Behavior of FRP-confined concrete in annular section columns

This paper presents and interprets the results of a series of axial compression tests on short FRP-confined circular columns with an inner void to examine the behavior of concrete in such FRP-confined annular sections. This study was motivated by the need to understand and model the behavior of concrete in a new form of double-skin tubular columns (DSTCs) composed of a steel inner tube, an FRP outer tube and a concrete infill between the two tubes. To this end, three types of specimens were tested: FRP-confined solid cylinders (FCSCs), FRP-confined hollow cylinders (FCHCs), and short DSTCs. The main parameters examined include the section configuration, the void ratio, the diameter-to-thickness ratio of the inner steel tube, and the thickness of the FRP tube. The test results show that the presence of an inner void reduces the effect of external FRP confinement, but this loss of confinement effectiveness can almost be completely compensated for through the provision of a suitable steel tube. As a result, the concrete in hybrid DSTCs is very effectively confined by the two tubes and the load-axial shortening behavior of concrete in hybrid DSTCs is very similar to that of FCSCs. For an approximate analysis of DSTCs with a suitable steel tube and a reasonable void ratio, an existing stress-strain model for FCSCs may be used.