Rehabilitation of defected RC stepped beams using CFRP

Stepped beam is an example of non-prismatic beams that provides stress concentration at the stepped joint and requires an adequate detailing for such joint in order to avoid premature failure. Although, fiber reinforced polymer (FRP) materials are widely used for the strengthening and retrofitting of concrete structures and bridges, the choice of effective FRP-strengthening configuration for special RC members such as stepped beams is still a challenging issue. This paper presents both experimental and analytical investigations undertaken to evaluate the ability of externally bonded (EB) CFRP strips and sheets to restore the ultimate capacity of defectively detailed stepped beams. Five beams were reinforced with insufficient bond length flexural reinforcement and were designed to be failed by de-bonding. Two additional beams were adequately reinforced with two different correct details and served as control specimens. Before testing, four defected beams were strengthened using hybrid system of both EB-CFRP strips and sheets with different configurations. The strengthening strategy of these beams was based on analyzing their stepped joints using strut-and-tie model with all possible arrangements of tension and compression members and then applying the strengthening strips and sheets parallel to the obtained tension ties. Test results showed that, in contrary to the observed behavior of the correctly detailed beams, the un-strengthened defected beam exhibited premature splitting failure due to slippage of the main reinforcement and, its load carrying capacity was decreased by about 77% compared with that of the correctly detailed beam. However, strengthening the defected beams with EB-CFRP had not only restored the defected beams flexural capacity but also, prevented the early steel reinforcement de-bonding and consequently enhanced the flexural performance of the strengthened beams. Finally the most efficient EB-CFRP strengthening configuration is proposed.