Experimental investigation of slender circular RC columns strengthened with FRP composites

• Response of CFRP wrapped circular RC columns under eccentric compression is studied.
• Columns of 150 mm diameter having three heights: 600, 900 and 1200 mm, were tested.
• Columns were strengthened using three schemes of hoop and longitudinal CFRP wraps.
• Hoop wraps provide lateral support to longitudinal fibers and increase column strength.
• An expression for the slenderness limit of FRP-strengthened RC columns is proposed.

The relevant design code provisions for Fiber Reinforced Polymer (FRP) strengthened RC columns are restricted to the short RC columns strengthened with FRP jackets. These design provisions are thus strictly not applicable to those long RC columns where second-order/slenderness effect is substantial. In the present study, the effectiveness of hoop and longitudinal Carbon FRP (CFRP) wraps in reducing the lateral deflections and improving the strength of slender circular RC columns has been studied experimentally. A total of 12 small-scale circular RC columns of 150 mm diameter were cast in three groups, each group containing 4 columns of the same height. The columns of the first group belonged to short columns of 600 mm height, whereas the columns of second and third groups of 900 and 1200 mm heights respectively represented slender columns. Columns of each group had one control and 3 strengthened columns. The strengthened columns were prepared using three different strengthening schemes. In the first strengthening scheme, the columns were wrapped using a single layer of hoop CFRP sheet, whereas other strengthening schemes employed 2 and 4 longitudinal CFRP sheets in addition to one layer of hoop CFRP wrap. The columns were tested under monotonic compression with initial eccentricity of 25 mm. In general, CFRP-strengthening improves the strength and ductility of slender RC columns substantially. The test results indicate that the CFRP hoop wraps provide confinement to concrete and lateral support to the longitudinal fibers and thus increase the strength of the RC columns. In slender columns, the effect of longitudinal FRP fibers in carrying the load in post-yielding stage is more significant than hoop FRP fibers. The existing ACI expression of slenderness limit for RC columns was extended to propose a simple analytical equation for the slenderness limit of FRP-strengthened RC columns. The proposed expression for slenderness limit is valid for both RC and FRP-confined columns and matches well with the experimentally observed slenderness limit.