Innovative Hybrid Adhesive System for Enhanced Bond Performance in NSM FRP Strengthening of Concrete Structures

The use of near-surface mounted (NSM) fibre-reinforced polymer (FRP) has become as an effective strengthening technique for improving the performance of concrete structures. While epoxy adhesives (EA) are commonly used in NSM FRP systems, they present significant drawbacks including toxic emissions and reduced mechanical strengths under high temperatures and moisture. To address these limitations, this study introduces an innovative hybrid adhesive system for NSM FRP strengthening, combining epoxy and cement-based adhesives to leverage the high bond strength of polymers with the durability and ductility of cementitious materials. A new cement-based adhesive (CBA) formulated from commercially available components and two hybrid adhesives (by combining 25% and 50% of EA with CBA) were developed and evaluated through comprehensive pull-out testing. Results demonstrated that hybrid adhesives achieved maximum bond stresses 1.5-2.2 times higher than pure cement-based adhesives and 1.1-1.6 times higher than conventional epoxy systems, while exhibiting 3-6 times greater fracture energy than epoxy adhesives. The hybrid adhesives achieved 6.70-10.02 MPa bond strength with exceptional ductility (19.96-33.14 N/mm fracture energy), representing a noticeable improvement over pure epoxy and cement-based adhesives.  Multi-scale interface characterisation identified significant optimisation potential, with NSM systems utilising less than 50% of available FRP/adhesive capacity. Analytical validation also confirmed the superiority of ANN-based prediction models developed for cement-based systems over conventional empirical formulations mainly developed for epoxy.