Seismic performance of high-strength concrete columns cast in stay-in-place FRP formwork

The use of high-strength concrete (HSC) in seismically actively regions poses a major concern because of the brittle nature of material. The confinement requirements for HSC columns may be prohibitively stringent since they require proportionately higher 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, which can fulfill multiple functions of; i) formwork, ii) confinement reinforcement, and iii) protective shell against corrosion, weathering and chemical attacks. This paper investigates the use of stay-in-place FRP formwork as concrete confinement reinforcement for HSC columns with square and circular cross-sections. Large-scale specimens with 270 mm cross-sectional dimension and 90 MPa strength concrete, were tested under constant axial compression and incrementally increasing lateral deformation reversals. FRP casings were manufactured from carbon fiber sheets and epoxy resin. One of the square columns was equipped with internal FRP crossties, a new technique introduced by the authors, to provide well-distributed lateral restraints along the column face, thereby improving the mechanism of confinement. The 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 FRP-confined square sections can be substantially increased with the use of FRP crossties.