The behavior of damaged concrete beams strengthened by externally bonded steel plates is investigated experimentally. The variables tested were the thickness of the external steel plate, length and location of the interfacial crack, and the degree of surface preparation of the steel plate. In all five beams the thickness of adhesive was kept constant. The results indicate that (i) the behavior of the damaged open sandwich beam is similar to that of a singly reinforced concrete beam when no debonding occurs between the concrete and the adherent steel plate, (ii) the case with interfacial crack between the steel and the adhesive is more critical man that the case when the interfacial crack is between the adhesive and the concrete, (iii) the failure load and the mode of failure are dependent of the degree of the surface preparation of the steel plate. The results suggested that the interface separation (debonding) is the critical mode of failure of the open sandwich beam if a large interfacial crack between the steel plate and the adhesive exists. An analytical investigation to predict the interfacial debonding is summarized, and its results suggest that linear elastic fracture mechanics is suited for predicting the failure load for the case of debonding in open sandwich beams.