Experimental study of flexural behavior of high‐strength concrete beams with hybrid reinforcement under repeated loading

Abstract

This study investigates the flexural behavior of high‐strength concrete beams reinforced with steel, basalt fiber‐reinforced polymer (BFRP), and hybrid steel–BFRP systems under repeated loading. The experimental program included nine geometrically identical beams tested under four‐point bending, comprising one steel‐reinforced beam, four BFRP‐reinforced beams, and four hybrid beams with varying steel‐to‐BFRP ratios. Repeated loading was applied at 70% of the static ultimate capacity with constant amplitude, and the loading cycles were continued until the mid‐span deflection reached 70%–80% of the ultimate deflection obtained from monotonic tests, followed by monotonic loading to failure. The study evaluated load–deflection response, residual deflection, stiffness degradation, cumulative‐energy dissipation, crack propagation, and post‐loading flexural capacity. Results show that hybrid beams exhibited significantly improved performance compared to BFRP‐reinforced beams, with substantially lower residual deflection and reduced stiffness degradation under comparable loading cycles. For beams subjected to approximately 24–25 cycles, stiffness degradation ranged from about 9% to 38% depending on the reinforcement ratio, while residual deflection was reduced to as low as 2.60 mm in steel‐dominant hybrid configurations. Hybrid beams retained up to 98% and 96% of their original static capacity after repeated loading, indicating strong resistance to damage accumulation. This improved performance is attributed to the combined action of steel and BFRP reinforcement, where steel provides crack control, tension stiffening, and ductility through yielding, while BFRP contributes high tensile strength and corrosion resistance. Based on the experimental results, a cumulative damage index incorporating stiffness degradation, residual deflection, and energy dissipation was proposed to quantify damage progression. The model was further used to predict the equivalent number of cycles required to reach a critical damage state (DI = 0.99) under the applied loading protocol.