From Neat Epoxy to Nanocomposites: Innovations in Bonding Systems for FRP-based Concrete Retrofitting: A Mini Review

Abstract

Structural retrofitting with Fiber-Reinforced Polymer (FRP) systems has become a vital approach for enhancing the strength, ductility, and durability of deteriorating concrete structures. The efficiency of these systems largely depends on the adhesive layer, where conventional Neat Epoxy (NE) adhesives often suffer from brittleness and limited crack resistance. Recent advancements in Nanomaterial-Modified Epoxy Adhesives (NMEAs) have led to notable improvements in their mechanical, thermal, and interfacial properties. Incorporating carbon-based nanomaterials such as Carbon Nanotubes (CNTs), Carbon Nanofibers (CNFs), and graphene has been shown to enhance fracture toughness, tensile strength, and load transfer. For example, introducing 0.1 wt.% single-walled CNTs resulted in a 13% increase in fracture toughness and a 3.5% improvement in compression-after-impact strength, while 0.5 wt.% multi-walled CNTs achieved up to 7% higher elastic modulus and 10% greater tensile strength compared to NE. Similarly, silicon-based nanomaterials, including silica nanoparticles and nanoclays, enhance stiffness, minimize porosity, and improve adhesion efficiency in both Externally Bonded Reinforcement (EBR) and Near-Surface Mounted (NSM) FRP systems. Spectroscopic and microstructural analyses reveal that nanoparticles influence cross-linking and crystallinity within the epoxy matrix, leading to more stable and durable adhesive bonds. Beyond mechanical enhancement, eco-friendly nanomaterials, such as rice husk ash-derived silica and biomass-based graphene, contribute to sustainability by lowering embodied carbon and extending the structural lifespan. This mini-review synthesizes recent developments, identifies critical research gaps, and outlines future directions toward resilient, high-performance, and sustainable FRP-retrofitting systems.