Research On Time-Dependent Mechanical Performance Of Reinforced Concrete Beams Strengthened With Externally Bonded CFRP

In recent years,due to the good cost performance,fiber reinforced polymers are widely used in concrete structure reinforcement engineering.Under sustained load,the creep characteristics of the component materials(concrete,steel,adhesive,FRP,etc.)of RC beams strengthened with FRP will affect the long-term mechanical behaviors of the strengthened structure.However,the current design theory of FRP reinforced concrete structures in China is based on short-term tests,and the time-dependent evolution of mechanical behaviors of reinforced structures under long-term loads is not clear.Therefore,based on the experimental and theoretical research,this dissertation discusses the influence of CFRP strain hysteresis effect and structural time-dependent characteristics on the long-term mechanical behaviors of RC beams strengthened with externally bonded CFRP.The main contents are as follows:1.Aiming at the generation mechanism and loading requirements of CFRP strain hysteresis effect,a FRP loading strengthening test method was proposed to make the tensile surface of RC beams upward,and the corresponding reverse loading test device was developed,which was then used to complete the experiment of long-term loading test of CFRP reinforced RC beam.Several conclusions were drawn: 1)with the increase of pre-load,the time-dependent deformation(deflection and strain)of reinforced structure is decreased;2)under sustained loading,the existing cracks continue to develop with the increase of time,resulting in the decrease of the structural section stiffness,the speed of which,however,will be reduced with the increase of the pre-load;3)under sustained loading,the time-dependent growth of strain in the compressive zone(concrete and compressive reinforcement)of the reinforced structure is significantly faster than that in the tensile zone(CFRP and tensile reinforcement),resulting in the move of the position of neutral axis to the tensile zone,and the height of the compressive zone of the section increases with time;4)under sustained loading,the stress redistribution of the cross section caused by the creep characteristics of the material makes the strain of CFRP increase with time,resulting in the increase of the shear stress of CFRP-concrete interface.2.The bending-debonding tests of the long-term loading test group and the short-term contrast test group were completed.The following experienmemtal conclusions were drawn:1)the height of compressive zone first increases with time under sustained loading,and then obviously decreases after the yield of tensile reinforcement;2)the FRP strain hysteresis effect reduces the yield load of the strengthened beam,while the structural time-dependent characteristics have little effect on the yield load of the strengthened beam;3)the strain of CFRP increases rapidly after the yield of tensile reinforcement,and the strengthened beam enters to the stage of unstable development with main cracks generated,where there is significant difference between the growth rate of CFRP strains on both sides of the cracks,leading to the local concentration of interfacial shear stress in cracked areas;4)under sustained loading,the strain hysteresis effect of CFRP and the time-dependent characteristics of structure will aggravate the local concentration of interfacial shear stress between adjacent bending main cracks,so as to accelerate the local debonding of bonding interface,and then reduce the flexural debonding bearing capacity of CFRP strengthened beams.3.Based on the material creep constitutive relation,plane section assumption,deformation compatibility condition and equilibrium equation,the normal section analysis method and the stress relaxation solution were used to analyze the instantaneous response of the reinforced structure under loading and the time-dependent behavior evolution under sustained loading.For two cases with or without pre-load,the calculation formula of the time-dependent evolution law of the section strain and the time-dependent law of the section stress redistribution of the reinforced structure are derived.The calculation process of the height of the section compression zone with time is compiled by MATLAB program.With the increase of pre-load,the height of the section compression zone of the reinforced structure decreases,and the concrete participating in compression creep decreases,which leads to the weakening of the overall time-dependent characteristics of the reinforced structure.4.For two cases with or without pre-load,the calculation formula of the instantaneous interfacial shear stress distribution between the adjacent bending main cracks of the reinforced structure is derived.Based on the time-dependent behavior analysis model of reinforced structure,the calculation formula of shear stress increment with time at CFRP-concrete interface is deduced.On this basis,combined with the maximum interfacial shear stress and the interfacial failure mechanism of intermediate crack(IC)debonding,the CFRP-concrete interfacial debonding failure judgment model considering the influence of CFRP strain hysteresis effect and structural time-dependent characteristics is established.It is found that when concrete of the original structure is older,the CFRP strain hysteresis effect and the time-dependent characteristics of the structure reduce the critical debonding load of the bond interface of the reinforced structure.5.Combined with the actual reinforcement process,the calculation formula of sectional stiffness of phased degradation is derived according to the curvature method.On this basis,the calculation formula of phased deflection is derived according to the virtual work principle.The coefficients of pre-load and pre-damage are introduced to modify the existing time-dependent deflection calculation model.A time-dependent deflection calculation model considering the strain hysteresis effect of CFRP and the loading history of the original structure is proposed,and the values and application scope of each parameter in the model are given.Based on the idea of phased superposition,a more accurate and efficient theoretical calculation method for long-term deflection of RC beams strengthened with CFRP is proposed.It is shown that the time-dependent deflection of the reinforced structure decreases significantly with the increase of the pre-load.