Fibre reinforced plastic (FRP) products were first used in construction during the 1950s. Over the following two decades the quality of the FRP materials improved considerably while the material costs decreased as manufacturing methods became more automated. Reinforced concrete bridges, however, were not externally reinforced with FRP materials until 1980s. FRP can be manufactured to provide a wide range of varied characteristics [2]. The ranges of ultimate tensile strength and, or the modulus of elasticity for FRP materials, for example, are much wider than the corresponding ones for steel. Since the composite behaviour of reinforced concrete elements strengthened with external FRP plates is affected by the characteristics of the materials forming the composite element, considerable variation in the behaviour of composite elements is expected with significant changes in the FRP material characteristics [3]. Different modes of failure are expected to happen with different beam configurations and material properties. For example, over-reinforced beams (either internally or externally) are expected to fail in a brittle manner with less deformation compared to under-reinforced beams. There are distinctly different characteristics associated with each mode of failure [4].

In this paper, eight different modes of flexural failure, associated with the externally strengthened reinforced concrete beams, are identified. While two of these modes are unlikely to occur when steel plates are used with another two modes being unlikely to occur for FRP plated beams, there does exist four common modes of failure which can happen for beams strengthened with either of these two materials. The equilibrium and compatibility equations for all the identified eight possible modes of failure associated with steel/FRP plated beams are developed in this paper.

1

D.J. Oehlers, "Reinforced Concrete Beams with Plates Glued to their Soffits", Journal of Structural Engineering, ASCE, 118(8), August, 2023-2038, 1992. doi:10.1061/(ASCE)0733-9445(1992)118:8(2023)

2

L.C. Hollaway and M.B. Leeming, (Editors),"Strengthening of reinforced concrete structures using externally-bonded FRP composites in structural and civil engineering", Woodhead Publishing Limited, Cambridge, England, 1999.

3

M. Raoof and M.A.H. Hassanen, "Peeling failure of R.C. beams with FRP or steel plates glued to their soffits", Proceedings of the Institution of Civil Engineers, Structures and Buildings, 140, 291-305, August 2000. doi:10.1680/istbu.2000.32601

4

M.A.H. Hassanen, "Behaviour of R.C. beams upgraded with externally bonded steel or FRP plates", PhD Thesis, Loughborough University, UK, 2000.

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