Shear Behavior and Failure Mechanisms of Hybrid Structural Beams Comprising Pultruded GFRP and Rubberized Concrete

This study investigates the shear behavior and failure mechanisms of innovative hybrid structural beams fabricated by filling pultruded glass fiber-reinforced polymer (GFRP) box sections with waste rubber-reinforced concrete (RuC). Environmentally friendly concrete was produced by replacing natural aggregate with recycled tire-rubber fibers at proportions of 0%, 5%, 10%, and 15%. Twelve hybrid beam specimens were tested to evaluate the synergistic effects of rubber content and stirrup spacings of 16, 20, and 27 cm on shear capacity, ductility, and crack propagation. The experimental results revealed that the reference specimen (S16-0%) exhibited the maximum shear capacity of 154.41 kN and a brittle failure mode, while an increase in rubber content to 15%, combined with wider stirrup spacing, significantly reduced this capacity to a minimum of 96.89 kN (S27-15%). However, the 5% rubber replacement ratio achieved an optimal performance balance by preserving sufficient load-carrying capacity while enhancing flexural deformation and ductility, particularly in specimens with 16 cm stirrup spacing. Damage analysis demonstrated that longitudinal splitting cracks initiated in the mid-span tension zone at the bottom of the pultruded profiles, with final localized damage concentrated at the geometric corners of the box section. Crucially, the outer pultruded GFRP profiles provided substantial structural confinement, effectively mitigating the strength loss associated with high rubber incorporation and controlling the progression of sudden brittle failure. These findings highlight that combining pultruded GFRP profiles and optimized RuC offers a structurally viable and sustainable solution for modern infrastructure applications.