An extensive experimental study has shown that the external bonding of Carbon Fibre Reinforced Polymer (CFRP) laminates or Carbon Fibre Sheets (CFS) to the tension face of concrete beams is an effective strengthening method to increase the flexural load capacity and the stiffness. The strengthening mechanism relies on composite action between the concrete and the laminate. If composite action is not fully achieved, the strengthening effect will not be realized. Test resulted have indicated that the local failure of the concrete layer between the bonded and longitudinal reinforcement in retrofitted beams is predominant. In this study, two modes of debonding were observed: (1) laminate end debonding due to a thick or stiff CFS laminate, and (2) flexural cracks initiated in the vicinity of the loading point which would induce debonding away from the laminate end. Also, the ductility factor of the strengthened beam has, in general, been reduced. The strengthening effect was found to increase proportionally with the number of CFS; however, the peeling-off (debonding) mechanism of the CFS limits the composite laminate thickness, beyond which failure occurs without achieving the maximum laminate strain. Premature failure due to debonding always occurs before the full composite action of the bonded laminate to the strengthened beam is reached. The ultimate tensile strength or strain of the laminate may not be fully utilized due to the debonding failure; however, through proper design of the strengthening system, an effective (maximum) laminate strain of the bonded CFS can be achieved. This effective laminate strain can be used for the prediction of the flexural behaviour of the strengthened beam. A design-oriented expression for determining an effective laminate strain has been developed. This expression accounts not only for the laminate length and the laminate width but also for the stiffness of the laminate, as well as the stiffness of the member of which the laminate is bonded. In summary, a method has been developed for determining the point at which bonded CFS delaminates from the concrete beam. This method provides a very useful tool for engineers to assess and design a strengthening system for concrete beam rehabilitation in industry.