Journal of Composites for Construction

This paper presents the conceptual development of a new form of hybrid structural column, termed the hybrid fiber-reinforced polymer (FRP)-concrete-steel multiple-tube concrete column (MTCC). A MTCC consists of an external FRP tube and number of internal steel tubes, with the space inside all the tubes filled with concrete. The new column form allows the use of small circular steel tubes, readily available in the market, to build columns of a much larger scale. In the column, the steel tubes are well-protected from corrosion by the external FRP tube, and the concrete is well-confined by both the internal circular steel tubes and external FRP tube. A series of stub column tests involving 10 MTCC specimens and two concrete-filled FRP tube (CFFT) specimens are presented to demonstrate the advantages of the new column form. The test variables included the type and configuration of internal steel tubes as well as the thickness of external FRP tube. The test results confirmed that the concrete in the test columns was very effectively confined, and that buckling of the steel tubes was effectively prevented, leading to a very ductile column response. The test results also showed that MTCCs generally have a much larger ultimate axial strain than that of CFFTs when the same FRP tube is used. A simple analytical model proposed for the axial load-strain curve of MTCCs is also presented and is shown to provide reasonable and conservative predictions of the test results.

The effect of size on strength of fiber-reinforced polymer (FRP)-confined cylindrical concrete columns is studied. The specimens studied are self-similar, i.e., all column dimensions as well as FRP wrap thickness are scaled proportionately with specimen size. Unlike many previous studies, a significant size effect is found in large specimens. A simple analytical model is used to explain the observed behavior. The mode of failure is seen to be critical in determining the size effect. Whereas cylinders of small size fail because of FRP rupture induced by plastic dilation in the concrete, in larger columns, failure may occur because of fracture and shear banding. A size effect is observed whenever failure is fracture dominated. The confining pressure mobilized by the FRP wrap is found to be critical. The reasons why several previous studies did not predict a size effect are investigated, and the size-effect sensitivity of different models is studied.