Re-evaluation of shear contribution of CFRP and GFRP sheets in concrete beams post-tensioned with unbonded tendons

Contemporary design guidelines for the shear resistance of post-tensioned concrete (PC) beams retrofitted with fibre reinforced polymer (FRP) sheets are developed from the results of reinforced concrete (RC) beams. The accuracy of these guidelines for calculating the shear resistance of PC beams wrapped with FRP sheets is still questionable due to the difference in the shear behaviour of RC vs PC beams. The effects of the essential factors (e.g. tendon profile and anchorage system) on the shear resistance of PC beams have not been sufficiently studied, particularly for unbonded post-tensioned concrete (UPC) beams. This study presents an experimental investigation on the shear behaviour of UPC T-beams and the shear contribution of CFRP/GFRP sheets. The experiment consisted of 26 large-scale UPC T-beams with various factors including the tendon profile (harped or straight), concrete strength, FRP anchors, CFRP vs GFRP sheets, and various wrapping configurations. The experimental results showed that CFRP/GFRP sheets increased the shear resistance of UPC beams by 4-26%, which was much lower than that in RC beams. In comparison with the UPC beams with straight tendons, those with harped tendons exhibited a reduction in the shear contribution of CFRP/GFRP sheets (approximately 40%) and showed a more ductile failure mode with an improvement of the beam deformation (up to 2.3 times). FRP anchorage systems significantly enhanced the FRP shear contribution and energy absorption increased by 88% and 42% regarding those without anchors, respectively. Besides, it was observed from the test data that the widely-used method to determine the experimental shear contribution of FRP sheets in PC beams based on the difference between the shear resistance of strengthened and unstrengthened beams was inappropriate because it failed to consider the interactions between FPR sheets and other shear components, i.e. concrete and stirrups. Meanwhile, the experimental shear contribution of FRP sheets is recommended to be determined from the actual strain data of the FRP sheets across the main shear crack.

Automated summary

This study investigates the shear behavior and shear contribution of CFRP/GFRP sheets in unbonded post-tensioned concrete (UPC) T-beams through experiments. The results show that CFRP/GFRP sheets increase the shear resistance of UPC beams, although to a lesser extent compared to reinforced concrete (RC) beams. T-beams with harped tendons exhibit lower shear contribution of CFRP/GFRP sheets but show improved ductility and beam deformation. FRP anchorage systems significantly enhance the shear contribution and energy absorption of the FRP sheets.