Ductile behaviour of prestressed CFRP plate strengthened RC beams: Concept and experimental verification

Carbon fibre-reinforced polymer (CFRP) plates are high-strength, lightweight, and corrosion-resistant composite materials that have been extensively utilized for flexural strengthening of reinforced concrete (RC) structural members . Due to the elastic-brittle nature, RC beams strengthened with CFRP plates typically exhibit brittle failure modes, either through debonding at the CFRP-to-concrete interface or rupture of the CFRP plate. Consequently, current design codes consider the contribution of CFRP primarily as a safety margin, thereby limiting its full potential as a high-performance material. This paper proposes an unbonded external prestressing method for strengthening RC beams using CFRP plates. The proposed method incorporates a mid-span supporting technique previously developed by the authors, which alters the application of prestressing forces and simplifies the strengthening process. The mid-span supporters are designed as elastic-plastic, maintaining the CFRP plate at a high stress level below its tensile strength during loading to postpone brittle failure. Firstly, the concept of the proposed method is presented in this paper. Subsequently, a series of parametric studies were conducted using finite element (FE) analysis to determine the optimal elastic-plastic behaviour of the supporters. Based on the FE results , two types of elastic-plastic supporters were designed, and experimental investigations were carried out on both strengthened and unstrengthened beams. The strengthened beams with elastic-plastic supporters demonstrated up to a 72 % increase in flexural strength compared to the unstrengthened control beam. Moreover, the tensile stress in the CFRP plate was successfully maintained below 97 % of its tensile strength until concrete crushing occurred. The ultimate deflection reached 3.2 times that of the beam with rigid supporters. The intactness of the CFRP plate confirms the success of the proposed strengthening method. • New strengthening method for RC beams using prestressed CFRP plates. • Ductile failure mode of prestressed CFRP strengthened RC beam. • Mid-span supporting prestressing technique incorporating nonlinear supporters.