Effect of Fiber Reinforcement on the Shear Behavior of Reinforced Concrete Beams

Results from an experimental program aimed at evaluating the shear behavior of steel fiber reinforced concrete (SFRC) beams without stirrup reinforcement are presented. A total of 28 simply supported beams with a test shear span-to-effective depth ratio of approximately 3.5 were tested under a monotonically increased concentrated force up to failure. Among the 28 beams tested, four beams were constructed with regular concrete, three without stirrups and one with stirrups satisfying the minimum stirrup reinforcement requirements in the 2008 ACI Building Code. Three types of steel fibers were evaluated, all with hooks at their ends. Test variables were: 1) beam depth (455 or 685 mm); 2) fiber length-to-diameter ratio (55 or 80); 3) fiber tensile strength (1100 or 2300 MPa); 4) fiber volume fraction (0.75, 1.0 or 1.5%); and 5) longitudinal tension reinforcement ratio (1.6, 2.0 or 2.7%). All beams were designed so that a shear failure would ultimately develop, either prior to or after flexural yielding initiated. Regular strength concrete was used in all beams, with cylinder strengths at test day ranging between 29 and 51 MPa. Test results showed that the use of hooked steel fibers in a volume fraction greater than or equal to 0.75% led to multiple diagonal cracks and a substantial increase in shear strength compared to regular concrete beams without stirrup reinforcement. All SFRC beams failed at a shear stress greater than or equal to 0.33√ f’ , MPa, where f’ is the concrete cylinder strength. Also, the fiber c c reinforced concrete beams exhibited higher shear strength and better diagonal crack distribution compared to the beam with minimum amount of stirrups. No significant difference in average shear stress at shear failure was observed with an increase in beam depth from 455 to 685 mm. Although changes in fiber length did not lead to appreciable changes in beam overall behavior, maximum diagonal crack width prior to shear failure was found to be on the order of 5% of the fiber length. A simple model to predict the shear strength of SFRC beams is proposed based on the assumption that shear is resisted by fibers crossing diagonal cracks and shear carried in the concrete compression zone. Reasonable agreement between experimental and predicted strengths was found when applying this model to SFRC test beams reported in the literature. 1 Gustavo J. Parra-Montesinos, Hai H. Dinh, and James K.