Flexural strengthening of reinforced concrete beams with NSM-CFRP bars using mechanical interlocking

Flexural strengthening of reinforced concrete (RC) beams using near-surface-mounted (NSM) technique has become an attractive alternative for rehabilitation using fiber reinforced polymer (FRP) materials. Previous studies have recommended using available anchoring techniques to overcome premature bonding failure. In this study, mechanical interlocking grooves were utilized to delay or prevent debonding failure. The first part of the study aimed to investigate the bond characteristics for NSM-CFRP bars by conducting several pullout tests on No. 6, No.10, and No.13 CFRP bars. Results indicate that mechanical interlocking grooves can significantly enhance the bonding capacity and prevent or delay premature bonding failure. In the second part, the proposed NSM CFRP strengthening technique was used to strengthen nine RC beams. In addition to longitudinal grooves, the proposed technique consisted of 6 mm wide lateral grooves (or mechanical interlocking) placed at 76 mm on center along the entire length of the strengthened beams. Steel reinforcement ratios of 0.7% and 0.4% were selected. All beam specimens were tested under four-point bending until failure. Results showed that strengthening was more effective for specimens with a lower steel reinforcement ratio. Percentages of enhancement in flexural strength were between 34-68% and 60–128% for specimens with 0.7% and 0.4% steel reinforcement ratios, respectively. Finally, a simple empirical model was created for the experimental results. Theoretical results showed reasonable agreement with the experimental results. However, the maximum load carrying capacity and flexural stiffness were overestimated for beams with a total reinforcement ratio (steel plus CFRP) larger than 1.1%.