Direct versus indirect identification of FRP-to-concrete interface relationships

Several proposals have been carried out for modelling FRP-to-concrete interface behaviour. The bi-linear formulation is one of the most widely accepted relationship between shear stress and relative slips developing throughout the adhesive interface. However, various expressions, rather diverse indeed, are currently available within the scientific literature for determining the key parameters which define such a bi-linear model depending on the mechanical properties of basic materials (namely, concrete, epoxy resin and composite element). Moreover, neither standard testing procedures nor unified calibration and identification techniques are currently recognised. The present paper deals with the latter aspect and is basically aimed to examine the differences in model calibration possibly arising from direct or indirect identification techniques. Finally, comparisons with different proposals even available within the scientific literature for the interface relationship are also performed.