Effect of steel fibers on minimum shear reinforcement of high-strength concrete beams

Minimum shear reinforcement is required for reinforced concrete (RC) flexural members to prevent brittle shear failure considering uncertainty of concrete shear strength in current design codes. In ACI 318-14, the use of steel fibers for minimum shear reinforce-ment is permitted within limited ranges of design parameters such as a concrete compressive strength lower than 40 MPa (5.8 ksi), beam depth smaller than 600 mm (24 in.); and fiber volume ratio over 0.75%. In this study, the effect of steel fibers on the shear strength was studied for high-strength concrete beams (60 MPa [8.7 ksi]). The main test parameters were the concrete strength, use of steel fibers, and use of stirrups. The test results showed that steel fibers with a volume fraction of 0.75% significantly increased the shear strength of high-strength concrete beams. This is mainly because the high-strength concrete increased the contribution of the steel fibers by increasing the tension zone depth. Such effect of steel fibers was confirmed by results from previous studies. The test results also showed that the limitation of concrete strength can be increased from 40 to 60 MPa (5.8 to 8.7 ksi).