Durability of NSM CFRP-to-concrete bond after six years of exposure to laboratory and outdoor environments

The long-term durability of near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthening systems remains a critical issue for reinforced concrete structures exposed to environmental actions. Although CFRP exhibits high mechanical performance and corrosion resistance, the durability of the CFRP–concrete bond is significantly affected by the ageing behaviour of the bonding agents. This study investigated the long-term durability of NSM CFRP-to-concrete bond systems following a six-year period of exposure to both laboratory-controlled and natural outdoor degradation environments. Two laboratory conditions (Environment E1: 20 °C and 55% RH; Environment E2: continuous water immersion at 20 °C) and four outdoor exposure sites in Portugal were considered: Lisbon (E3), Serra da Estrela (E4), Elvas (E5), and Viana do Castelo (E6). Mechanical characterisation was performed on concrete, epoxy adhesive, and CFRP laminate, together with single-lap shear tests on NSM CFRP–concrete bond specimens. Therefore, after six years of ageing: the CFRP tensile strength varied between −4% and + 9% relative to the reference (initial) condition, while elastic modulus variations remained between −2% and + 4%; epoxy adhesive tensile strength decreased by up to 76% under water immersion, while outdoor environments produced reductions of approximately 8–24%; concrete compressive strength varied between −12% and + 4%, depending on the exposure environment; and the maximum pull-out force increased by approximately 10% under controlled laboratory conditions (E1) and decreased by up to approximately 9% under water immersion (E2) and in the Serra da Estrela outdoor mountain environment with freeze–thaw exposure (E4). Adhesive-related failure modes were predominant across all environments. The results provide rare long-term experimental evidence under both laboratory and real outdoor exposures, highlighting the critical influence of constituent materials, including adhesive durability, on the NSM CFRP bond performance. The findings support improved durability assessment and more reliable long-term design of NSM CFRP strengthening systems.