Ultra-short-term Creep And Axial Compressive Stress-strain Relationship Of Fly Ash Concrete Filled Steel Tube

Concrete-filled steel tubular(CFST)composite members have been widely used in the field of civil engineering because of their high bearing capacity,ultraior seismic performance,convenient construction and excellent economic performance.Meanwhile,fly ash as a substitute material for cement,the study of the application of fly ash concrete in the composite structure is not only of great significance to save resources and protect the environment,but also in line with the concept of sustainable development of civil engineering.As the focus of structural aging research,creep has a significant effect on the mechanical properties of CFST structures.Under continuous load,the creep of core concrete will affect the interface bond between steel tube and concrete,and the mechanical properties of CFST will change accordingly.At present,there is a lack of systematic research on creep and mechanical properties of fly ash concrete(FAC)and steel tube fly ash concrete(ST-FAC)after creep.In view of the viscoelastic development of concrete,a series of experimental and theoretical studies are carried out in this paper.The main research contents are as follows:1.According to the equal strength design of fly ash concrete in the 28-day curing age,C60 concrete with three kinds of fly ash content(0%,20%,40%)is prepared.With different load holding time and fly ash content as the main influence parameters,the ultra-short-term creep and creep recovery tests are carried out on C60 fly ash concrete specimens.By data analysis,the variation and development rules of initial deformation,creep,hysteresis recovery deformation,nominal residual deformation and elastic modulus of fly ash concrete are obtained.In order to more purely reflect the viscoelastic properties of the material itself,the load-holding time is controlled in three different time dimensions: seconds,minutes and hours.2.The calculated results of the existing creep and creep recovery models are compared with the experimental results,and the characteristics and differences of the existing models in the prediction of ultra-short-term creep are pointed out.Based on the fractional-order viscoelastic theory,a unified calculation model for ultra-short-term creep and creep recovery is proposed.Combined with the experimental results of fly ash concrete,the fractional-order viscoelastic model is verified and its parameters are analyzed.Finally,a fractional viscoelastic creep and creep recovery prediction model with load holding time and fly ash content as variables is obtained.3.In order to systematically study the application of fly ash concrete in the structure,with the same mixture ratio of fly ash concrete preparation of C60 fly ash concrete filled steel tube(ST-FAC).The ultra-short-term creep and creep recovery tests are also carried out in three time dimensions of seconds,minutes and hours,so as to obtain various characteristic deformations of the composite members and the variation laws of the loading and unloading combination elastic modulus.4.The obtained fractional creep and creep recovery prediction model of fly ash concrete is introduced into the composite member,and the test results are compared with the existing creep and creep recovery model of concrete filled steel tube,and the applicability of the existing model is discussed.Meanwhile,considering the viscoelastic development law of composite members under the action of low stress level,a unified fractional order viscoelastic model for ultra-short-term creep and creep recovery of composite members is proposed,and the parameters are analyzed with the experimental data.5.The stress-strain relationship of fly ash concrete and fly ash concrete filled steel tube after ultra-short-term creep is established.Firstly,destructive tests are carried out on the specimen after creep recovery and the reference specimen according to the relevant code,and the stress-strain relationship curve and the variation rule of the characteristic points such as peak stress,peak strain and secant elastic modulus are obtained.The plastic damage(CDP)model of fly ash concrete and core confined fly ash concrete are established,and the accuracy of the models is verified by finite element simulation.