Seismic Performance of Square High-Strength Concrete Columns in FRP Stay-in-Place Formwork

The use of high-strength concrete (HSC) in seismically active regions poses a major concern because of the brittle nature of the material. The confinement requirements for HSC columns may be prohibitively stringent since they require proportionately greater confinement than columns of normal-strength concrete. An alternative to conventional confinement reinforcement is the use of fiber reinforced polymer (FRP) casings, in the form of a stay-in-place formwork. This paper investigates the use of stay-in-place FRP formwork as concrete confinement reinforcement for HSC columns with square cross sections. Large scale HSC building columns, encased in FRP casings, were tested under simulated seismic loading. The columns had $270\mathrm{mm}$ square sections and concrete strengths up to $90\mathrm{MPa}$ . The casings were manufactured from carbon FRP and epoxy resin. The unique aspects of the test program were the introduction of the corner radius as a test parameter, and the presence of internally placed FRP crossties, integrally built with column casings, to improve the effectiveness of concrete confinement. Results indicate that the deformation capacity of HSC columns can be improved significantly by using FRP casings. The results further indicate that the confinement effectiveness of columns is significantly affected by the corner radius of casings. Additionally, the confinement efficiency can be improved with the use of FRP crossties. The columns developed inelastic drift capacities of up to 11%, demonstrating the usefulness of FRP stay-in-place formwork in improving deformability of HSC columns.