| The structural strength and durability of existing old building cannot be satisfied due to the long service life and serious structure ageing.Therefore,the repair and reinforcement of old buildings has become an urgent problem to be solved.FRP has excellent mechanical properties,i.e.,light weight,high strength,durability and high fatigue resistance.At present,the technology of externally bonded FRP laminates is widely used in practical projects.However,in the experimental study and engineering application,it is found that the internal force difference between the two sides of the reinforcement layer at the crack will be formed during the reinforcement process,leading to the FRP stripping of the mid-span pure bending section.In this paper,ECC was introduced into the concrete beams strengthened with FRP laminates as the transition layer.ECC with excellent crack control ability and strain hardening characteristics could effectively inhibit the crack development of the concrete layer and reduce the interfacial stress concentration to delay the debonding of FRP laminates and even change the failure mode of the beams strengthened with FRP laminates.In this paper,two new materials CFRP laminate and ECC were employed to the reinforced structure.Two groups of reinforced beams were carried out,including CFRP plate-ECC composite reinforced concrete beams and CFRP sheet-ECC composite reinforced concrete beams.Based on the phenomenon analysis and theoretical research,the calculation model of the bearing capacity of concrete beams strengthened with CFRP laminate-ECC composite is established.The main contents of this paper include:(1)PE-ECC with good performance was perpared by domestic PE fibers and local raw materials.The tensile strength and compression strength of ECC were were conducted.Through the optimization design of mixed proportion,the ultimate tensile strain of PE-ECC proposed in this paper can reach about 6%,which exhibits the excellent tensile properties.This mixed proportion of PE-ECC is used as the reinforcement material of RC beams in subsequent study.(2)The flexural performance of concrete beams strengthened by CFRP plate and ECC is conducted under four-point bending test.Parameters varied different reinforcement methods,CFRP width and matrix type.In addition,the variation of characteristic load value,crack development,deflection analysis,failure mode and strain distribution of CFRP plate of the reinforced beam are analyzed in the whole process.The research results show that the beam strengthened by CFRP plate and ECC have high bearing capacity and small ductility relative to the normal RC beam.The composite beams strengthened by PE-ECC matrix and CFRP plate exhibit excellent flexural performance.(3)The flexural behavior of reinforced concrete beams strengthened with CFRP sheets and ECC is conducted under four-point bending.PE-ECC is selected as the reinforcement matrix type,and the parameter variable is different layers of carbon fiber sheets.The experimental results show that using PE-ECC as the transition layer can not only effectively delay the delamination of the sheet,but also change its failure mode.The FRP sheet fractured at failure in the beam strengthened by PE-ECC and a layer of CFRP sheet.The utilization rate of the sheet is greatly improved.(4)The reinforcement efficiency of reinforced concrete beams with different types of laminates is compared and analyzed.Results show that the composite reinforcement of CFRP and ECC has good flexural performance.The beam strengthened by CFRP sheet increased crack laod and ultimate load by 23.3%and 20.3%,respectively,relative to the beam strengthened by CFRP plates and PE-ECC.The utilization of CFRP sheets increased by 130.4%relative to the CFRP plate.(5)Through the theoretical analysis of the whole process of flexural performance of concrete beams strengthened with CFRP laminate and ECC,the calculation model of the bearing capacity of composite reinforced beams is theoretically deduced in two cases.For the reinforced beams with flexural failure,the theoretical value deduced based on the concrete bending theory can be highly consistent with the experimental value.For the reinforced beams with central debonding failure,the Lu Xinzheng model can well predict the bearing capacity. |
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