Abstract:The object of this research is to investigate the flexural behaviour of concrete beams strengthened with NSM CFRP. A particular focus is the efficiency of the NSM strengthening system in beams with different proportions of internal steel reinforcement. A total of five full-scale concrete beams, including a beam with NSM reinforcement but no longitudinal tensile steel, two reinforced concrete control beams, and two reinforced concrete beams strengthened with NSM CFRP reinforcement, are tested under monotonic static loading. The beams with NSM CFRP reinforcement all fail due to the de-bonding of the CFRP. The strengthening efficiency of the NSM CFRP does not appear to be greatly influenced by the amount of internal steel reinforcement. Two existing calculation models recommended by ACI Building Code 318 and ACI Building Code 440F are discussed, but are found to have certain limitations. A new theoretical model to explain the de-bonding phenomenon is proposed, and a method of calculating the deflection of the NSM CFRP reinforced concrete beams based on the summation of discrete rotations at flexural cracks and the NSM CFRP bond-slip relationship is developed. A comparison between the new theoretical predictions and the experimental results is made, and the results indicate a good agreement.