Tests and analysis of reinforced concrete beams under torsion retrofitted with FRP strips

The aim of the present study is twofold. First, the results of an experimental investigation on the torsional response of reinforced concrete (RC) beams retrofitted with epoxy-bonded carbon fibre-reinforced-polymer (FRP) strips as external transverse reinforcement are presented. Second, an analytical approach for the estimation of the torsional capacity and the entire behaviour of RC beams retrofitted with FRP fabrics is attempted. The experimental program comprises eight beams with a rectangular cross-section tested under pure torsion. Based on the measured values of the torsional moment at cracking and at ultimate and the behavioural curves of the beams, useful conclusions concerning the effectiveness of the FRP strengthening technique are indicated. Further, in order to describe the entire torsional behaviour of RC beams retrofitted with FRP, a method that employs the combination of two different analytical theories is used. The prediction of the elastic till the first cracking part is achieved using a smeared crack analysis for plain concrete in torsion, whereas for the description of the post-cracking response a modified softened truss model is proposed. The well-known softened truss model is properly modified to include the influence of the FRP fabrics on the torsional response as external reinforcement. Analytical torsional behaviour curves are compared with experimental ones from the present study and the literature, and first concluding remarks are mentioned.