Behavior of Short and Slender Concrete Columns Reinforced with GFRP Bars and Square Spirals under Concentric and Eccentric Loads

Abstract

With growing interest in sustainable reinforcement, this thesis investigates the structural behavior of GFRP-reinforced concrete columns, emphasizing the effects of square spirals and ties under concentric and eccentric loading. A combination of experiments, analytical modeling, finite element analysis, and database evaluation is used to address gaps in current design provisions. The experimental program involved 21 column specimens (200 × 200 mm) with slenderness ratios of 20, 40, and 60, subjected to load eccentricities of 0%, 15%, and 30% of the section height. Key variables included spiral pitch, tie spacing and overlap length, and longitudinal reinforcement ratio. Results showed that increasing the tie overlap length by 40% beyond the code minimum shifted the failure mode from brittle, crack-free failure to gradual, post-peak degradation. While square spirals enhanced the stability of short columns under moderate loads, their confinement effectiveness diminished in slender columns and at higher eccentricities. To better understand the role of transverse reinforcement, a novel experimental method was developed by removing the concrete cover, allowing direct observation of failure modes in both longitudinal and transverse GFRP bars. These tests were complemented by 3D finite element (FE) models using concrete damage plasticity (CDP) for concrete and the 3D failure criterion for GFRP. The findings demonstrated that reducing tie spacing and increasing overlap length improved post-peak strength retention. A new deformability index was proposed, and multiple FE modeling approaches were evaluated. Due to the absence of standardized methods for compressive testing of GFRP bars, a novel fixture was developed. A total of 61 specimens with three bar sizes (15M, 20M, 25M) and length-to-diameter ratios of 2, 4, and 6 were tested. The results showed that compressive strength was approximately 86% of tensile strength at a ratio of 2, decreasing with increasing length, while tensile and compressive moduli were nearly identical. Analytical modeling included cross-sectional and second-order analyses. The axial load–moment (N–M) interaction diagrams, developed using equilibrium and strain compatibility, suggest that equivalent stress block approach provides lower-bound estimates; applying a scaling factor of 1.1 could bring the estimates into closer agreement with the analytical results. A second-order analysis using sine-shaped deformation curves yielded a practical slenderness limit of 14 for design. Finally, a comprehensive database of 274 GFRP-RC column tests was compiled to evaluate and refine design code predictions. A new empirical formula was proposed to estimate peak load capacity and effective flexural stiffness, incorporating eccentricity and slenderness effects and addressing limitations of ACI 440.11-22.