Study On Mechanical Properties Of Steel-concrete Composite Beams Strengthened With CFRP Sheets In Negative Moment Region

Composite steel-concrete bridge girders are ubiquitous in nations with developed civil infrastructure.This structural form can maximize the superior performance of steel and concrete.In order to improve the traffic comfortableness of highway bridges and the continuity of the superstructure,continuous composite structures are always the best choice in multi-span bridges.However,under the action of negative bending moment,the cross-section rigidity and service life of multi-span continuous composite beams will decrease continuously due to the cracking of concrete slabs.When the structure is damaged for various reasons,the use of appropriate materials and technologies for reinforcement is more economical and time-saving than demolition and reconstruction.CFRP(Carbon Fiber-Reinforced Polymer)material has great potential in structural reinforcement by virtue of its excellent performance.Therefore,in this paper,the mechanical properties of composite beams strengthened with CFRP sheets under negative bending moment are studied by using experimental,theoretical and finite element analysis methods.The main work and conclusions of this paper are as follows:(1)Eight steel-concrete composite beams with the same cross section were designed.CFRP sheets were bonded to the top of the concrete slab.Four-point bending reverse loading tests were carried out.The material strain,mid-span deflection,load,crack width and interface slip were tested.The test results show that the specimens without CFRP reinforcement show typical bending failure mode of steel-concrete composite beams,and the ductility of composite beams decreases with the increase of shear connection.All specimens strengthened with CFRP sheets can play a greater role after the steel beam yield,and the failure mode of the strengthened specimens is CFRP strip failure.The use of CFRP reduces the ductility of steel-concrete composite beams.Increasing the shear connection degree and the number of CFRP layers can improve the peak load of the strengthened beam,reduce the crack spacing and width,and reduce the slip value of the beam end and the deflection of the service stage.Under the same number of layers,the composite anchorage technology further improves the peak load and the utilization rate of CFRP sheets,and further reduces the crack width.(2)Based on the analysis of the measured strain distribution of CFRP sheets and the observation of the failure phenomenon,aiming at the debonding failure problem of the specimen reinforced with CFRP sheets,the debonding failure mechanism of reinforced beams caused by intermediate bending cracks is studied,and the reliability of the existing calculation model of debonding strain and the current reinforcement specification is analyzed.The calculation formula of CFRP strain in the mid-span of composite beams strengthened by composite anchorage method is proposed when the peak load is reached.The calculation formula of bearing capacity of composite beams under negative bending moment is established considering shear connection degree,CFRP layers and composite anchorage measures.The research shows that the existing calculation model and the current reinforcement specification are relatively conservative when calculating the debonding strain of CFRP sheets.In addition,the calculation method of flexural capacity of reinforced beams in negative bending moment zone deduced in this paper can better consider the influence of shear connection degree,CFRP layers and anchorage measures,and the calculation results are close to the experimental values.(3)The finite element software ABAQUS was used to simulate the static performance test of reinforced beams under negative bending moment based on the constitutive relationship measured by the material property test and the results of the reverse loading test,considering the influence of CFRP sheets on the deformation behavior of reinforced beams.In the proposed finite element model,the mixed mode cohesive law is used to describe the interface behavior under the combined action of normal stress(mode-I loading)and shear stress(mode-II loading).The bilinear tractionseparation model is used to represent the interface behavior under pure mode-I loading and pure mode-II loading.The quadratic stress criterion is used to define the damage initiation,and the linear power law criterion based on the fracture energy is used to define the damage evolution.In addition,the nonlinear properties of materials,geometry and contact are considered in the model.By comparing with the experimental results,the validity of the proposed model is verified in terms of load deflection relationship and bending capacity.