Construction and Building Materials 17 (2003) 27–41 0950-0618/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0950-0618 Ž 02 . 00091-0 Shear capacity of FRP-strengthened RC beams: FRP debonding J.F. Chen *, J.G. Teng a, b Institute for Infrastructure and Environment, School of Engineering and Electronics, Edinburgh University, The King’s Buildings, a Edinburgh EH9 3JN, UK Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China b Abstract Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre- reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Fibre reinforced polymer; Fibre reinforced plastic; FRP; Debonding; Reinforced concrete beams; Shear design; Shear strength; Shear strengthening 1. Introduction A recent innovation for the shear strengthening of reinforced concrete (RC) beams is to externally bond fibre-reinforced polymer (FRP) composite plates or sheets. This method has become popular because of the advantages of FRP composites such as their high strength-to-weight ratio, good corrosion resistance, and versatility in coping with different sectional shapes and corners. Many studies on this theme have been carried out since the early 1990s w1–24x. These studies have established clearly that such strengthened beams fail in shear mainly in one of the two modes: tensile rupture of the FRP; and debonding of the FRP from the sides of the RC beam, depending on how the beam is strengthened w25x. Common methods of strengthening include side bond- ing, U-jacketing and wrapping (Fig. 1). Both FRP strips and continuous sheets have been used. The fibres in the FRP may also be oriented at different angles. The combination of different bonding configurations, fibre distributions and fibre orientations can result in many different strengthening schemes. Symbolic repre- sentations are used here as in Chen and Teng w7x when *Corresponding author. presenting the test database so that each shear strength- ening scheme is identified by a set of clearly defined symbols. Each of the shear strengthening schemes can be denoted by one symbol representing the bonding configuration (S for side bonding, U for U jacketing and W for wrapping), followed by a second symbol representing the fibre distribution (S for strips and P for plates ysheets) and followed by two sets of numbers representing the first and second fibre orientations (Fig. 1). For example, US45 y135 represents U jacketing with FRP strips at 45 and 1358. A more detailed discussion is given in Teng et al. w25x. Available experimental data indicate that almost all beams strengthened by wrapping failed due to FRP rupture (although debonding most likely occurs first, FRP rupture controls the shear capacity in this case). Some beams strengthened by U jacketing w6x also failed in this mode. A predictive strength model and a design proposal for this failure mode are given in Chen and Teng w7x. In contrast, almost all beams strengthened by side bonding only, and most strengthened by U jacket- ing, failed due to FRP debonding. Fig. 2 shows the possible debonding zones for both U jackets and side plates. Once the FRP starts to peel off, the beam can fail very quickly. The ductility of beams failing in this mode is usually very limited.