Load-Bearing Performance of Reinforced Concrete Beams Subjected to Secondary Strengthening with Carbon Fiber Reinforced Polymer

The potential for carbon fiber reinforced polymer (CFRP)-strengthened reinforced concrete (RC) beams to achieve satisfactory load-bearing performance through secondary strengthening, after reaching their ultimate capacity and experiencing CFRP failure, has been infrequently explored. As a result, the feasibility of secondary strengthening and the associated mechanical behavior remain inadequately understood. This paper presents an experimental study aimed at investigating the secondary strengthening of RC beams with CFRP subsequent to the occurrence of CFRP debonding or rupture under loads significantly exceeding the ultimate capacity of conventional RC beams. Four-point bending tests were conducted to evaluate key indicators for the secondary-strengthened beams, including load-bearing capacity, stiffness, crack development, and strain distribution. The following observations were made regarding the secondary-strengthened test beams in this study: the ultimate load-bearing capacity of the secondary-strengthened beams is approximately 35.83% to 62.99% greater than that of an ordinary RC beam, and 2.8% lower to 14.75% higher than that of a primary-strengthened beam. The number of cracks in the secondary-strengthened beams remains largely unchanged, while existing cracks from the primary-strengthened system exhibit slight widening, with negligible growth in length. Additionally, the secondary-strengthened beams demonstrate enhanced overall stiffness compared to the primary-strengthened beams. A zero-strain concrete zone is identified within the cross-section of the secondary-strengthened beams, with a height ranging from approximately 0.25h to 0.8h when flat CFRP sheets are employed for strengthening. These experimental results indicate that CFRP-strengthened RC beams may retain the potential for secondary strengthening even following the failure of the initial CFRP system. The findings provide preliminary data that could facilitate further research into the secondary strengthening of RC beams using CFRP, and they may also offer a new exploratory perspective for re-evaluating the reliability and load-carrying performance of primary CFRP strengthening.