An Approach Of Steel Plate-FRP Hybrid Strengthening To Reinforced Concrete Beams

The technique of external bonding of fiber reinforced polymers(EB-FRP)to strengthen the reinforced concrete structure has been extensively used in civil and hydraulic engineering.But the bonding interface between the FRP materials and the concrete surface is so weak that the debonding of FRP laminate is likely to occur to the concrete structures strengthened with EB-FRP technique easily.The failure of FRP premature debonding results in tiny utilization of the tensile strength of the FRP material,greatly limiting the performance improvement of the concrete structures strengthened with FRP materials.Therefore,scholars conduct a large number of research and exploration on the anti-debonding technology and mechanism in order to improve the interfacial bond strength to enhance the strength utilization of FRP materials.Based on the improved HB-FRP technology that combined with the traditional EB-FRP technique and the improved mechanical fastening system,this paper proposes the Steel Plate Hybrid Bonded FRP(SPHB-FRP)technique.To examine the effectivity of SPHB-FRP technology,this paper performs experimental studies on 23 strengthened concrete beams to discuss the impacts of this technology on the ultimate bearing capacity,ductility,failure mode,crack distribution and quantity and the distribution of FRP strain.In order to verify whether the SPHB-FRP method has more advantages than HB-FRP technology,the two techniques are applied to respectively reinforce concrete beams and conducted under three-point bending tests.The test parameters include the number of FRP layers,the interval of mechanical fasteners,the way of interfacial treatment and the strengthened method.Results show that SPHB-FRP technique presents no better anchorage to the strengthened beam with smooth interface,and it cannot prevent the debonding failure when the interval of anchors is 200 mm.However,for the strengthened beams with rough interface,they show greater and more stable ultimate bearing capacity and better ductility than those strengthened with HB-FRP method when they are strengthened by the same layer number of FRP materials with SPHB-FRP,and the bearing capacity of the HB-FRP strengthened beam is greatly affected by the anchor spacing,which,therefore,the interval can be moderately increased when using SPHB-FRP technology.The steel plate in SPHB-FRP technology can not only increase the effects of anchoring and prevent the premature debonding of FRP laminates,but greatly improve the rigidity of the SPHB-FRP strengthened beam and make cracks distribute evenly and narrowly.For the reinforced concrete beams strengthened with 8 layers of CFRP strips,the experimental results show the failure of the beam strengthened with the HB-FRP method was characterized by FRP rupture.Although SPHB-FRP technology has not achieve the goal of preventing CFRP debonding,compared to HB-FRP technology,the ultimate bearing capacity and the ductility of the strengthened beams can be positively enhanced.What's more,the ultimate bending moment of SPHB-FRP strengthened beams can be computed with simplified methods and the calculated results are well consistent with the experimental results.