Axial behavior of square RC columns confined with FRP strips at corners and FRP battens at sides

The application of fiber reinforced polymer (FRP) composites has been successfully promoted for external strengthening of reinforced concrete (RC) columns. Recent studies showed that when these jackets are formed in a wet lay-up process for confinement of columns, an average rupture strain of FRP in specimens substantially fall below those of flat coupon tensile tests. The problem is more important in square columns because of premature rupture of FRP at corners due to stress concentration.
In this paper, a new method is introduced for confinement of square concrete columns as using FRP strips at corners and FRP battens at sides. In this method, FRP battens does not experience any curvature in confining of section and its behavior is very close to flat coupons. To compare the new proposed method with other FRP-confining techniques, 11 square RC columns with 133 x 133 mm cross section and 500 mm height were experimentally tested under uniaxial compression. The test parameters included influence of confining method, continuity or discontinuity of corner strips along height of column and the volume of fibers used. The experimental results clearly demonstrated that confining battens in the new method of corner strip-batten uniformly stretched under the tension stresses of confinement; therefore, uniform distribution of confining pressure on section occurred and the stress concentration at corners was eliminated. Thus, in the proposed method, better performance of FRP in confinement was observed and the compressive behavior of the strengthened column was significantly improved compared to those confined using conventional FRP wraps.