Influence of concrete compressive strength on the debonding failure of externally bonded carbon fiber reinforced polymers

This paper investigates the interactions between externally bonded FRP and high strength concrete substrate in small scale reinforced concrete beams strengthened with CFRP. The bond mechanism was assessed through beam-tests adapted from ASTM, designed to simulate normal and shear stresses induced at the FRP/concrete interface. The beams were 600 mm long, with a square 150 mm × 150 mm cross-section, two 5 mm steel reinforcing bars, 6.3 mm steel stirrups spaced 75 mm apart, one single layer of externally bonded CFRP, and concrete compressive strength of 30 MPa, 50 MPa, 80 MPa, or 90 MPa. The mechanisms of damage initiation and debonding propagation showed to be highly dependent on the load transferring and the stress redistribution after concrete cracking. The use of high concrete compressive strength affected the pre-cracking response, the stress transfer, and the overall ductility at failure, resulting in a less effective FRP strengthening scheme. For service conditions, the strains in the FRP were lower, the higher the concrete compressive strength. The FRP strains at failure were below the maximum strain level to avoid intermediate crack-induced debonding determined according to current design standards