Improving strength and deformation characteristics of concrete-filled steel tubular elements in bending

The article is devoted to the investigation of the type of monolithic reinforced concrete structures, known as Concrete-Filled Steel Tubular (CFST) elements. They are mostly used in compression and rarely in bending, due to their high deformability. However, there is still a trend in the development of CFST structures with both compressed and bent elements. While bending the reactive lateral pressure transmits from the steel tube to the concrete core creating a favorable condition - volumetric stresses in the concrete, which increases the flexural strength of the material significantly. 
Experimental studies have been carried out to determine the bearing capacity and deformability of steel-reinforced concrete (composite) structures filled with fiber-reinforced concrete (FRC) of different compositions. The optimal composition of the concrete core for CFST elements in bending was selected, it helped to increase the strength and deformation characteristics of the element. To choose this optimal composition of the concrete core among others, a series of specimens were prepared and tested in compression, tensile and bending tests, and together with that, the modulus of elasticity and adhesion between the concrete core and the metal casing were studied. As the optimal solution for the concrete core – a mixture of concrete modifier (CM) and metal fibers were selected, due to an increase in the adhesion between the concrete core and the metal casing. 
As a result, of experimental studies, graphs of load-deflection dependencies were built and presented. The research results proved that with an increase in the physical and mechanical properties of concrete together with the degree of its adhesion to steel, transverse deformations decrease. For further research development, numerical modeling might be created to describe and foresee the behavior of such elements with different parameters in more detail, which will lead to a better understanding of the combined operation of the tube and core.