Mechanical Properties of Hybrid Fiber-Recycled Concrete and Flexural Performance of Its BFRP-Reinforced Beams

The combined use of recycled aggregate concrete (RAC) and basalt fiber-reinforced polymer (BFRP) bars offers a promising sustainable and corrosion-resistant solution for reinforced concrete structures. However, the inferior quality of recycled aggregates and the relatively low elastic modulus of BFRP bars can compromise the mechanical and flexural performance of RAC members. To address these issues, hybrid fiber-reinforced recycled aggregate concrete (HFRAC) incorporating polyvinyl alcohol (PVA) and steel fibers was developed, and its mechanical and flexural performances were experimentally investigated. The basic mechanical properties of conventional Portland cement concrete (PC), fiber-free RAC, and RAC with hybrid fiber (HF) contents of 0.6%, 0.9%, 1.2%, and 1.5% were first evaluated. A total of nine beams were subsequently tested under four-point bending to investigate the effects of HF content (0-1.5%) and BFRP reinforcement ratio (0.48-1.98%) on flexural behavior. The results showed that an HF content of 1.2% provided the best performance among the investigated fiber contents at both the material and structural levels. At the material level, compared with RAC, 1.2% HF increased the cube compressive strength, axial compressive strength, elastic modulus and splitting tensile strength by 19.44%, 23.08%, 11.39% and 32.55%, respectively. The incorporation of HF effectively mitigated the mechanical deterioration caused by recycled aggregates, allowing HFRAC to achieve comparable or improved basic mechanical properties relative to RAC. At the structural level, compared with the fiber-free RAC beam, the beam with 1.2% HF exhibited increases of 132.51% and 11.92% in cracking and ultimate loads, respectively, and a 47.9% reduction in crack width, while also demonstrating improved flexural performance compared with the PC beam under the investigated conditions. Three failure modes were observed, namely BFRP bar rupture, balanced failure, and concrete crushing, with balanced failure occurring at a reinforcement ratio of approximately 1.0-1.1%. The hybrid fibers effectively refined cracks through a bridging effect, demonstrating superior crack control compared to increasing the reinforcement ratio alone. This study offers valuable insights into improving the performance of RAC and facilitating the wider adoption of BFRP bars in structural applications.