Axial Compressive Behavior Of Recycled Aggregate Concrete Filled Elliptic Steel Tubular Stub Columns

The thesis aims to study recycled concrete with elliptical steel tube which not only has general virtues of concrete filled steel tube but also is more advantageous than ordinary ones because of its elliptic section and recycled core concrete.The elliptic section not only has the merits of good architectural aesthetic effect and reasonable application of primary and secondary axes,but also can effectively reduce the lateral impact load of fluid and other objects on the recycled concrete members of the elliptic steel tube with its unique streamlined boundary.Up to now,there is still a lack of systematic basic research on the test,parameter analysis and design method of the recycled concrete with elliptical steel tube at home and abroad,which limits its wide application in practical engineering.Therefore,it is of great significance to study the basic properties and design methods of the recycled concrete members with elliptic steel tubes.In the test,the axial pressure properties of 4 recycled concrete filled steel tubes and 1 elliptical concrete filled steel tube short column were studied.Secondly,through the experimental study of 9 cubes and 27 prismatic recycled concrete test blocks,the experimental data of the recycled concrete with the replacement rate of 0%,30% and 50% recycled aggregate and the change rule of its basic mechanical properties were obtained.On the basis of the relevant data recorded in the test,the finite element numerical analysis software is used for the numerical and comparative analysis of the components in the test.At the same time,the influence of the ratio of the long and short axes of the elliptic section and the change of the area of the section on the recycled concrete members of the elliptic steel tube is analyzed by numerical analysis.The main conclusions of this paper are as follows:(1)With the increase of the replacement rate of recycled aggregate,the cubic compressive strength standard value,axial compressive strength standard value,elastic modulus and poisson’s ratio of recycled concrete all decrease to different degrees,and the influence rule of recycled elastic modulus is roughly linear.(2)The load-displacement curve of the recycled concrete members with elliptic steel tube is similar to that of ordinary concrete filled steel tube.(3)The steel tube thickness increased from 2mm to 3mm,and its ultimate bearing capacity increased by 46.5%.From 3mm to 4mm,its ultimate bearing capacity increased by 15.4%.Compared with the thickness of 4mm steel tube,the ultimate bearing capacity of the members increased by 69.0%.The thickness of steel tube has a great influence on the bearing capacity of members.The more the thickness of steel tube increases,the greater the bearing capacity will be,and the axial stiffness of the members also increases slightly.According to the curve fitting of the least square method for the ultimate bearing capacity data of the members with different steel tube thicknesses,the rule of the influence of different steel tube thicknesses on the members is roughly linear.(4)The replacement rate of recycled aggregate increased from 0% to 30%,the ultimate bearing capacity decreased by 2.5%.From 30% to 50%,the ultimate bearing capacity decreased by 1.9%;Compared with the replacement rate of 50% and 0%,the reduction rate of ultimate bearing capacity is only 4.3%.The influence of the replacement rate of recycled aggregate on the bearing capacity of components is relatively small but cannot be ignored.As the replacement rate increases gradually,the bearing capacity of components decreases gradually.At the same time,with the increase of the replacement rate of recycled aggregate,the axial stiffness of the component also decreases slightly.(5)Based on numerical analysis,the ratio of the length of axis increased from 1.5 to 3.0,its ultimate bearing capacity decreased by 6.8%,the ratio of the length of axis of the axial compressive ultimate bearing capacity for component has a certain influence,is gradually increasing the ratio of the length of axis,the ultimate bearing capacity of the component then gradually decreases,and the load-displacement curve of the decline has gradually become the steep.In practical engineering,small elliptic section with long and short axes should be chosen as far as possible under the precondition of satisfying the functional requirements.(6)Based on the existing literature,the comprehensive influence of steel tube thickness t and recycled aggregate replacement rate r on the bearing capacity of members is taken into account,and a formula for calculating the axial bearing capacity of elliptic recycled concrete short columns is obtained.