Research On The Design Method Considering Time-dependent Behavior Of Concrete Beam Strengthened By Fiber Reinforced Polymer

In recent years,CFRP are more and more widely used in the reinforcement of bridges,tunnels,buildings and other concrete structures or components.Concrete,adhesives and CFRP in reinforced structures all have time-dependent characteristics,in the long-term service,the peak stress and effective bond length at the end of reinforced beams will be affected by time-dependent characteristics.The peeling load at the end of the CFRP is directly affected by the peak stress,and the contribution value of shear bearing capacity of CFRP is directly affected by the effective bond length.At present,scholars at home and abroad still lack sufficient research on this field.The finite element analysis model of flexural strengthening of concrete beams and double-sided shear test with CFRP are established by Abaqus in this paper to research on the above problems.The research results are applied to practical engineering case analysis.The main research work and achievements include:(1)The current research status,existing problems and deficiencies of CFRP reinforced concrete structures at home and abroad are summarized.(2)The finite element analysis model of flexural strengthening of concrete beams and double-sided shear test with CFRP are established by Abaqus,and the creep properties of adhesives and concrete are also considered.The results of finite element analysis show that only when creep properties of concrete are considered,the peeling normal stress increases by 34% and the peeling shear stress at the interface increases by 47% after 90 days.When both the creep properties of adhesives and concrete are considered,the peeling normal stress increases by 14% and the peeling interfacial shear stress increases by 24% after 90 days.Creep of concrete increases peak peeling normal stress and peak shear stress of the interface at the end of CFRP,while creep of adhesives decreases peak peeling normal stress and peak shear stress of the interface at the end of CFRP.(3)Based on the analysis results of 16 groups of the finite element model of CFRP flexural strengthened beams with time-dependent material properties,the average increment coefficients of peeling normal stress of the interface at the end of CFRP after 100 days of different grade loads,such as 0.3,0.5,0.7 and 0.9 times ultimate load,are 21.75%,17.43%,15.41% and 13.10% respectively.The average increase coefficients of peak shear stress of the interface are 24.68%,19.43%,16.94% and 14.67% respectively.In structure's design,this paper suggests that when the time-dependent properties and safety of materials are considered,the peak normal stress increasing coefficient is 25%,and the peak shear stress increasing coefficient is 29%.(4)Based on the analysis results of 36 groups of the finite element model of double-sided shear specimens with time-dependent material properties,The average increment coefficients of effective bond length after 100 days of different grade loads,such as 0.3,0.5,0.7 and 0.9 times ultimate load,are 28.98%,30.30%,31.97% and 32.13% respectively.The effective bond length will not change significantly with the increase of external load.In structure's design,this paper suggests that when the time-dependent properties and safety of materials are taken into account,the effective bond length increase factor is 31%.(5)The nominal principal stress of concrete under biaxial stress and the peeling failure criterion at the end of CFRP are revised considering the time-dependent properties of materials.The theoretical formulas of peeling failure loads of reinforced beams under concentrated load,two-point symmetrical load and uniformly distributed load in midspan are deduced considering the time-dependent properties of materials.An example of CFRP flexural strengthened beam is given,and the peeling failure load of this example is calculated considering the time-dependent material.(6)An example of CFRP shear strengthening is given.The modified effective bond length is substituted into the theoretical formula of CFRP shear bearingcapacity.The shear bearing capacity of CFRP considering the time-varying properties of materials is calculated.The effect of increasing shear bearing capacity of CFRP considering time-varying properties is discussed.