Estimation of pull-out and shear strength of FRP spike anchors

Spike anchors are a promising way to enhance the maximum load and post-peak load-strain response of externally-bonded fibre-reinforced polymer (FRP) materials for structural retrofitting. To date, the effectiveness of these anchors has been proved by experimental research, but little work has been conducted to provide an analytical basis for design of anchored reinforcements. A major concern for engineers working with spike anchors is the identification of the parameters that govern their behaviour, in order to develop a predictive basis for calculation. This paper presents an analytical model to predict the maximum load of single spike anchors, considering two main stress configurations: direct pull-out and shear; the model aims to provide engineers with an analytical tool in terms of reduction of the tensile strength so that calculation can be based on flat-coupon tests or material properties. For direct pull-out configurations, an anchor's capacity can be calculated as any other post-installed anchors taking into account the specific shear strength and strain behaviour of the epoxy-to-concrete interface. For anchors under shear stress or a combination of tension and shear, the model assumes that the reduction due to bending is conditioned by: dowel angle, bending ratio (defined as the inner bending radius divided by the anchor's diameter) and embedment depth; the embedment depth has thus been included in the expression as it affects the confinement provided by the concrete surrounding the anchor dowel.