Strain localisations in FRP-confined concrete: new insights

Circumferential confinement of reinforced concrete columns with externally bonded fibre-reinforced polymer (FRP) wraps is now widely implemented as a method to strengthen and/or enhance the deformability of deficient or damaged reinforced concrete columns. A fundamentally important parameter in the design of these members is the so-called strain efficiency of the FRP wraps - the ratio of the tensile hoop strain in the FRP at failure to the average failure strain observed in direct uniaxial tensile coupon tests. Available experimental hoop strain data obtained using localised strain gauges provide little insight into the variation of hoop strain over the surface of FRP wraps, the factors influencing strain efficiency and the resulting strain efficiency that should be assumed in design, and these remain essentially unknown. In this paper, a novel digital image correlation technique is used to provide a detailed experimental quantification of the axial and hoop strain variation over the surface of FRP confined concrete cylinders. Potential implications for the analysis and design of FRP confined concrete are discussed.