A novel FRP–dual-grade concrete–steel composite column system

Abstract

This paper presents an experimental study that was designed to investigate the compressive behavior of a new type of fiber reinforced polymer (FRP)-concrete–steel composite column system. This composite column system has recently been developed by the author as a special form of double-skin tubular columns (DSTCs), with the column manufactured using two different grades of concrete (referred to as dual-grade concrete (DGC) DSTCs in this paper). In this system, the annular section of the column (i.e. the section between FRP and steel tubes) is filled with normal-strength concrete (NSC) and the core section inside the steel tube is filled with a higher grade concrete mix. Based on the understanding that confinement demand of concrete increases with its strength, the system was designed to maximize the effectiveness of the composite column. To establish the performance levels of this new column system with respect to those of the conventional DSTCs manufactured with normal- and high-strength concrete (NSC and HSC), a comprehensive experimental investigation was undertaken where 28 circular and square DSTCs were tested under concentric compression. Results of the experimental program indicate that the proposed DGC DSTC system exhibits a superior compressive behavior compared to those of the conventional NSC and HSC DSTCs. It is observed that both circular and square DGC DSTCs exhibit extremely high axial load and deformation capacities, which in turn results in a much higher system and concrete ductility compared to those in companion single-grade concrete DSTCs. The results also show that companion square and circular DGC DSTCs can be designed to develop similar axial load capacities under the same level of FRP confinement. It is found that, under these conditions, the axial deformation capacities of square DGC DSTCs exceed those of their circular counterparts.