A model to predict the torsional stiffness of ‘U-wrapped’ reinforced concrete beams

Wrapping is one of the effective ways of strengthening concrete elements. Several researchers have reported on the effectiveness of wrappings made from glass-fibre-reinforced polymers and carbon-fibre-reinforced polymers for improving the strength of concrete elements. Wrapping on three sides (‘U-wrapping’) is an effective method for enhancing the strength of beams supporting slabs, but scant literature is available on the strength enhancement of ‘U-wrapped’ concrete elements subjected to torsional loads. In the investigation reported in this paper, an attempt was made to quantify the improvement in the twist of U-wrapped rectangular concrete members subjected to torsional loads. Ferrocement was used as the wrapping material. Beams were cast with different amounts of reinforcement in the core and different wrapping portions, and the core concrete grade and wrapping mortar strength were also varied. The wrapped beams were analysed with the help of MARS software. An analytical model was subsequently developed to predict the secant stiffness at cracking torque (SSCT) using skew bending theory incorporating material properties. The model predictions for SSCT were found to be in good agreement with experimental test results.