Study On Structural Behavior Of Concrete Flexural Members With CFRP Tendons Under Fire Action

Fiber reinforced polymer(FRP)composites have many potential advantages compared with traditional steel products,these include relatively high tensile strength,corrosion resistance and light weight.It is widely used in buildings,bridges and industrial structures,especially in strengthening and retrofitting structures.However,there are few literatures about the concrete members prestressed with CFRP materials during and after elevated temperatures,and the properties of degradation is not clear.Therefore,it is imperative to systematically study its fire resistance characteristics.On the basis of collecting a large number of literature about high temperature performance of FRP materials and prestressed concrete members in the world,the fire resistance tests and theoretical studies of CFRP tendons and prestressed concrete members with tendons are carried out during and after elevated temperatures.The specific research contents and results are as follows:(1)Experimental study on high temperature mechanical properties of CFRP tendonsIn this paper,a comprehensive experimental study is carried out on high temperature mechanical properties of CFRP tendons,which includes expansion coefficient test,elevated temperature test(constant temperature loading and constant load heating)and post-elevated temperature test.The variation of mechanical properties of CFRP tendons with temperature are obtained.The test results show that: a)The longitudinal thermal deformation of CFRP tendons is different from that of ordinary steel bars,which is shrinkage deformation.The deformation decreases with the increase of temperature;b)The ultimate tensile strength and elastic modulus of CFRP tendons decreases with the increase of temperature.In the comparative study of constant temperature loading and constant load heating,it is found that the latter has lower mechanical properties than the former due to the existence of initial stress.However,the difference is not significant.The main reasons is that the temperature stability of CFRP tendons for an additional 30 minutes in constant temperature loading test and the surface temperature of CFRP tendons is higher than the internal temperature in constant load heating test;c)When the maximum temperature is not more than 200?,the mechanical properties of CFRP tendons recover greatly in the tensile strength test of CFRP tendons after elevated temperature;d)According to the high temperature test data of CFRP tendons,the high temperature mechanical properties of CFRP tendons were fitted by polynomial model,hyperbolic tangent function model and exponential function model.The fitting expression is in good agreement with the experimental results,which can be used as the constitutive equation of high temperature mechanical properties of CFRP tendons.(2)Experimental study on fire resistance of prestressed concrete members with CFRP tendonsFirstly,the static characteristics of prestressed concrete member with CFRP tendons are tested at room temperature.The results show that: a)The midspan deflection-load curve of prestressed concrete members shows four stages changes,which are the reverse arch stage,the elastic stage,the cracking elastic stage and the plastic stage;b)From the beginning of loading to the failure of the member,it is obvious that the stress increment of CFRP tendons has a linear relationship with the midspan deflection.Secondly,according to the ultimate bearing capacity of concrete members at room temperature,fire resistance tests of 9 prestressed concrete members with CFRP tendons were carried out during elevated temperatures.The research parameters include fire time,tension control stress,loading level and coating thickness.The test results show that: a)When the cross section size,coating thickness and fire time of concrete members are the same,the temperature field of each member changes similarly.The loading level and tension control stress have little effect on temperature field;b)The deflection of concrete members and the stress of CFRP tendons increase with the time of fire.When the stress of CFRP tendons exceeds the ultimate tensile strength during elevated temperature,the fracture of CFRP tendons leads to the failure of concrete members;The stress and deflection increase with the increase of loading level.However,the fire resistance time of concrete members decreases;The reduction of tension control stress does not effectively increase the time of fire.It is mainly attributed to the transverse expansion of CFRP tendons during elevated temperature,resulting in the shedding of fire retardant coatings or large cracks in fire retardant coatings.When the thickness of the coating is thin the protective effect is limited for the fire retardant coating on the surface of CFRP tendons.Therefore,the fire prevention thickness should be increased appropriately;c)When the fire time is short,the prestressed concrete members with CFRP tendons which have not been damaged are tested after fire.The load-deflection curves of concrete members after fire show a two-stage development rule,which is different from the four-stage load-deflection curve of concrete members at room temperature.Before the steel bar yield,the load-deflection increases linearly.However,after the steel bar yields,the load change little and the deflection increases rapidly.At the same time,the development rule of midspan deflection-stress increment after fire is similar to that of static load test at room temperature.Both of them have a linear relationship.(3)Analysis of temperature field on prestressed concrete membersBased on the rule that the temperature changes in concrete members are generally not affected by load and deformation,the finite element temperature model of prestressed concrete members is established by using ABAQUS commercial software.The results of finite element are in good agreement with the experimental results,which lays a foundation for the analysis of structural mechanical properties of prestressed concrete members.(4)Structural performance analysis of prestressed concrete members with CFRP tendons during fireFirstly,according to the high temperature constitutive relationship of each materials in concrete members,the finite element mechanical model is estabished by ABAQUS commercial software.In order to accelelate the computational efficiency,a quarter of the prestressed concrete members were modeled.The calculated results of stress and deflection obtained by finite element agree well with the experimental results.Based on the validated finite element model,the main influence parameters of concrete members are analyzed.It is found that the influence rule of the thickness of protective layer is obtained about stress and deflection.Secondly,in order to further understand the structural performance of the prestressed concrete flexural members during fire,based on the coupled curvature of load and temperature,a comprehensive calculation method of four-parameter iterated conjugate beams is proposed for the performance of prestressed concrete members.The stress of CFRP tendons and midspan deflection of concrete members are analyzed with Python language.The calculated results are in good agreement with the experimental results,which verifies the validity of the modified theoretical method.(5)Structural performance analysis of prestressed concrete members with CFRP tendons after fireThe nonlinear analysis of the structural properties of prestressed concrete members after fire is similar to that during fire,except for the calculation of the temperature field.In order to analyze the structural performance of concrete members after fire,it is necessary to obtain the maximum temperature distribution of the concrete section.Then,according to the constitutive relationship of each materials after elevated temperature,the concrete members are analyzed.Firstly,the maximum temperature of concrete members is obtained by using the ABAQUS subroutine USDFLD.Secondly,the maximum temperature field is introduced into the finite element mechanical model and the modified theoretical model,and the calculated results are in good agreement with the experimental results.At the same time,in order to facilitate the calculation of engineers,a simplified calculation method of prestressed concrete members with CFRP tendons after fire is proposed,and the obtained results also meet the accuracy requirements.