Repair of heat-damaged RC flat slabs using a hybrid CFRP rope–sheet technique

The punching shear behavior for the flat slabs is considered the most important parameter that might be affected when the slab is subjected to elevated temperatures, which makes it important to use a repairing technique for such damaged slabs. Additionally, the use of NSM-FRP ropes proved their efficiency in improving the structural and mechanical performance of the heat-damaged RC structural elements, especially when being applied in combination with EB-CFRP sheets. The efficiency of using hybrid repair techniques in recovering the structural properties of the heat-damaged flat slab has been investigated in this study. A total of eight RC flat slabs with a square shape were used in this study. Each specimen had a side length of 1000 mm and a thickness of 120 mm. Additionally, a central square-shaped RC column positioned at the middle of the span, with a side length of 150 mm, was used to ensure a punching shear failure for all specimens. One specimen was designated as an intact specimen, four specimens were heated at 600 °C for three hours, one of them was kept as a control, and the remaining three specimens were repaired using CFRP ropes anchored by CFRP sheets, and additional three specimens were preloaded up to 50% of the ultimate capacity of the control specimen and then heated at 600 °C for three hours, then repaired using CFRP ropes anchored by CFRP sheets. Test results revealed that the heating process harmed the ultimate capacity and the stiffness of the RC flat slab, where the ultimate capacity and stiffness decreased by 13.8% and 24.3% relative to the control specimen, respectively. For the non-preloaded heat-damaged flat slabs, the most effective technique in recovering the ultimate capacity and stiffness was to use inclined CFRP ropes on the first punching perimeters anchored by CFRP sheets, where the ultimate capacity and stiffness increased by 91.3% and 18.8%, respectively. On the other hand, the use of inclined CFRP ropes on both punching perimeters anchored by CFRP sheets was the most effective technique in recovering the ultimate capacity and stiffness of the pre-loaded heat-damaged flat slab, where the ultimate capacity increased by 74.5% and the stiffness increased by 21.2%.