Investigating the effects of steel fibers and notch depth on the fracture behavior of reinforced concrete (RC) beams using both matrix-cracked and through-thickness cracked approaches

Abstract

Steel fibers are commonly used to compensate for the tensile weakness of concrete. However, the interaction of steel fibers with conventional reinforcing bars after a crack has been formed is poorly understood. Most fracture studies use a cut through the thickness of the material that cuts through any fiber in its path, whereas a real crack would maintain the bridging action. In this study, the true mode I fracture toughness (KIC) of steel-fiber-reinforced concrete beams with longitudinal steel bars is determined using a matrix-crack (MC) approach where short fibers are not broken through a natural pre-crack, and compared with companion beams prepared with a conventional through-thickness crack (TTC). Reinforced concrete (RC), normal strength concrete (NC), and fiber-reinforced concrete (FRC) beams with a steel fiber volume fraction of 1.0%, two ratios of notch depth to beam depth (a/d=0.25 and 0.5), and two reinforcement layouts were tested under four-point bending. Incorporation of steel fibers delayed crack propagation and improved ductility, increasing the ultimate load-carrying capacity by up to 50% and the modulus of toughness by nearly 94% relative to fiber-free beams. All cases showed higher first-cracking loads and better post-cracking load maintenance for MC specimens than for TTC specimens, confirming that the MC approach provides a more realistic representation of fiber bridging. KIC increased with a/d in plain reinforced beams, due to the growing moment arm of the steel closing force, but decreased with a/d in fiber-reinforced beams, indicating that fiber-bridging efficiency weakens as the crack deepens. The results support the MC method as a more reliable approach for assessing the intrinsic fracture toughness of fiber-reinforced RC members.