Study On Creep Performance Of Steel Tube Multi-time Expansive Concrete

In view of the obvious shortcomings of traditional expansion agents when applied to concrete-filled steel tubular structures,domestic scholars have recently created a new type of multi-time expansive material with controllable shrinkage compensation.By accurately matching the type,time and size of concrete shrinkage in the tube,Compensation of shrinkage deformation of concrete in the pipe in stages and in the whole process,so that the concrete in the tube has no shrinkage at all ages,and maintains a certain pre-compression stress in the concrete.To a certain extent,it properly solves the problem of debonding and voiding of concrete-filled steel tubular,and has broad application prospects.As another time-dependent deformation of concrete that is always accompanied by shrinkage,creep will increase structural deformation and cause significant internal force redistribution between steel tube and concrete,which has a significant impact on the mechanical properties of concrete-filled steel tube structures.Therefore,it is an important prerequisite for its popularization and application to study the long-term creep performance of concrete-filled steel tube with shrinkage-compensated multi-time expansive concrete.To this end,this paper presents a more in-depth study of the axial creep performance of steel pipe multi-time expansive concrete.The main work and conclusions are as follows :(1)Taking the total mass content of multi-time expansive agent and the confinement effect coefficient of steel tube as the test parameters,the longterm axial compression creep test of 6 plain concrete/steel tube concrete specimens for 365 days and the shrinkage test of 16 control specimens were carried out to study the creep characteristics and parameter influence law of multi-time expansive concrete filled steel tube.The test results show that the concrete mixed with multi-time expansive agent will not only produce obvious expansion effect,but also increase the creep of the concrete itself and the corresponding concrete-filled steel tube composite members,indicating that the multi-time expansive agent will change the creep constitutive relationship of the concrete.At the same time,the steel tube may have a large hoop effect on the expansive concrete.(2)The concrete creep coefficient calculation model obtained by fitting the measured results is used as the creep constitutive model of concrete in the tube.Based on the axial and circumferential deformation coordination relationship and the age-adjusted effective modulus method(AAEM),a timestep-incremental theoretical calculation method for the creep effect of multitime expansive concrete in steel tubes considering time-varying hoop stress is established.The influence of creep on the mechanical properties of multi-time expansive concrete in steel tubes and the influence of parameters such as expansive agent and steel ratio are analyzed.The results show that the predicted results obtained by the established theoretical calculation method are in good agreement with the measured data.If the influence of steel pipe hoop stress is ignored,the theoretical calculation value will be 15% lower.In addition,creep will cause obvious stress redistribution in the component.When the loading age is 338 days,the stress of the steel tube increases by about 46% and the stress of the concrete decreases by about 41%.On the premise that other influencing parameters are consistent,creep increases with the increase of stress level and concrete strength grade.It decreases with the increase of steel ratio.(3)Combined with AAEM and finite element theory of bar system structure,the finite element calculation format and specific algorithm of bar system for long-term creep effect analysis of concrete filled steel tube structure are established.The long-term creep effect of a long-span concrete filled steel tube arch bridge is analyzed by using the algorithm and the multi-time expansion concrete shrinkage and creep constitutive obtained above,and compared with the structure of ordinary concrete filled steel tube.The results show that when the age is 750 days,under the influence of the creep of the multi-time expansive concrete,the structural stress redistribution of the concrete-filled steel tube arch bridge is obvious,and the vault deflection increases by 0.25 m,which is 14.9%higher than that of the ordinary concrete-filled steel tube structure.