Experimental Study On FRP-confined Circular Steel Tube Machine-made Sand Concrete Short Column

In order to solve the problem of the increasing demand for construction sand and the shortage of natural river sand resources,this paper suggests to use manufacture sand to replace natural river sand.The quality of manufacture sand separated by rock crushing and screening is stable and controllable,with better particle shape and reasonable grading,which can meet the performance requirements of various types of concrete.However,compared with natural river sand,its surface is rough and its edges and corners are clear.The strength of the replaced concrete is improved,but its ductility is less than that of ordinary concrete,which is more prone to brittle failure.Therefore,this paper proposes the form of steel pipe machine made sand concrete composite structure,which gives full play to the restraint effect of steel pipe on machine made sand concrete,and alleviates the defect of steel pipe buckling to the interior due to force,and improves the mechanical performance of the structure as a whole.On this basis,in order to avoid the disharmony of transverse deformation between steel tube and manufacture sand concrete in the elastic stage and the deterioration of mechanical properties of composite structures caused by the void in the early compression stage,this paper adopts the method of adding FRP constraint outside the steel tube to strengthen the constraint on the internal core manufacture sand concrete,so as to achieve the effect that the manufacture sand concrete and steel tube are closely bonded and jointly stressed,thereby improving the performance of members.In this paper,C50 manufacture sand concrete is designed and trial mixed after analyzing the material properties of each raw material.On this basis,wet bulk density test and axial compression test are carried out for the manufactured sand concrete with this mix ratio to explore the influence of the internal structure evolution of the manufactured sand concrete with the wet bulk density as the main parameter on the bearing capacity of the specimen.In order to explore the mechanical properties of the composite structure of FRP,circular steel tube and manufacture sand concrete,based on the material performance tests of circular hollow steel tube,CFRP and GFRP respectively,this paper conducted an experimental study on the FRP confined circular steel tube machine made sand concrete short column members,designed 50(10 unrestrained,20 GFRP constrained,20 CFRP constrained)steel tube machine made sand concrete short column members.The influence of different parameters such as the diameter thickness ratio D/t,the internal structure of manufactured sand concrete with wet packing density as an important characterization parameter,the type of FRP,and the number of FRP layers on the mechanical properties of the member under axial compression is studied.The failure mode,the load displacement(strain)curve of the whole process,and the difference of the ultimate bearing capacity under different parameters are analyzed in combination with the changes of the internal structure of manufactured sand concrete.The experimental research shows that the replacement of manufacture sand can increase the bulk density of concrete,reduce the porosity,and improve the compressive strength of specimens;The lateral deformation,splitting brittle failure and buckling deformation of the steel tube can be effectively reduced by adding FRP constraints or(and)reducing the diameter thickness ratio of the steel tube,and the overall bearing capacity of the specimen can be improved;Finally,the bearing capacity prediction formula of FRP confined CFST short columns with manufactured sand is proposed and compared with the existing industry specifications.The strength predicted by the formula in this paper is basically consistent with the test results.