Flexural performance of RC beams strengthened with inorganic adhesive bonded NSM CFRP bar

Organic adhesives (e.g. epoxy) have been widely utilized in near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthening technique for reinforced concrete (RC) structural members. Their inevitable shortcomings, such as poor fire resistance, emission of toxic fumes, and incompatibility with concrete substrate, necessitate the advancement of eco-friendly inorganic adhesives. In this study, two types of inorganic adhesives, including ordinary Portland cement mortar (OPCM) and alkali-activated slag mortar (AASM), were adopted in the NSM CFRP bar strengthening method for RC beams. The inorganic adhesive type, bonding length, and additional anchorage device on CFRP bar were considered as parameters in the strengthening scheme. One control beam and four strengthened beams were fabricated and tested, followed by comparison of their failure modes, loading behaviour, and reinforcement strains. The results show that all the strengthened beams exhibit higher ultimate loads than the control beam. Comparing to the OPCM, the AASM is more effective in mitigating debonding of CFRP bar and enhancing the ultimate load of beams. Furthermore, it is essential to provide a sufficient bonding length for AASM bonded NSM CFRP bars to guarantee their strengthening effectiveness. The addition of anchorage device for CFRP bars can further enhance the synergistic action between CFRP bar and adhesive, leading to a further enhancement in flexural strength and CFRP bar strain. In addition, the flexural strengths of OPCM or AASM bonded NSM CFRP bar-strengthened beams can be accurately predicted by the equations provisioned in ACI 440.2R-2017.