Experimental Study on the Behavior of High-Strength Reinforced Concrete Beams with a Longitudinal Void, with and Without Micro Steel Fibers, at Different Shear Spans

This study examines the impact of creating a longitudinal void in high-strength reinforced concrete (RC) beams to decrease concrete volume. Testing involved three groups: solid beams (SB), beams with a 75 mm void (VB), and beams with a void incorporating 1.2% micro-steel fibers (VBSF), across various shear spans (a/d) of 1.8, 2.7, and 3.6. Results indicated that increased a/d led to a 54.2% reduction in peak loads and a 119.2% increase in deflection for solid beams. The void negatively affected load-bearing at a/d = 1.8 with a 25% peak load reduction and brittle failure. At a/d = 2.7, effects were minimal, while a/d = 3.6 exhibited enhanced ductility but excessive deflection. Micro-steel fibers delayed crack propagation and improved ductility, with VBSF beams achieving peak-load increases of 39%, 19%, and 9.1% at a/d = 1.8, 2.7, and 3.6, respectively. However, when comparing VBSF to the SB beams, the combined effect of the void and steel fibers showed a higher increase in the load capacity for the intermediate and longer spans (peak-load increases of 5%, 22% and 20% at a/d = 1.8, 2.7 and 3.6 respectively). Overall, the results show that the reduction of such material can only be structurally viable under carefully controlled conditions and that steel fibers are effective in reducing the stiffness and flexure-shear penalties associated with the longitudinal void. Also, the post-peak response was successfully tuned to a fully plastic state by using the steel fibers for the intermediate and long shear spans.