Performance and Parametric Analysis of Flexural Strengthening for RC Beams with NSM-CFRP Bars

The flexural strengthening of concrete structures by the near-surface-mounted (NSM) method has proven to be a reliable alternative to the existing externally bonded (EB) method using carbon fiber–reinforced polymer (CFRP) composites. However, the NSM method has not been used as widely as the EB method, and many factors influencing the effectiveness of NSM-CFRP bars still need to be investigated and clarified. Therefore, a total of eight concrete beams were constructed and tested. The effects of different parameters, including the number of NSM-CFRP bars, prestressing in NSM-CFRP, and the type of end treatment—for reduced/delayed end bond failure—on the performance of the strengthened beams were investigated. The test results demonstrated that the use of NSM-CFRP bars is effective in increasing the flexural capacity of concrete beams. However, when the number of CFRP bars was increased from one to two, only the postyielding cross-section stiffness of the beam displayed a better performance. The prestressed beams exhibited a higher cracking load and a higher yield load compared with the nonprestressed beams. The prestressing considered in this regard was 40% of the ultimate capacity. Three end treatments were considered: U-shaped steel-plate hoops, expanded groove at the ends of the bar, and reduced resin modulus at the bar ends when prestress is released. Of these, the end treatment using a U-shaped steel-plate hoop developed in this article was simple and effective and was capable of increasing the ductility of both prestressed and nonprestressed beams. By addressing the effects of the aforementioned factors on the NSM strengthened beams, this paper offers valuable information for the increased implementation of this technique.