Bond mechanisms of various shapes of NSM CFRP bars

The use of the near-surface-mounted (NSM) fibre reinforced polymer (FRP) strengthening technique to retrofit existing concrete structures is now mainstream, with many examples across the world. The NSM FRP technique involves bonding FRP bars into pre-cut grooves in the concrete cover of a structural member to be strengthened, using an adhesive. It offers many advantages over external bonding of FRP reinforcement, for example, increased bond capacity and protection against external damage. Even though there are several guidelines available across the world for use of FRP as structural materials, none of them covers all the design aspects associated with these advanced technologies. Lack of experimental data, design tools, and analytical models addressing those design issues create obstacles for the efficient use of these advanced polymer materials. The particular objectives of this research project are to investigate the bond behaviour between the NSM FRP bars and concrete, and to develop a rational analytical model to predict the anchorage length requirements for NSM CFRP bars. Variables such as bond length, size and shape of bar, surface texture of bar, groove size (thereby resin thickness), concrete strength and resin type have been considered. In particular, attention has been focussed on the effect of bar shape on bond behaviour. Bond behaviour of round, rectangular and square FRP bars has been compared, whilst maintaining a constant bonded surface area (i.e. the product of bar perimeter and bond length). The various modes of anchorage failure are discussed in detail along with their underlying mechanisms.