Pre-cracked RC beams shear strengthened with prestressed CFRP laminates applied according to the NSM technique

Abstract

Several reinforced concrete (RC) structures requiring strengthening already have cracks of different width, some of them of shear nature. However, there is an extra lack of knowledge on the effectiveness of using fiber-reinforced polymer (FRP) composites for shear strengthening of pre-cracked RC structure. To bring knowledge in this domain, RC T-cross section beams with maximum pre- induced shear crack widths of 0, 0.5 and 1.0 mm were strengthened in shear using near-surface-mounted (NSM) technique with passive and prestressed carbon fiber-reinforced polymer (CFRP) laminates. The effects of maximum width of the pre-cracks, passive or prestressed CFRP laminates, and CFRP shear strengthening ratio on the beam’s load carrying capacity, deflection performance, stiffness degradation and CFRP strain were analyzed. The ability of prestressed CFRP to restrain the pre-cracks in the beams while they are subjected to the sustained pre-load was also investigated. The test results showed that prestressed CFRP reduced the pre-crack width of RC beams under sustained pre-load by 12.4–50.4 %. The pre-cracking, despite to have decreased the beam’s stiffness, had minimal influence on the growth rate of the maximum shear crack width, and the decrease of the beam’s load carrying capacity was limited to 3.0 % and 14.5 % for a pre-crack width of 0.5 and 1.0 mm, respectively. Applying prestress to CFRP laminates and decreasing CFRP spacing were effective in reducing the maximum shear crack width, enhancing the load carrying capacity and improving stiffness of strengthened beams. A formulation for calculating the contribution of prestressed NSM CFRP for the shear capacity of RC beams was proposed and its good predictive performance is demonstrated.