Preparation And Performance Influencing Factors Of Concrete Filled Steel Tubular With Preplaced Aggregate

Concrete-filled steel tube composite structure has the advantages of high bearing capacity,good seismic performance,simple construction and remarkable economic benefits,and has a broad application prospect in bridge construction in complex mountain areas.The key to good mechanical properties of concrete-filled steel tube lies in the close bond between the steel tube and the concrete in the pipe.Due to the shrinkage characteristics of the concrete in the pipe and the existing pouring process,it is inevitable that there is debonding or even void between the concrete in the pipe and the steel tube.Preplaced aggregate concrete is the concrete formed by presetting coarse aggregate in the formwork and reperfusion with high fluidity slurry.With high volume content of coarse aggregate and interlocking of aggregate,it can give full play to the skeleton function of coarse aggregate,and the volume stability is significantly improved compared with that of conventional concrete due to the small amount of rubber material.The preplaced aggregate concrete is used as the concrete in pipe of steel tube to form the preplaced aggregate concrete of steel tube,increase the volume stability of concrete in pipe,ensure the joint work of steel tube and concrete in pipe,give full play to the advantages of concrete-filled steel tube and preplaced aggregate concrete,and promote the development and application of concrete-filled steel tube composite structure.At present,the strength of preplaced aggregate concrete applied in engineering is generally low,and its combination with steel tube cannot give full play to the mechanical property advantage of high bearing capacity of concrete filled steel tube,so it is necessary to develop high strength preplaced aggregate concrete.This paper focuses on the influence of coarse aggregate composition and grouting characteristics on the performance of preplaced aggregate concrete,and puts forward the coarse aggregate grading composition,void ratio requirements,as well as the preparation method of high strength,high flow and low shrinkage grouting material,and successfully prepares ultra-high strength preplaced aggregate concrete filled steel tube.The research contents and main achievements are as follows:（1）Research on the composition of preplaced coarse aggregate:the influence of the composition of preplaced coarse aggregate on the strength of concrete filled steel tubular with preplaced aggregate is studied,and the design method of coarse aggregate for concrete filled steel tubular with preplaced aggregate is proposed.The test results show that the higher the strength of the concrete filled with preplaced aggregate steel tube is,the more continuous the grading type is,the particle size range is 10-25 mm,and the lower the porosity is.Considering the influence of the porosity on the pouring difficulty and strength,the porosity should be controlled between 34.7%and 40.7%.When the gradation design is 10-16mm:16-25mm=3:7 and the void ratio is 34.7%,the strength of concrete filled with preplaced aggregate steel tube can reach a higher level.In addition,using coarse aggregate with high strength of parent rock and layered and vibration grouting is beneficial to improve the strength of concrete filled steel tubular with preplaced aggregate.（2）Study on preparation of high performance grouting material:the effects of mineral admixture system,mixture ratio parameters,amount of expansion agent and cement strength grade on the properties of grouting material are studied,and the preparation method of high performance grouting material for prefilled aggregate concrete filled steel tube is put forward.The test results show that choosing 10mm～16mm:16mm～25mm=3:7（voidage 34.7%）was used as pre-filled coarse aggregate gradation,and high performance composite mortar（P·O 42.5 cement,water-binder ratio 0.28,cement-sand ratio 1.2,microsphere content 20%,silica fume content 8%,expansion agent content 6%）was used as grouting material.A single vibration free forming process can produce pre-filled aggregate steel tubular concrete（CFST）with 28d compressive strength of 115.9MPa,28d elastic modulus of 56300MPa and 90d drying shrinkage of 165×10^-6.Compared with conventional C100 concrete,the single-side raw material cost can be reduced by 46.4%.In addition,on the basis of constant coarse aggregate composition,high performance composite net slurry（P·O 42.5 cement,water-binder ratio 0.15,microsphere content 15%,silica fume content 10%,expansion agent content 6%）was used as slurry material.A single vibration free forming process can produce pre-filled aggregate steel tubular concrete with 28d compressive strength of 114.2MPa,28d elastic modulus of 42600MPa and 90d drying shrinkage of 264×10^-6.Compared with conventional C100 concrete,the single-side raw material cost can be reduced by 25.7%,without mechanical sand,which can effectively alleviate the shortage of sand and gravel resources.Both of them have good economic benefit and environmental harmony.