The Application of Expansion Concrete in Steel Pipe

Concrete pipe is a combination of steel and concrete materials. When concrete filled steel tubular axial compression, before approaching limit load, on the one hand, the volume becomes large steel pipes bound due to the generation of micro cracks in the concrete core, in a three-way complex stress state under pressure to improve the bearing capacity of concrete; On the other hand, after the steel tube filled with concrete, it reinforced steel geometric stability to avoid or delay the local buckling of steel. Such compressive strength of concrete steel pipe and concrete compressive strength was significantly greater than the simple sum. 
The formation of expansive concrete filled steel pipe into a better form of expansive concrete composite structures. Expansive concrete common is added alunite classes in ordinary concrete expansion agent, producing crystalline sulphoaluminate caused swelling. The expansion of steel tube confined concrete due to the presence of ettringite generated in the expansion of the role of being able to squeeze into the pores and crystalline calcium silicate with fibrous woven into a mesh, so that the organizational structure of cement more is dense, the mechanical properties has been some improvement. At the beginning of the concrete core, it is in charge of lateral compression by the state, which can make steel concrete confining force appeared too late defects. In the initial expansion of the concrete pipe under load, the steel and concrete walls to withstand external loads each joint stiffness. Under stress is small, can be approximated by the elastic properties of the concrete. When CFST long axial compression, core concrete creep occurs, leading to unload concrete, steel force increases. Core concrete longitudinal compression and lateral deformation were increased, making the load and stress redistribution between steel and concrete. Between steel and concrete due to the lead core concrete creep load decreases, and will lead to increased confining force, so late creep decreases. Than ordinary concrete reinforced concrete and steel, the overall effect of creep of concrete pipe expansion is not very prominent. 
Weakness of high-strength concrete is brittle, ductile poor, which hinders its practical engineering application, especially in complex stress state, controlled by brittle failure, the reliability of its work is greatly reduced. If high strength steel pipe concrete poured concrete forming high strength carbon steel pipe, high strength concrete pipe are effectively bound, which will greatly enhance the ductility. In addition, under complex stress state, steel has a strong shear and torsional capability, so that it can effectively overcome the brittleness and ductility of high strength concrete difference weaknesses through a combination of both. Expansive concrete pipe in the early Loaded confining force greater axial compression, the ordinary steel in concrete by the early Dutch trend, steel and concrete Poisson's ratio is different, both from each other there. Mixed with bulking agents, so that by the early Dutch have a large concrete pipe confining force, the concrete in the three-state pressure to improve the compressive strength of concrete to maximize the mechanical properties of steel concrete. High modulus of elasticity of concrete pipe expansion, favorable for the long columns bearing steel concrete structures mostly have large slenderness ratio, so the damage is mainly controlled by the stable. Thus, the elastic modulus of concrete pipe is especially important to improve, improve the elastic modulus can be a corresponding increase in stability capacity of steel concrete.