Study On Shrinkage And Creep Of Concrete-filled UHPC Tube Composite Column

The excellent properties of ultra-high performance concrete（UHPC）can be further improved after high-temperature steam curing,so it is very suitable for precast structures.Concrete-filled UHPC tube（CFUT）is an innovative composite structure composed by precast UHPC tube and core concrete,which was showed with excellent mechanical properties and durability,and has great application prospects.However,UHPC exhibits almost no posttreatment shrinkage and minimal creep after steam curing,which is quite different from that of normal concrete（NC）.The difference in creep and shrinkage characteristics between UHPC and NC will cause the internal force redistribution of the composite structure under long-term load,which has a certain influence on long-term performance.In order to study the shrinkage and creep properties of CFUT structure and the redistribution of internal forces,firstly,the shrinkage and creep tests of NC,UHPC and CFUT were carried out,and the shrinkage and creep laws of three kinds of specimens were determined.For subsequent theoretical and finite element analysis,the prediction model with the highest accuracy was selected after comparison.Secondly,the stress state calculation method of core concrete in CFUT was obtained through theoretical derivation.On this basis,the theoretical calculation formula of shrinkage and creep of CFUT was deduced based on AEMM method.Then,the numerical simulation of shrinkage and creep test was carried out by the finite element software ANSYS,and the parameter analysis was carried out.The effects of reinforcement ratio,proportion of UHPC area,loading age and stress level of core concrete on the long-term performance of CFUT were discussed.Finally,based on an actual engineering,the application scheme of precast UHPC tube was designed and analyzed.The main conclusions are as follows:（1）The experimental results show that the creep and shrinkage of CFUT columns（S-CC,C-CC）are obviously smaller than those of NC columns（S-NC,C-NC）.The shrinkage strain of S-CC is about 61.9%of that of S-NC.The nominal creep coefficient of C-CC is about 36.0%of that of C-NC at 433 days after loading.（2）According to the strain curves measured by the test,the shrinkage strain and creep strain increased rapidly in the early stage,and gradually slowed down in the later stage.In addition,it can be seen that the shrinkage and creep were obviously affected by ambient temperature and humidity.Moreover,the B3 model can accurately predict the shrinkage and creep of concrete.（3）Through theoretical and finite element analysis,the confining force of the precast tube with stirrups on the core concrete is calculated,and the stress state of the core concrete in CFUT is clarified.The results indicate that the core concrete is approximately under uniaxial stress state.（4）The long-term strain of the specimens is calculated by finite element analysis and theoretical formula,and the calculated results are in good agreement with the experimental values,which indicates the correctness of the theoretical formula and the finite element method for simulating shrinkage and creep.At the same time,the stress variation of NC and UHPC caused by redistribution of internal force is analyzed.（5）The parameter analysis results show that as the reinforcement ratio and the proportion of UHPC area increase,the long-term strain,creep coefficient and NC stress of CFUT all decrease.When the reinforcement ratio is increased from 1%to 5%,the creep coefficient is reduced by 6.5%to 25.9%compared with the case without reinforcement.When the proportion of UHPC area increased from 10%to 50%,the creep coefficient decreased by 18.0%to 53.9%.The effect of loading age on the long-term performance of CFUT is more obvious when the age is relatively small.When the loading age increases from 7 days to 28 days and 90 days,the final strain value decreases by 5.9%and 15.0%,respectively,and the degree of internal force redistribution also decreases.As the stress level increases,the degree of internal force redistribution decreases.However,the stress level does not affect the development of creep strain.The results also show that creep and stress are approximately linear only when the stress is low.（6）When the precast UHPC tubes are used to form composite piers and applied to high-pier bridges,it can effectively reduce the deformation of the piers in each construction stage.Compared with ordinary reinforced concrete piers,the vertical displacement of the pier top of the composite pier was reduced by 27.9%during the construction of the main girder,81.3%during the ten years after the completion of the bridge,and the long-term deflection of the mid span was also reduced.