Response of self-compacting concrete filled tubes under eccentric compression

Self-Compacting Concrete (SCC) use is spreading worldwide and it is becoming a regular solution in some special applications, including steel–concrete composite columns. In the particular case of Concrete Filled Tubes (CFT), the main advantage from a practical point of view in the use of SCC consists in employing the steel tube as a formwork to directly cast concrete inside it, without the need of vibration. The study of three different concretes for structural applications as composite elements is presented, each of them designed for a 28-day cylindrical compressive strength of 50 MPa: (i) a Normal Vibrated Concrete, (ii) a Self-Compacting Concrete, (iii) an expansive SCC (with the goal of an increase in bond strength as a consequence of the expansion). CFT with critical length ranging from 131 cm to 467 cm have been experimentally and analytically investigated in uniaxial compression. In each case the steel case presents a cross section of 139.6 mm of external diameter and 4.0mmthickness and with a fixed eccentricity of the applied load equal to 25 mm. The bond strength at the steel–concrete interface is reported for each of the three mixes. The experimental and analytical results show that the behavior of eccentrically loaded columns is governed by the bending moment–axial load interaction. As a consequence, perfect bond at the interface can be assumed and the axial capacity of the column is only a function of its geometry and of the mechanical properties of the materials. A numerical procedure is proposed to evaluate the increase in the axial capacity of the composite columns consequent to the confinement of the internal concrete in case of zero-eccentricity of the applied axial load with respect to the column’s axis. Finally, the obtained numerical results are introduced into code provisions to evaluate modified axial force N-bending moment M interaction diagrams to predict the axial capacity of the column in the particular test configuration.