Behaviors of Concrete Columns Confined with Both Spiral and Fiber Composites

Concrete columns confined with high-strength fiber reinforced polymer (FRP) composites can enhance the strength as well as the ductility of such structures. In recent years, the use of FRP composites to repair and strengthen existing reinforced concrete (RC) structures has been widely used. When the columns of existing RC structures are wrapped with FRP composites, the core concrete of such columns is confined not only by the FRP composites but also by the existing steel reinforcing ties (or spirals). Therefore, it is necessary to understand correctly the compressive response of concrete confined with both steel spirals and FRP composites in order to predict the behavior of such RC columns. Since the behavior of the reinforcing steel spiral ties and the FRP composites are different from each other, the behavior of concrete columns confined with both steel spiral and FRP composites is expected to be different from that of concrete columns confined with only steel spiral or FRP composites. In this paper, 24 RC small-scale columns were tested under pure axial compression in order to study the response of RC columns confined with steel spiral ties and FRP composites. Two main parameters were considered in this investigation: the confinement type (one material (FRP composites or steel spiral) versus two materials (FRP composites and steel spiral)) as well as the lateral confining pressure. The test results showed that the compressive response of concrete when confined with two materials (steel spiral ties and FRP) was quite different from the compressive response of concrete when confined with only one material. The study also compares the experimental results to the predicted results obtained using current constitutive relationships found in the technical literature.