Concrete softening effects on the axial capacity of RC jacketed circular columns

Among the different strengthening techniques to repair RC structures, reinforced concrete (RC) jacketing is one of the most commonly adopted, especially for columns. Its wide application is due to its easy application and relatively reduced cost with respect to other methods (e.g. FRP wrapping, Shape Memory Alloy active confinement). The target of RC jacketing is to increase axial, flexural capacity and ductility of weak existing members by means of two main effects: confinement action provided by the jacket and composite action between external jacket and inner concrete. Different theoretical studies have been carried out to calculate the strength enhancement due to confinement action and most of these are based on the adaptation of classical models for confinement evaluation in RC members. However, experimental investigations on compressive behaviour of RC jacketed sections have shown that the actual axial capacity could be substantially lower than that analytically evaluated with classical confinement models. This fact could be explained by taking into account the presence of tensile stresses developing in the jacket, due to the different dilatation of the inner core and the external jacket. This phenomenon is relevant especially in members where the concrete properties of the jacket are different with respect to those of the core, causing the premature failure of the external layer. This paper presents a simplified approach able to evaluate these effects. In particular RC jacketed circular columns are analysed as case study and the member is studied by considering the different concrete properties of core, jacket and cover. Circumferential and radial stresses are firstly calculated under the assumption of linear elastic behaviour and plane strain state. The model is extended in the non-linear range by adopting a secant constitutive law. Finally, comparisons are made with experimental data available in the literature, showing good agreement and explaining some experimental results