Confinement model for FRP-bonded capsule-shaped concrete columns

Fiber-reinforced polymer (FRP) systems have been effectively used to increase the axial load capacity of circular and near square concrete columns by providing lateral confinement. Recent research works however showed that such confinement is less significant in rectangular columns with sectional aspect ratio more than 1.5. To improve the confinement effect, rectangular sections may be re-profiled as capsule-shaped sections by adding two semi-circular segments at the short ends before the bonding of FRP reinforcement. A test program was conducted on FRP-confined capsule-shaped plain concrete columns with section aspect ratios ranging from one (circular) to four and confined with transverse carbon FRP sheets of up to three plies. The columns were tested under monotonically increasing axial load. Test results showed significant enhancement in confined compressive strength of columns by up to 143%, 72%, 22% and 28% for column section aspect ratios of 1, 2, 3 and 4 respectively. A confinement model using a modified effective confinement co-efficient for circular, rectangular and capsule-shaped section is proposed. The proposed model reasonably predicted the confined compressive strength of FRP-confined capsule-shaped columns.