Creep Of Concrete-filled Steel Tube And Its Effect On Bearing Capacity

Concrete-filled steel tube(CFST)is widely used in important projects such as space structure,high-rise and super high-rise buildings and bridges.During long-term loading,the shrinkage and creep of the core concrete may jeopardize the use and safety of the structure.In this paper,a series of studies on the shrinkage and creep behavior of CFST and its influence on ultimate bearing capacity are carried out.The main contents include:(1)The change law of humidity of core concrete.The temperature and humidity probes were used to monitor the variation of temperature and humidity inside the sealed concrete filled steel tube column,partially sealed steel tube concrete column and plain concrete column with age.The results show that Under different sealing conditions(unsealed,partially sealed,sealed),the humidity inside the specimen decreases with age and eventually stabilizes;the lowest internal humidity decreases is the concrete filled steel tubular specimens.The initial humidity was reduced by 4 percentage points,and the internal humidity of the concrete filled steel tube was maintained at a stable value for about 50 days.This fully shows that the steel pipe wrapped around the concrete limits the exchange of moisture between the core concrete and the outside world,reducing the loss of water.In addition,the experimental results also show that the steel pipe has no effect on the temperature change inside the concrete,and the temperature change inside the core concrete is basically consistent with the temperature change of the environment.According to the test results,the internal humidity of concrete under different sealing conditions is fitted with the change of age,and the prediction function expression for predicting the change of internal humidity of concrete with age is proposed.(2)The variation law of the strength of the core concrete.Through the test specimens of standard cubes,plain concrete columns and core concrete columns in concrete-filled steel tubes,the variation of compressive strength of these three specimens was studied.The analysis results show that the age of the concrete is 7 days before the concrete-filled steel tube.The strength of core concrete develops rapidly,and the development of compressive strength in late age tends to be slower,and finally its strength tends to be stable.According to the experimental results,the compressive strength of the core concrete of CFST was fitted with the change of age,and the logarithmic function was used to predict the core concrete age of CFST.(3)The shrinkage and creep of the core concrete.The shrinkage and creep effects of 24 concrete-filled concrete short columns were studied.Combined with the variation law of temperature and humidity and the variation law of core concrete strength,the shrinkage and creep of concrete filled concrete were predicted by correcting the shrinkage and creep models of four classical plain concretes.And compare with the actual value of the test.It is concluded that the modified GL2000 model and ACI209 model overestimate the creep deformation of CFST core concrete.The calculated value of the modified B3 creep model is close to the actual value,which can reflect the actual creep.Very high precision.According to the experimental results,the shrinkage value and creep value of CFST are fitted.The logarithmic function and the three-term multiple are proposed to predict the shrinkage value and creep value.(4)The ultimate bearing capacity of concrete filled steel tube before and after creep.The concrete-filled steel tubular members of the one-year creep test were unloaded,and the CFST shrinkage test piece and the creep test piece were subjected to the axial compression test to study the relationship between the ultimate bearing capacity and the concrete filled steel tube under axial pressure.The relationship between axial force and longitudinal strain and transverse strain is studied.The failure mode and macroscopic deformation characteristics of the specimen are studied.The research results show that the creep will increase the bearing capacity of the CFST members.