The Effect of Compression Reinforcement on the Shear Behavior of Concrete Beams with Hybrid Reinforcement

Abstract This study examines the impact of steel compression reinforcement on the shear behavior of concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars, steel bars, and hybrid reinforcement of GFRP and steel bars. Nine beams, measuring (1200 mm x 150 mm x 250 mm), were tested under a three-point loading system. The beams were divided into three groups based on the GFRP-to-steel reinforcement ratio in the tensile region: T1 (100% steel), T2 (50% GFRP, 50% steel), and T3 (100% GFRP). Each group had three beams that differed in the amount of compression reinforcement as a percentage of tensile reinforcement (0%, about 50%, and about 100%). Beams fully reinforced with GFRP showed a significant decrease in ultimate load capacity compared to those with hybrid or steel reinforcement. Beams with partial or full GFRP reinforcement exhibited wider cracks and higher deflection than steel-reinforced beams. Conversely, adding compression reinforcement increased ultimate load capacity, particularly in beams with the highest compression reinforcement amount, and reduced deflections, strains, crack widths and crack formation across all groups. This behavior underscores the lower stiffness of GFRP and the benefits of adding compression reinforcement, which enhanced the stiffness and shear strength of the tested beams. Hybrid reinforcement of GFRP and steel bars in the tensile region, paired with an adequate amount of compression reinforcement, provides an optimal reinforcement strategy. This approach balances stiffness and ductility, and enhances the shear capacity and overall performance of concrete beams compared to those reinforced only with GFRP.