Numerical investigation of concrete columns with external FRP jackets subjected to axial loads

The application of fibre reinforced polymer (FRP) composites as an external reinforcement for concrete columns has proved to be an efficient method of confinement for strengthening and retrofitting existing structures needing a performance enhancement. In recent years, many different numerical models have been proposed to evaluate the behaviour of FRP confined concrete, also focusing on different constitutive laws for concrete. In this paper, a damage model has been developed to simulate and predict the response of concrete columns under uniaxial loading, externally confined with carbon FRP (CFRP) jackets, mainly focusing on the role of cross-section shape and internal steel bars in the strengthening intervention efficiency. Particularly, the constitutive behaviour of concrete has been here formulated via a modified Mazars' damage law, which allows to evaluate the three-dimensional confinement effects in the columns during monotonic axial compression loads. Many analytical models included in the design codes as well as previously developed numerical models do not relate the external FRP jackets performances to the internal steel reinforcements: the present study pursues this task, trying to understand the mechanisms from a detailed model. Its overall good agreement with experimental results gathered from literature proves the correctness of the suggested formulation.