Study of Resin Durability Intended for Strengthening Steel Structures with UHM CFRP Plates

The use of fiber-reinforced polymer (FRP) composites for retrofitting concrete structures is well established, but their use in steel structures remains limited due to insufficient research on FRP/steel bond durability. The recent development of next-generation ultrahigh-modulus (UHM) carbon-FRP (CFRP) plates has significantly expanded the range of potential strengthening applications, particularly for structural steel elements. This study presents the first experimental investigation into the durability of adhesively bonded UHM-CFRP systems for strengthening steel elements under severe environmental conditions. The study investigates the environmental durability of two commercially available epoxy adhesives, TYFO TC and UREPOX 2C/2B, used in UHM-CFRP strengthening systems. Accelerated aging tests, including water immersion, saltwater immersion, and ultraviolet (UV) exposure under ambient and elevated temperatures, were conducted on 15 specimens per adhesive. TYFO TC demonstrated lower water absorption (2.6%–2.9%) than UREPOX 2C/2B (2.4%–3.5%), while UREPOX 2C/2B exhibited higher tensile strength, reaching up to 30 MPa. However, TYFO TC showed superior ductility, with ultimate strain values up to 7.1 times greater, and achieved a higher glass transition temperature (84°C versus 70°C). Both adhesives experienced UV-induced surface and chemical degradation, but TYFO TC maintained better elongation and ductility under these conditions. These findings demonstrate that TYFO TC is the recommended adhesive for UHM-CFRP applications on steel surfaces exposed to harsh environmental conditions, due to its superior elongation and overall durability.