Interfacial Bonding Characteristics And Flexural Performance Of Hybrid-Bonded FRP Strengthened RC Beams

China’s infrastructure has experienced a wave of construction for decades.Many long-span high-section concrete bridges are overwhelmed that having suffered environmental erosion and overload for a long time.A large number of structures are in service with diseases and need to be reinforced urgently,otherwise may cause severe disaster.Fiber composite materials have been widely used in structural repair and reinforcement due to its characteristics of lightweight,high efficiency and environmentally friendly.Externally bonded FRP composite material is the most popular reinforcement method to solve the problem of insufficient bearing capacity,but it’s premature debonding is a key factor affecting the performance of the material,which also limits its larger-scale application.Hybrid bonded FRP reinforcement technology is an important breakthrough in the field of FRP reinforcement.This technology combines the FRP external bonding method and mechanical fastening,which solves the key technical problems of premature debonding of FRP.This system can activate more mechanical fasteners to achieve higher bonding strength,and greatly improve the structural bearing capacity.However,the composite effects of various bonding surfaces in the hybrid bonded system cause its mechanism complex,which can not be directly revealed through the test at present.The simple and reliable bond-slip model has not been obtained,nor has it established a calculation method for its debonding capacity.The flexural design method of hybrid bonding reinforcement has not been formed,and its bearing capacity to suppress debonding cannot be predicted,which limits the application of this technology.FRP reinforcement ratio,mechanical fastener’s size,and distribution are the most important factors on the flexural performance of hybrid bonded FRP strengthening concrete beams.However,there is little literature on these issues.The slip between FRP and concrete will significantly affect the load and deformation development of reinforced beams.At present,it is not yet possible to effectively calculate the amount of slip,nor can it achieve a design that meets strength and ductility.Therefore,this paper carried out study on the interface bonding characteristics and flexural behavior of concrete beams strengthened with hybrid bonded FRP.The main work and achievements of this paper include:(1)Designed test to separate the hybrid bonding into three parts including FRP bonded concrete,friction under positive pressure and FRP bonded steel(dowel action).In the hybrid bonded FRP reinforcement,the dowel action participates in different time sequences,causing a delay in the development of the bond,so its action can not be ignored.The trilinear model can well reveal the interfacial bonding constitutive characteristics under hybrid bonding.But due to the dowel action,the delay of peak stress caused by dowel slip needs to be considered.Adopting the standard test method for the bond-slip relationship proposed in this paper,the bond-slip relationship at the position of the mechanical fastener can be directly tested.For the problem that the dowel effect in hybrid bonding is difficult to quantify,a non-bonded steel plate system is proposed,in which the system is simple in design,effective in anchoring,uniform in force and short in the construction period.The bond-slip calculation formula of the composite interface can be obtained based on the superposition of external bonding and friction.(2)Based on the trilinear bond-slip model and the bonding characteristics of the through-length hybrid bonding,the bonding characteristics of the discontinuous stress at the mechanical fastener are analyzed,the development law of interfacial stress is revealed and the interfacial debonding load of hybrid bonding is predicted.Increasing the width of the steel plate and the length of the external bonding can increase the ultimate load and the corresponding slip.However,when the length of the external bonding is longer than the effective bonding length,the ultimate load will no longer increase.(3)The direct shear test of the hybrid reinforcement system was carried out,and the complete bond-slip constitutive relationship of the interface was obtained.A calculation method of hybrid bonded reinforcement system was proposed based on PI(Partial interaction)model.The effect of spacing,quantity and distribution mode of mechanical fasteners on the tensile performance of the interface was analyzed XIV systematically.According to the characteristics of the bond-slip relationship of hybrid bonding and external bonding,the critical distance of non-synergistic interaction between adjacent mechanical fasteners is proposed;Based on the calculation formula of the debonding bearing capacity of a single mechanical fastener,the calculation formulas of the debonding bearing capacity of the reinforcement system of multiple mechanical fasteners were obtained,which were verified by tests and PI numerical analysis methods.(4)It was obtained that local debonding due to sufficient crack width and the bond force is smaller than the shear force caused by external load at interface are required for global debonding;Hybrid bonding FRP reinforcement solves the problem of brittle debonding of external bonding and improves its failure mode;A complete flexural design process for hybrid bonding FRP reinforcement is proposed,and the formula for calculating the bearing capacity of hybrid bonding FRP to suppress debonding is obtained.(5)Based on tests,the failure mode of flexural strengthening with different geometry and distribution of mechanical fastener were compared and analyzed.A three-dimensional finite element model considering the bond-slip relationship between steel bars and concrete,strands and concrete,and the bond-slip relationship between FRP and concrete was established.The effect of beam length on the mechanical fasteners’distribution was analyzed,the geometry and design parameters including fiber reinforcement ratio,steel plate width,anchor size and fastener spacing on the load-bearing capacity and material utilization efficiency of the flexural strengthening beams were revealed.In the design of flexural strengthening,the above-mentioned geometry and design parameters need to be designed item by item and consider the relationship between various factors in order to meet the design requirements of load capacity and failure conditions.(6)The distribution of mechanical fasteners affects the slip strain between FRP and concrete,thus affecting the failure mode and ductility of the control section of mid-span;Due to slip,the flexural capacity of FRP to achieve effective tensile strain is improved;FRP achieves effective tensile strain and concrete crushing at the same time to distribute mechanical fasteners that can meet both strength and ductility requirements.A semi-theoretical and semi-numerical method based on finite element analysis was proposed to determine the distribution of mechanical fasteners,an implementation process was established and it was applied to the strengthening of prestressed concrete T-bridge.