Study On Axial Bearing Capacity Of Recycled Coarse Aggregate Self-compacting Concrete-filled Steel Tube (CFST) Slender Columns

The engineering application of self-compacting recycled concrete filled steel tube can not only solve the environmental pollution caused by construction waste,but also effectively reduce the exploitation and use of natural resources,which is in line with the requirements of environmental protection concept of sustainable development in our country,and can give full play to the mechanical properties advantages of two materials,which has an important research value.In this paper,the mix design method and material mechanical properties of selfcompacting recycled concrete are studied in depth,and the axial compression failure mode and bearing capacity of self-compacting recycled concrete filled slender column members are analyzed.The main content of this paper is as follows:(1)Experimental study on working and mechanical properties of self-compacting recycled concrete.The combination of fixed sand and stone volume method and free water cement ratio method is used to determine the mix ratio of self-compacting recycled concrete.24 sets of slump expansion tests and 24 sets of cube axial compression tests were carried out to study the effect of recycled aggregate replacement rate(ratio of recycled coarse aggregate mass to total coarse aggregate mass)on the flow performance and axial compression strength of self-compacting reclaimed concrete.The results show that: With the increase of the regenerated coarse aggregate substitution rate,the fluidity and filling property of the self-compacting regenerated concrete mix decreased;The failure of self-compacting recycled concrete is mainly due to the failure of strength between old mortar and new mixture;As the substitution rate increases from 0 to 100%,the axial compressive strength decreases by 15.2%.(2)Axial compression test analysis of self-compacting recycled concrete filled steel tube slender columns.The axial compression tests of self-compacting recycled concrete filled steel tube slender columns were carried out,and the influences of parameters such as the replacement rate of recycled concrete,the thickness of steel tube and the slenderness ratio of member on the bearing capacity were analyzed.The influences of various variables on the bearing capacity,ductility and energy dissipation of member were explored.The results show that: The failure mode of self-compacting recycled concrete filled steel tube slender columns is elastoplastic instability failure.The thickness of the steel tube has significant influence on the bearing capacity,ductility and peak strain of the member.As the thickness of the steel tube increases from 1.5mm to 4mm,the bearing capacity of the member increases by 33.6%,the ductility increases by 26.8%,and the peak strain changes by 26.2%;The replacement rate of recycled concrete has a significant effect on the change rate coefficient of bearing capacity of components.(3)Finite element analysis on axial compression of self-compacting recycled aggregate concrete-filled slender columns.Based on the performance test of self-compacting recycled aggregate concrete and the concrete damage model,the finite element model of self-compacting recycled aggregate concrete filled steel tube is established.According to the test results,the calculation formula of peak strain of long column in self-compacting recycled concrete filled steel tube is modified.The results show that: The stress of the slender column members is mainly distributed in the middle and two ends of the members;The constitutive relation and finite element model used in this paper agree with the experimental results and are suitable for the finite element analysis of self-compacting recycled aggregate concrete-filled slender columns.(4)Revised calculation formula of axial compressive capacity of concrete-filled steel tube member.Based on The yield surface of the double-shear unified strength theory and the material yield criterion,the ultimate bearing capacity calculation formula of the self-compacts short recycled concrete tubular column was modified by introducing the thickness to diameter ratio coefficient.Based on tangential modulus theory,the reduction coefficient of self-compacting recycled aggregate concrete-filled slender columns is modified.The results show that,the formula in this paper is suitable for the calculation of the bearing capacity of the thin-walled steel tubular concrete composite members,and the calculated results are in good agreement with the experimental results.