A MODEL FOR THE MECHANICAL BEHAVIOUR OF THE FRP CONFINED CONCRETE COLUMNS Athanasios I. Karabinis 1 and Theodoros C. Rousakis 2 1, 2 Department of Civil Engineering, Democritus University of Thrace, Xanthi, 67100, GREECE ABSTRACT The FRP material used as confining mean, have shown to significantly enhance the mechanical behaviour of concrete. For the analytical prediction of the increase in strength and ductility of FRP confined concrete a constitutive model based on the non-associative plasticity theory is proposed. The model has been applied to steel confined concrete and is modified and calibrated so as to incorporate the dilation characteristics of the FRP confined concrete. The model provides the stress-strain curves of axially loaded circular columns that are confined by FRP reinforcement. It can be used in FRP tube encased concrete as well as in FRP sheet wrapped concrete. Satisfactory correlation of experimental and analytical results is remarked. KEYWORDS FRP, confinement, plasticity theory, columns, axial loading, external reinforcement, wrapping, concrete, constitutive model. INTRODUCTION The need for rehabilitation or strengthening of degraded concrete structures (due to earthquakes, corrosion, overloading) has directed the research to the enhancement of the mechanical behavior of the concrete structural members, mainly columns. Confinement by FRP sheet has been proved to considerably increase the strength and ductility of concrete when loaded axially (Harries et al. 1998), (Karabinis and Rousakis, 2000). Similar is the effect on the behavior of FRP encased concrete (Harmon et al. 1995).