to be the strongest beam (128.13% improvement was obtained). Model S1.5 was in the second order and the last was NF. The results of load versus deflection and angle of twist for all specimens indicated a linear behavior at the earlier stage of loading (approximately (21–34) % from the ultimate load), deformation/twisting after that was increased gradually till failure. Nevertheless, elastic stiffness didn’t significantly affected by fiber content. Impact of fiber adding was evident at the nonlinear stage, beams can sustain higher load at the same deflection/twisting rate when fibers are used. No significant strain has been recorded in the flexural reinforcement (S1&S2) and no main flexural cracks had been appeared but a large wide diagonal cracks had been generated in the middle twisted region and near supports, the highest obtained strain were in the stirrups S4 and S5, although the applied load was torsional-flexural, all beams fail by excessive twisting.