Advances in bonding agents for retrofitting concrete structures with fibre reinforced polymer materials: A review

Retrofitting, strengthening, and repairing of deteriorated concrete structures have adopted epoxy adhesives for decades to bond fibre reinforced polymer (FRP) materials to the substrate of the deteriorated concrete elements. Despite being the most commercially used solutions, epoxy adhesives show several disadvantages, in terms of the performance of the strengthened elements and the toxicity of the material. Cement-based adhesives (CBAs) have been also considered for retrofitting concrete elements, but their use is very limited due to their poor performance, for instance, their low mechanical properties (i.e. compressive and tensile strength) compared to epoxy adhesives. The nanomaterial-modified epoxy adhesives and nanomaterial-modified CBAs (i.e. innovative high-strength self-compacting cement adhesives (IHSSC-CA)) have been effective substitutes with their extraordinary mechanical and thermal properties. This paper overviews the development of adhesives for FRP-strengthened concrete elements, in particular of epoxy and CBAs with the addition of nanoparticles. Both the mechanical and thermal properties of strengthening FRP and bonding interfaces with various adhesives are firstly investigated and compiled, and then the performance of retrofitted/strengthened-concrete elements with the FRP composites using neat and nanoparticles-modified adhesives is studied to examine the effectiveness of the modified adhesives over neat adhesives for retrofitting. Based on the compiled developments, further developments are identified, and recommendations are made for future research and innovations in this subject area.