Ultimate behaviour of FRP wrapped sections under axial force and bending: Influence of stress–strain confinement model

Abstract In the present work, the influence of the adopted confined concrete constitutive law, among those available in the technical literature, on the flexural strength and curvature ductility of reinforced concrete sections strengthened by FRP (fibre reinforced polymer) wrapping is investigated. An important issue to be underlined is that the stress–strain relationship of confined concrete depends not only on the number of layers and on the type of FRP used for wrapping, but also on the size and the shape of the section. By using the main constitutive laws proposed in the technical literature to model the confined concrete behaviour, the moment–curvature diagrams have been evaluated for a significant number of study cases by means of a specifically developed computer program based on a refined fibre model. The results show that even if the different constitutive laws exhibit large differences in the resulting stress–strain behaviour, they lead to negligible differences in terms of flexural resistance, but to very significant differences in terms of curvature ductility. Therefore, the accurate evaluation of the ultimate strain seems of paramount importance compared to the whole stress–strain curve. In addition, the influence of pre-existing loads acting on the structure at the time of the strengthening intervention has been investigated showing that it affects the knee region of the moment–curvature relationship, while the ultimate flexural resistance remains almost unaffected.

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