Core confinement and cover spalling in square and rectangular SCC and SFRC columns

Abstract

To ensure adequate strength and deformability, modern concrete codes require the use of closely-spaced confinement reinforcement in columns. In ductile columns, these requirements result in congested columns which are difficult to construct; the use of self-consolidating concrete (SCC) and steel fiber reinforced concrete (SFRC) can facilitate the construction of such columns. However, limited research exists on the axial behaviour of SCC columns. Moreover, further research is needed to understand the influence of fibers on core confinement in SCC columns. This research aims at better understanding the influence of transverse reinforcement and fibers on core confinement in SCC columns. To achieve this objective, this paper presents the results from thirteen columns built with SCC, steel fibers and varied amounts of transverse reinforcement. The columns, which had square (250 mm × 250 mm) and rectangular (350 mm × 175 mm) cross sections, were built without cover in order to examine the competing influence of transverse reinforcement and fibers on core confinement. The results confirm that an increase in transverse steel-ratio improves the axial strength and ductility of SCC columns. The findings also show that fibers improve core behaviour in SCC columns, with a more significant effect in columns with low transverse steel-ratios compared to well-confined columns. The results also point to a relative loss in confinement and fiber efficiency when the sectional shape changes from square to rectangular. The influence of fibers on the cover contribution was also studied using the results from companion columns containing identical reinforcing cages but constructed with cover. The results show that fibers transform the cover spalling mechanism from sudden to gradual, allowing the cover to contribute significantly to column capacity and ductility. However, bar buckling plays an important role in the SFRC cover spalling mechanism, particularly in columns with large tie spacing. The paper also presents an analytical study which predicts the axial response of the columns using different confinement models. The results show that the Cusson-Paultre and Aoude models can predict the axial response of plain and fiber-reinforced SCC columns, however recalibration of the models is recommended for columns having rectangular cross sections.