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Tube reinforced concrete

Steel possesses high strength and ductility. Steel, as a structural member, especially as a compressive member such as columns, will have a limited stiffness. On the contrary, the concrete performs very well as a compression member. Concrete has excellent compressive strength and high stiffness. The combining of a steel tube and concrete to form a tube reinforced concrete will enhance the member strength, stiffness, ductility, and overall capacity. concrete-filled steel tube (CFST) is not just a steel tube with a concrete core. CFST will speed the construction process and enhance the load-carrying capacity and ductility of the structural members. 

Figure 1

The tubes used for CFST can be a circular, square, or rectangular shape. However, the performance of circular CFST is better than other shapes. Also, the processing of a circular CFST is easier and cheaper than other shapes. A circular tube with a thickness of less than 20 mm will be welded using a spiral welding method. When the thickness of the tube is more than 20 mm, longitudinal butt welding will be used to form the circular tube. While two or even four welds required to form the box section tube. It is obvious that the cost of circular tube processing is cheaper than that for square tubes.

CFST has been used for many structures. The using of CFST can contribute to cutting the cost of the project. The steel tube will be used as formwork for casting concrete, which reduces the cost of materials and labors. Also, using of CFST will enhance concrete compressive strength. The confinement of concrete inside the steel tube will improve the concrete compressive strength. Concrete in CFST will increase the member stiffness and reduce the buckling.
On the other hand, steel will reinforce the concrete and contribute to resisting the tensile stress, bending, shear, and providing sufficient ductility for the structure. The CFST is suitable for arch structures. The high compressive strength of CFST will be suitable for resisting the high compressive forces generated in arch structures such as bridges. THE CFST used to Wushan Bridge in china with a span length of 460 m. 
Figure 2

Recently, Hollow CFST attracted significant attention in both research and application. Hollow CFST has many advantages over solid CFST. The using of hollow CFST will reduce the amount of materials used, which reduces the cost of the work. In addition to reducing the cost, the using of hollow CFST will increase the member capacity by the increase of moment of inertia. Also, it will provide a place for installing utilities such as cables or drainage pipes. The significant advantages of hollow CFST result in a rapid increase of using hollow CFST in the construction industry. Steel tubes with concrete forms a unique structure. Under normal conditions, concrete will develop a strong bond with the confining steel tubes. However, sometimes the concrete separate from the tube due to concrete shrinkage or bleeding. Therefore, expansive concrete is recommended to eliminate the separating of concrete from steel tubes.

Figure 3


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