Research on the Flexural Behavior of Hybrid Fiber-Reinforced BFRP Lightweight Aggregate Concrete Beams

To simultaneously address the deterioration of mechanical properties in lightweight aggregate concrete (LAC) and the insufficient deformation control capacity of hybrid fiber-reinforced polymer (BFRP) bars, an experimental study on the flexural behavior of hybrid fiber-reinforced BFRP-LAC beams was conducted. A total of eight beams with dimensions of 120 mm × 200 mm × 2000 mm were fabricated. The effects of hybrid fibers and BFRP reinforcement ratio on the flexural performance were investigated. Four-point bending tests were performed to analyze the failure modes, load–deformation responses, crack development patterns, and sectional strain distributions. The results indicate that two failure modes were experimentally observed in the BFRP-reinforced hybrid fiber LAC beams, namely concrete crushing and BFRP bar rupture, whereas balanced failure was considered a theoretical failure condition. The failure mode was strongly dependent on the reinforcement ratio. At a low reinforcement ratio (ρ = 0.68%), tensile failure governed by BFRP bar rupture occurred. At a moderate reinforcement ratio (ρ = 1.02%), a relatively ductile concrete-crushing failure was observed. When the reinforcement ratio increased to 1.56% and 1.81%, brittle concrete-crushing failure dominated. The incorporation of hybrid fibers improved the ductility and optimized the failure process. Both the hybrid fiber content and the BFRP reinforcement ratio significantly influenced the load-carrying capacity and deformation behavior of the beams. Increasing the fiber content enhanced the cracking load and ultimate load, delayed crack propagation, and reduced crack width, whereas increasing the reinforcement ratio was more effective in improving the ultimate capacity. The load–deflection curves exhibited a typical two-stage response without a yielding plateau. The bridging effect of hybrid fibers effectively mitigated stiffness degradation and improved crack control performance. Moreover, the plane section assumption was validated for hybrid fiber-reinforced BFRP-LAC beams. This study provides a technical basis for enhancing the performance of LAC and promoting the application of BFRP bars in structural engineering.