Analysis Of Bond Performance Between Steel Reinforcing Bars And CFRP Layer To Concrete Under High Temperatures

In the reinforced concrete structures,the steel reinforcing bars or carbon fiber reinforced composite(CFRP)to maintain good bonding properties to concrete is the fundamental condition to ensure the normal work of the structural members and the bond is a kind of complex interaction process,which conveys stress of concrete between the steel reinforcing bars or CFRP to concrete and makes the deformation coordination.At elevated temperatures,the mechanical properties of concrete and reinforcing steel as well as the bond between the two materials may significantly deteriorate.The changes in the bonding behavior may influence the the stiffness,cracks extension or the moment capacity of the reinforced concrete structures.Previous research results show that:the concrete stress condition,the crackextension state,the mechanical properties changes of concrete,steel reinforcing bars or CFRP'and other factors will have a great effect on the mechanical bond-slip relationship.In recent years,the damage and degradation of bonding properties between concrete and steel reinforced rebars at elevated temperatures has attracted more attention to many scholars and have carried out a series of experimental study to obtain certain research results.But for the complex of influencing factors such as the damage and failure mechanism of bonding properties and the difficulty of high temperature test technology,so the investigation of bonding between steel rebars and concrete at elevated temperatures is quite difficult in practice.So it is necessary to carry out theoretical and experimental research on the bond performance between steel reinforced rebars and concrete at elevated temperatures.In this paper,based on the previous work,a further study is made on bond performance between steel and CFRP to concrete under high temperatures,the main research work is as follows:1.The mechanical bond-slip relationship for steel-reinforced concrete at elevated temperatures is investigated using an analytical model based on the the principle of fracture mechanics.A steel reinforced rebars-concrete composite structural member under high temperatures is considered as a generalized elastic body with both the applied load and the interfacial shear stress acting as boundary stresses,and the debonding process is modeled as crack propagation along the interface.The energy principle in fracture mechanics is used to derive the energy relationship in the process of debonding,and an energy-based criterion for reinforced steel rebars-concrete composite structures is proposed.The analysis is first performed for a special case with shear stress along the debonded interface and then for a general case with shear stress softening.The theoretical solution for the interfacial bond strength is obtained.The same expression for the bond strength is attained whether the softening is considered.The solution for the bond stress is influenced by material parameters of interlayer concrete,including fracture energy,depth,as well as shear modulus.The theoretical data with test data were compared in the example.2.According to the strength theory and the equilibrium condition of force and considering the effect of high temperatures on the mechanical properties of the material,the bonding shear stress and the relative slip values between steel reinforcing bars and concrete under high temperatures is deduced.Furthermore,the local shearing stress-slip relationship when ?1 ? ?T(x)? ?f under 20??190? in RC beams has been calculated,the bond stress-slip curve under different temperatures are given and the bond behavior between steel reinforcing bars and concrete under different temperatures were studied.3.Pull tests of 18 specimens have been carried out to determine the bond between steel reinforcing bars and the surrounding concrete under 20??190?,the bond stress and relative slip in the free end and the loading end is gauged under different temperatures.The measured data can be used to test the reasonable of theory and the comparative analysis showed that the results of the theoretical analysis are in good agreement with the experimental results.4.A simple theoretical model to estimate shear and peel-off stresses in a CFRP-strengthened concrete beam at elevated temperatures is proposed and the expressions of the maximum shear and peel-off stresses in the end region of the FRP plate under high temperatures are derived.5.Through the example of a reinforced concrete beam with different thickness of CFRP reinforcement concrete shear layer,the shear stress variation under different temperatures was calculated and analyzed,the critical shear stress in the adhesive bonding layer under high temperatures was given,the results are compared with the related calculation and the rationality of the theory is validated.