Finite element analysis of flexural behavior in ultra-high-strength concrete beams with BFRP and hybrid BFRP-steel reinforcement

Three-dimensional finite-element (FE) simulations were developed to assess the flexural response of ultra-high-strength concrete (UHSC) beams reinforced with basalt-FRP (BFRP) bars and with BFRP-steel hybrids. The models were calibrated against published tests and reproduced midspan force-deflection curves, reinforcement strains, and crack development with good fidelity. A parametric study examined (i) the BFRP reinforcement ratio relative to a balanced level and (ii) the BFRP-to-steel area ratio in hybrid sections. Results show that increasing the BFRP ratio raises ultimate flexural capacity but reduces deformability and energy dissipation. Hybrid reinforcement recovers inelastic reserve while preserving the durability advantages of FRP, yielding a more favorable balance between strength and serviceability. A yield-anchored modified ductility index tailored for hybrid sections is formulated to quantify post-yield rotational capacity. The findings provide design guidance for proportioning UHSC beams in conventional and aggressive environments: BFRP reinforcement ratios of ρ_f/ρ_fb ≥ 1.4 are suitable where strength and durability dominate, whereas hybrid layouts are recommended when controlled deflection and ductility are also required.