Assessment of fire-induced damage and CFRP retrofitting strategies for GFRP-reinforced concrete columns

Abstract

Fibre-reinforced polymers (FRPs) are increasingly used to improve the durability and sustainability of reinforced concrete structures. Glass FRP (GFRP) bars offer a corrosion-resistant alternative to steel reinforcement, making them attractive for harsh environments and long-term infrastructure. However, their performance under elevated temperatures remains a concern and has not been clearly investigated. GFRP-reinforced concrete (RC) members are vulnerable to thermal degradation, and the structural consequences of fire exposure (such as bond loss, strength reduction, and instability) are not yet fully understood. One promising repair method is external confinement using carbon FRP (CFRP) wraps, which provide high strength and can be efficiently applied in the field. This study investigates the use of CFRP jacketing to restore axial capacity in fire-damaged GFRP-RC columns. A two-phase experimental program was conducted on fourteen square columns (200 × 200 mm), including both short (1000 mm) and slender (1500 mm) specimens, with variations in fire exposure (one or two hours) and confinement level (one or two CFRP layers). Ambient specimens had strain gauges surface-mounted on the longitudinal GFRP bars prior to casting, while fire-tested Specimens were embedded with thermocouples to monitor internal temperatures during exposure. Nine columns were fire-tested in accordance with the CAN/ULC-S101 standard, and seven were subsequently retrofitted with CFRP. All specimens were tested under axial compression with pin-ended boundary conditions. Lateral displacement was measured using LVDTs positioned at mid-height in both orthogonal directions. The program evaluated how fire duration, slenderness, and confinement affect residual and regained axial capacity. Test results showed that fire exposure caused significant capacity loss, with reductions amplified by fire duration and slenderness. Two-hour specimens retained only a fraction of their original strength, and slender columns showed greater thermal penetration and instability. Internal temperatures exceeded critical thresholds for GFRP degradation, leading to visible damage in bars and concrete. CFRP jacketing enhanced strength and ductility in all retrofitted columns. Confinement became more effective with increasing fire damage, as retrofitted columns showed greater strength gains relative to their unwrapped counterparts. However, even with multiple layers, severely damaged columns remained well below ambient strength. These results highlight both the potential and limitations of CFRP confinement in post-fire rehabilitation of GFRP-RC compression members. KEYWORDS Glass fibre-reinforced polymer (GFRP), Carbon fibre-reinforced polymer (CFRP), Reinforced concrete, Fire Safety Engineering, Residual axial capacity, Regained axial capacity, Post-fire rehabilitation, FRP confinement, Thermal degradation, Fire-damaged columns, CFRP jacketing, CAN/ULC-S101, Slenderness effects, Ductility enhancement.