Strengthening of Flat Slabs for Punching Shear using Carbon Fiber Reinforced Polymer Sheets

Punching shear strength frequently governs the failure of slab-column connections. In general, the ultimate punching shear capacity of flat slabs can be increased by using thick slabs, drop panels, capitals, or shear reinforcement. In recent decades, steel fiber concrete has been also used to enhance the punching shear strength of flat plates and slabs. Concrete structures must be retrofitted and rehabilitated when the concrete has deteriorated or the reinforcing steel has been severely corroded. The use of fiber-reinforced polymer (FRP) is an innovative technology to strengthen concrete structures. In this paper, punching shear strengthening of flat slabs using Carbon Fiber Reinforced Polymer (CFRP) sheets is studied. Ten specimens of reinforced concrete slabs with the dimensions of 1000*1000*100 mm with different compressive strengths were manufactured and tested. Eight of them were strengthened by CFRP sheets and the others were kept as control specimens. The width of CFRP sheets varied in different specimens. The CFRP sheets were located at the tension side of the slabs in two perpendicular directions. Loading of slabs was applied downward using a hydraulic Jack. In the tests, the applied load and the displacement of specimens were measured using a computer aided data acquisition system. Based on the result of this experimental investigation, it can be concluded that the use of CFRP sheet increases the flexural stiffness and significantly improves the punching shear strength of slabs. The test results were compared with the equations proposed by ACI 318 and BS 8110 Codes. The ACI Code underestimates the punching shear strength of slabs and this underestimation becomes more pronounced with the increase in the flexural reinforcement. The BS 8110 Code appropriately accounts for the effect of flexural reinforcement on punching shear strength of slabs. However, for the strengthened slabs, an equivalent reinforcement ratio should be used to include the effect of both steel and CFRP flexural reinforcement. Based on the test results and code equations, new equations were proposed. It is shown that the proposed equations predict the punching shear strength of reinforced slabs with good accuracy.