Punching shear strengthening of flat slabs using prestressed carbon fiber-reinforced polymer straps

• Punching shear strengthening of existing flat slabs gains importance.
• Local prestressing around the column reduces residual rotations after preloading.
• Punching shear resistance was nearly doubled by a prestressed CFRP strap system.
• Two different strap anchorings (adhesive or steel frame) were successfully applied.
• The steel frame system activated unloading friction between anchor and slab surface.

An experimental study of full-scale reinforced concrete flat slabs crosswise strengthened with prestressed carbon fiber-reinforced polymer (CFRP) straps against punching shear was performed. The effects of two strap anchoring systems and of slab thickness on punching behavior were compared. In one system the anchors were adhesively bonded to the concrete surface, while an external steel frame balanced the horizontal strap force components in the second system. Strap activation and thus strap force increments were higher in cases with either lower prestressing or higher strap stiffness. The deformability of the steel frame allowed a balancing of the strap forces. In the cases with steel frame, the friction bond between the steel anchors and the concrete surface reduced the concrete compressive zone. The system stiffness was thus increased and the rotation capacity reduced compared to the case with bonded anchors. The thickest slab exhibited a reduced rotation capacity and thus strap activation. The slab therefore failed at the lowest load increase, which however was still 67% above the theoretical resistance of the non-strengthened slab.