Effect of Using a Hybrid Strengthening and Repairing Technique on the Punching Shear Performance of Undamaged and Damaged RC Flat Slabs

Abstract A hybrid system of near surface–mounted carbon fiber–reinforced polymer (NSM-CFRP) and externally bonded CFRP has proven efficiency in improving and recovering the structural performance of the undamaged and damaged structural elements, especially when used for the RC flat slabs. The main objective of this research was to propose strengthening and repairing techniques to enhance the punching shear performance of RC flat slabs. Specifically, a hybrid system of CFRP ropes embedded through an intact and preloaded RC flat slab surface and then anchored by CFRP sheets was used to investigate its efficiency in enhancing and restoring the structural properties of the undamaged and damaged flat slabs. Nine square RC flat slabs with 1,000-mm side length and 120-mm width were cast using normal-weight concrete with a compressive strength of about 26 MPa. The specimens were used as follows: one specimen was used as a control specimen, four specimens were strengthened using vertical and inclined CFRP ropes embedded through the flat surface anchored by CFRP sheets, and four specimens were preloaded up to 50% of the ultimate capacity of the control specimen and then repaired using vertical and inclined CFRP ropes embedded through the flat surface anchored by CFRP sheets. Test results indicated that the use of a hybrid system can improve the structural performance and punching shear behavior of RC flat slabs, whether undamaged ones or damaged by preloading. Additionally, the most effective strengthening technique in terms of improving the ultimate capacity of the flat slab was to apply the CFRP ropes inside the concrete surface vertically on both punching perimeters anchored by CFRP sheets; with this system, the ultimate capacity increased by 66.2%, whereas applying the same strengthening technique on the first punching perimeter only increased the stiffness, by 23.2%. The same performance was observed for the preloaded specimens, but when applying the rope with an inclination inside the flat slab, the percentage increase in the ultimate capacity when applying the repairing technique on both punching perimeters was 84.4%, and 14.2% increase was recorded in stiffness when applying the repairing technique on the first perimeter only. The use of the hybrid repairing and strengthening technique contributed to enhancing the failure mode performance of the flat slab; all specimens exhibited a debonding failure of the CFRP sheets, followed by a concrete cover separation.