The growing demand for durable and corrosion-resistant concrete structures has led to significant interest in Fiber-Reinforced Polymer (FRP) bars as a viable alternative to conventional steel reinforcement. However, the linear-elastic nature of FRP bars combined with the inherent brittleness of plain concrete frequently results in brittle, catastrophic failures under axial compression. To mitigate these limitations, this study presents a comprehensive three-dimensional (3D) numerical investigation of the performance of square FRP-reinforced concrete columns incorporating hybrid steel fibers (SF) and polypropylene fibers (PPF) under pure axial compression.

Finite Element Models (FEM) were developed in ABAQUS, employing the Concrete Damage Plasticity (CDP) constitutive model to capture the nonlinear compressive and tensile behavior of hybrid fiber-reinforced concrete. The numerical models were systematically validated against experimental data (load deformation curve and load-carrying capacity) from the literature to confirm their reliability. An extensive parametric study was subsequently conducted, examining four primary variables: (1) the ratio of steel to polypropylene fibers at a constant total volumetric content of 1%; (2) the longitudinal GFRP reinforcement ratio (1.0%, 1.5%, and 2.0%); (3) transverse stirrup spacing (100, 150, and 200 mm); and (4) column slenderness ratio (L/D = 4, 6, and 8).

The key performance indicators evaluated included axial load capacity, ductility index, energy absorption capacity, compression damage index (DAMAGC), and overall performance index. The results demonstrated that, among the investigated combinations, 0.75% SF + 0.25% PPF exhibited the best overall performance, attaining a peak load of 852 kN and a ductility index of 3.26, representing enhancements of 22.06% and 28.85%, respectively, compared to the control specimen. Increasing the GFRP reinforcement ratio from 1.0% to 2.0% yielded additional load capacity gains of up to 8.3%, whereas a denser stirrup spacing of 100 mm provided superior confinement. The slenderness analysis confirmed progressive strength degradation with increasing column height, with F5 retaining the highest absolute load at all slenderness levels. These findings provide insights into the design and optimization of FRP-reinforced hybrid fiber-reinforced concrete columns.

Keywords: FRP bars; Steel fibers; Polypropylene fibers; Hybrid fiber-reinforced concrete; Concrete Damage Plasticity; ABAQUS; Finite Element Analysis; Axial compression