Twelve tests were conducted on reinforced concrete beams with three steel fiber-volume fractions (0, 0.5, and 0.75%), three shear span-depth ratios (2, 3, and 4), and two concrete compressive strengths (31 and 65 MPa). The results demonstrated that the nom- inal stress at shear cracking and the ultimate shear strength increased with increasing fiber volume, decreasing shear span- depth ratio, and increasing concrete compressive strength. As the fiber content increased, the failure mode changed from shear to flexure. The results of 139 tests of fiber-reinforced concrete beams with- out stirrups were used to evaluate existing and proposed empirical equations for estimating shear strength. The test population included beams with a wide range of beam properties, but most of the beams were small. The evaluation indicated that the equations developed by Narayanan and Darwish and the equations proposed herein provided the most accurate estimates of shear strength and the onset of shear cracking. For the proposed procedure, the ratio of the measured strength to the calculated strength had a mean of 1.00 and a coefficient of variation of 15%.
The addition of steel fibers to a reinforced concrete beam is known to increase its shear strength and, if sufficient fibers are added, a brittle shear failure can be suppressed in favour of more ductile behavior.1,2 The increased shear strength and ductility of fiber-reinforced beams stems from the post- cracking tensile strength of fiber-reinforced concrete. This residual strength also tends to reduce crack sizes and pacings. The use of steel fibers is particularly attractive for high-strength concrete, which can be elatively brittle without fibers, or if conventional stirrups can be eliminated, which reduces reinforcement ongestion. The literature describes numerous studies of rectangular, fiber-reinforced beams without stirrups,2-21 of which 163-18 were reviewed by Adebar et al.2 Batson, Jenkins, and Spat- ney performed the first large experimental study of such beams,4 which included 42 tests of fiber-reinforced beams with- out stirrups that failed in shear. Subsequent investigations of normal-strength concrete6,7,9-17 (primarily in the 1980s) and high-strength concrete3,5,19,21(primarily in the 1990s) con- firmed the effectiveness of adding steel fibers and identified key parameters that affect shear strength. The increase in shear strength can vary drastically depending on the beam geometry and material properties. For example, in tests reported by Narayanan and Darwish,13 the increase in shear strength attributable to steel fibers varied from 13 to 170%.