Study On Mechanical Properties Of Expansive Concrete Filled Steel Tube

It is well known that under the action of three-way compressive stress,the strength,elastic modulus of concrete will be improved,and the plasticity and toughness properties will be greatly improved;and with the increase of side pressure,the performance will be improved more obviously.In the concrete filled steel tube structure,the steel pipe forms threedimensional constraint on the core concrete,that is,the steel pipe acts on the core concrete,which improves the compressive strength of the core concrete,and makes the concrete-filled steel tube structure have excellent mechanical properties.However,because the Poisson ratio of concrete is between 0.16-0.25 and that of steel pipe is between 0.26-0.3,the transverse deformation of steel tube is larger than that of internal core concrete in the initial elastic stage of compression load of concrete filled steel tubular column.Because of the shrinkage and creep of concrete itself,the bonding interface between steel pipe and concrete is separated,and this state is later The plastic stage will always exist,which will inevitably lead to the reduction of the binding force of the external steel pipe on the internal core concrete,which has a very adverse effect on the mechanical properties of the composite structure such as strength,rigidity and ductility.Therefore,in order to solve the above problems,strengthen the binding force of the external steel tube on the internal core concrete in the process of compression load of the concrete filled with steel tube and give full play to the combination effect of the two,this paper proposes that the expansion concrete is filled inside the steel tube,and the expansion effect is used to make the composite structure binding on the internal core expansion concrete before the external steel pipe is under compression load.At this time,the composite structure will be binding on the internal core expansion concrete The internal core concrete is subjected to threeway,namely,transverse,axial and circumferential pressure,which greatly improves the working and mechanical properties of the composite structure.Based on the above theoretical basis,this paper explores the influence of the thickness of steel tube,the replacement rate of expansive agent and the type of expansive agent on the mechanical and working performance of the composite structure of concrete filled steel tubular columns.After the design,three groups of tests are carried out,34 concrete-filled steel tubular columns are made,and the axial compression test is carried out to test the phenomena,failure mode,working performance and mechanics of the test piece The performance of the columns is analyzed and compared,and the influence of three test variables on the performance of the concrete filled steel tubular columns is summarized.The main contents and test results of this paper are as follows:1.the pre test of expansive concrete filled steel tube is carried out to investigate the expansion effect,concrete material and mechanical properties of concrete filled steel tubular column under different thickness and dosage of expansive agent.The test results show that the admixture of expansive agent makes the concrete exhibit volume expansion and makes the strength of the concrete filled steel tube column increase.2.based on the first group of pre experiments,18 concrete filled steel tubular columns were designed and manufactured with the parameters of steel tube thickness and more different admixture content.Six of them were cut and core concrete was taken out as the restraint concrete specimen.The expansion effect of expansive concrete,the contrast between concrete performance and pile density,the performance of concrete with or without restraint and the mechanical properties of concrete filled steel tubular columns are further studied and analyzed.The results show that the admixture of expansive agent makes the concrete structure more dense and the bulk density is bigger,and the proper amount of expansive agent makes the performance of the concrete filled steel tubular column improve.3.taking the thickness of steel tube and the type of expansive agent as parameters,8concrete filled steel tubular columns were designed and manufactured.The expansion effect of concrete mixed with three different types of expansive agent and the mechanical properties of the concrete filled steel tubular column were compared and analyzed.The results show that UEA composite expander has the best effect.