A method for predicting the flexural strength of RC beams strengthened with carbon fibre reinforced polymer

An extensive experimental study has shown that the external bonding of carbon fiber reinforced polymer (CFRP) laminates or carbon fiber 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 results have indicated that the local failure of the concrete layer between the bonded laminate and longitudinal reinforcement in retrofitted beams is predominant. 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 behavior 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 to which the laminate is bonded. Close agreement with the experimental results indicates the accuracy of the proposed method. It can predict correctly the debonding failures for all the tests in this stud