Strengthening reinforced concrete (RC) beams and slabs with prestressed carbon fiber-reinforced polymer (CFRP) composites is of high interest since the FRP material can be used more efficiently. Developing a non-mechanical prestressed strengthening system entails evaluating the bond resistance of prestressed FRP to the substrate. To increase the bond resistance of prestressed CFRP strips to the concrete substrate, a new method called externally bonded reinforcement on grooves (EBROG) was investigated and compared with the conventional externally bonded reinforcement (EBR) method. For this purpose, prestress force-release tests were performed to study the bond behavior of prestressed FRP to concrete. In-plane and out-of-plane deformations were measured through a three-dimensional digital image correlation (3D DIC) system. All the tests were carried out at Empa Structural Engineering Laboratory in Dübendorf, Switzerland.
Experimental results showed that the average bond resistances of EBR and EBROG joints in prestress force-release experiments were 34.4 and 81.4 kN, respectively; indicating an increased bond resistance for the EBROG method with a factor of 2.4. By using the EBROG method, the failure plane incorporated massive crack development in concrete bulk and therefore, the fracture energy and bond resistance of prestressed joints increased significantly. Besides, it was shown that prestressed joints experienced large out-of-plane deformations, emphasizing the fact the bond behavior of prestressed FRP to concrete should be treated as a mixed shear/tension fracture mode in the future studies.