Concrete interface relationships under monotonic and cyclic actions

External bonding of fibre reinforced polymer (FRP) composites has become a popular technique used worldwide for strengthening existing reinforced concrete (RC) structures. Interaction phenomena having place at the FRP-to-concrete interface deeply affect the overall behaviour of RC beams externally strengthened by FRP. Consequently, several proposals have been carried out for describing the relationship between FRP-to-concrete relative displacements and interface shear stress. The present paper is mainly devoted to comparing the effectiveness of different identification methods for shear-stress-relative-slips law throughout the FRP-to-concrete interface. Several experimental results (pull-out tests under monotonic and cyclic actions), both available in the scientific literature and carried out directly by the authors, are utilised with this aim. The first cyclic delamination tests, carried out by the authors, allow a preliminary investigation on the influence of cyclic external actions on the bond of FRP reinforcement to the concrete substrate.