Experimental Investigation on the Shear Performance of Concrete Beams Reinforced with Hybrid Steel–BFRP Stirrups and Basalt Macro Fibers

Abstract

This paper evaluates the shear behavior of reinforced concrete beams incorporating a novel hybrid shear reinforcement system. The system combines internal steel stirrups for ductility with external basalt fiber–reinforced polymer (BFRP) stirrups for corrosion resistance, along with basalt macro fibers (BMFs) in the concrete matrix. This configuration optimizes material placement according to its mechanical and durability advantages and assesses the interaction between hybrid stirrups and BMFs to further enhance structural performance and long-term durability. Nine beams were fabricated and loaded in four-point bending tests to examine the effects of transverse reinforcement type (steel, BFRP, hybrid), BMF volume fraction (0%, 0.75%, 1.5%), and shear span-to-depth ratio ( a/d = 2.0, 2.8, 3.5) on failure mode, shear capacity, crack control, and deformation behavior. The results revealed that, at an equivalent shear reinforcement ratio, beams with hybrid stirrups exhibited superior stress redistribution and ductile failure mode, demonstrating enhanced shear strength by 4% and 19% compared with beams with only steel or FRP stirrups, respectively. The incorporation of steel in the hybrid system improved stiffness and increased ductility by 42% relative to beams with pure FRP stirrups. The BMF inclusion led to enhanced postcracking tensile resistance, narrower crack widths, and an increase in the ultimate load capacity up to 46%. Reducing the a/d ratio from 3.5 to 2.0 produced a 53% decrease in maximum mid-span deflection and a 34% increase in ultimate load. Strain data demonstrated effective collaboration, with internal steel stirrups reaching their yield point, while external FRP stirrups remained intact. Furthermore, the shear capacity of the hybrid stirrup–reinforced beams was evaluated using the provisions of available design codes, with the most accurate estimate among these codes having an average experimental-to-predicted shear capacity ratio ( V _exp / V _pre ) of 1.23.