Effect of Steel-Fiber Shear Reinforcement on Postcracking Shear Fatigue Behavior of Nonstirrup UHPC Beams

Steel fibers in ultra-high performance concrete (UHPC) have been demonstrated to improve resistance to crack growth, decrease deflection, and increase fatigue life under cyclic loads. Using steel fibers as shear reinforcements, this paper investigates its effect on the postcracking shear fatigue behavior of nonstirrup UHPC beams. The study involved five UHPC beams comprising two static and three fatigue specimens to test the effects of the upper fatigue load, steel-fiber ratio, and stirrup ratio on the shear fatigue response. The test results showed that increasing the initial crack width in the nonstirrup UHPC beam from 0.05 to 0.2 mm (the equivalent of doubling the upper fatigue load) decreased the fatigue life from over two million to 24,875 cycles. The use of steel fibers in place of stirrups increased the residual-to-static strength and residual-to-initial stiffness ratios by 5% and 16%, respectively, after two million cycles. Compared with the specimen with stirrups, the steel-fiber-reinforced specimen had a lower midspan fatigue deflection-to-ultimate static deflection ratio owing to the relieved fatigue strain localization and increased tensile steel reinforcement-UHPC bond fatigue strength. The current codes predict the shear fatigue strength conservatively. A calculation model for accurately predicting fatigue deflection was proposed and compared with the existing test results. A future study on the wider parametric analysis and large-scale beam tests on the fatigue performance of these beams should be conducted.