Shear capacity of unbonded post-tensioned concrete T-beams strengthened with CFRP and GFRP U-wraps

Existing codes and design guidelines have not mentioned a procedure to calculate the shear resistance of unbonded post-tensioned concrete beams strengthened with fiber reinforced polymer (FRP) sheets. Up to date, the number of studies about the shear behaviour of post-tensioned concrete beams strengthened with FRP sheets in shear is very limited, particularly for unbonded post-tensioned beams. The effect of many factors on the shear resistance of such the beams has not been well investigated, for example, fiber factors (the type of fiber, the strengthening scheme, and the number of layers), the concrete strength, and the ratio of the shear span to effective depth. The study deals with the shear behaviour of unbonded post-tensioned beams strengthened with FRP U-wraps. The experiment consists of twenty-two post-tensioned beams with T section and unbonded tendons. The variables include concrete strength, number of FRP layers, FRP U-wraps scheme (continuous and spaced), types of FRP, and varied shear span to effective depth. The experimental results have shown that the FRP shear strengthening is more effective with higher concrete strength. The number of FRP layers, strengthening scheme, and type of FRP have a slight influence on the shear resistance of the beams but they significantly affect the ultimate deformation of the FRP sheets. The efficiency of FRP U-wraps considerably reduces with a reduction of the shear span to effective depth. As a result, the existing design guides may not yield reliable predictions since they have not considered this ratio. Moreover, a semi-empirical model is proposed to predict the shear resistance of unbonded post-tensioned beams strengthened with FRP U-wraps. The predictions from the proposed model fit well with the experimental results from other studies