Flexural behavior and design of under-reinforced concrete beams with BFRP and steel bars

•The service limit of hybrid under-reinforced concrete beams was improved compared with that of FRP-RC beams.•The upper bound of the reinforcement ratio for hybrid-RC beams in the transition region was determined.•Hybrid under-reinforced concrete beams demonstrated better overall performance factors.•Existing design methods regarding flexural strength, deflection and crack widths were modified for hybrid under-reinforced concrete beams. This study aims to investigate the flexural behavior of hybrid reinforced concrete beams and study the design methods. Seven concrete beams reinforced with basalt fiber-reinforced polymer (BFRP) bars and steel bars, including six under-reinforced concrete beams and one slightly over-reinforced concrete beam, were constructed and subject to four-point loading. The reinforcement ratio was the main test parameter. The experimental results showed a transition region between the tension and compression failure modes. The upper bound of the reinforcement ratio of this region was determined through a statistical analysis of available data. The under-reinforced concrete beams with BFRP and steel bars yielded large deflections and crack widths, and the deformability factors of these beams were significantly larger than those of over-reinforced concrete beams with hybrid bars reported previously. The American Concrete Institute (ACI) method and Xue’s equation were modified and employed to predict the flexural strength of under-reinforced concrete beams with hybrid bars. The experimental results were utilized to verify the accuracy of existing models to predict the pre-yielding and post-yielding deflections and assess the bond coefficients of the ribbed BFRP bars.