Structural behavior of thin-walled steel short columns filled with recycled aggregate concrete

This paper presents an experimental investigation to study the behavior of 12 recycled aggregate concrete-filled steel tubular (RACFST) short columns subjected to concentric axial loading. These columns formed from six different cross-sections involving: triangle, elliptical, and hexagon, whereas the other three sections included traditional forms for control purposes, involving: square, rectangular, and circular. The whole of the RACFST columns sections used is made of mild steel plates. All columns were divided into two groups and filled with recycled aggregate concrete. The steel tube thickness was the only parameter modified to study its effect properly. In addition, the study included the search for the best effective section with regard to the properties of stability and confinement, so these columns were designed so that the cross-sectional areas of steel tubes were approximately equal. Different data have been recorded in the experimental tests, including: the ultimate failure axial load, final failure stress, the reduction in the axial column length, failure patterns, and lateral displacement. Data obtained exhibited of RACFST columns with circular and elliptical sections, respectively, showed better stability, confinement for the concrete, and the ability to withstand greater final failure stress. On the other hand, the arrangement of all RACFST columns with polygonal sections in terms of bearing the ultimate failure stress was as follows: hexagonal (C.H.), square (C.S.), rectangle (C.R.), and triangle (C.T.). The reason for this was the increase in the number of corners of steel plates that formed the model. In another concept, this means that the greater the number of formed sides and the greater the angle between the sides (90° or more), the section can achieve more stability and confinement, respectively. In addition to these, the results showed, when the thickness of the steel tube increases, the concrete contribution ratio value decreases of the specimens examined.