Tests on Circular Thin-walled Steel Tubes Filled with Medium and High Strength Concrete

This paper presents the results from an experimental study performed on circular thin-walled steel tubes filled with medium and high strength concrete. Five different circular steel tubes were examined with diameter to thickness ratios ranging from fully effective to slender where local buckling of the steel tube dominates the behaviour. Two nominal concrete strengths of 50 MPa and 80 MPa were used to fill these steel tubes. The material properties, residual stresses and geometric imperfections were carefully measured. Four different loading conditions were examined to model different construction configurations and included axial and eccentric tests. Enhancement of the concrete strength by the steel tube occurred for all cocentrically loaded specimens filled with medium strength concrete, while enhancement of the high strength concrete only occurred with the thicker tubes. Two different failure modes were observed for the axially loaded specimens relating to the formation (or not) of a local buckle in the steel tube. Greater enhancement of the concrete was observed for specimens with a local buckle as greater restraint by the thin-walled steel tube was realised. The greatest concrete enhancement was found to occur by loading only the concrete with the steel tube used as pure confinement. Some confinement was observed for eccentrically loaded specimens. However, this was only at small eccentricities. For all specimens under all loading conditions, even the very thin-walled steel tubes improved the ductility of the concrete if not the strength.