Retrofitting concrete structures with fiber reinforced polymer (FRP) has today grown to be a widely used method throughout most parts of the world. Nevertheless, when strengthening a structure with external FRP, it is often not possible to make full use of the FRP. By prestressing, a higher utilization of the FRP material is made possible. The current thesis is dedicated to investigation of changes in strength, ductility and serviceability of RC beams strengthened with prestressed NSM CFRP strips. Five full scale simply supported beams were cast and tested at Isfahan University of Technology, including one reference (non-strengthened) specimen and four strengthened specimens with NSM CFRP strips over their full length. Among strengthened specimens, one was not prestressed and three other ones were imposed by three levels of prestressing force which made initial strain of 5, 20, and 30 percent of nominal ultimate strain in CFRP strips. All beams were reinforced by internal tensile steel reinforcement of 0.5? b (? b is the tensile balanced reinforcement ratio) and tested with three-point loading system. The experimental results indicated that the load carrying capacity of strengthened beams compared to that of control beam, had an increase of 10 percent in non-prestressed specimen, while it was 11.5 to 15 percent in prestressed ones. The tests showed a large increase in the crack and steel-yielding load. The prestressing level had a direct influence on both of these parameters: the prestressed strengthened specimens showed the cracking load 17 to 45 percent and the yielding load 5.5 to 9 percent higher than those of control beam. Together with higher crack loads, the cracks also were smaller in both width and depth, and propagated in more limited area. The non-prestressed strengthened beam showed almost the same behavior in cracking and yielding as non-strengthened one. It should be mentioned that the prestressed beams lost a part of their ductility at failure, which is because a part of the possible strain in the strips was used during prestressing, resulting in failure at smaller deflections. As a part of this study, an analytical evaluation was developed for the estimation of flexural behavior and ultimate load carrying capacity in strengthened specimens, using M-? diagram and concepts of prestressed concrete. Comparing experimental data with analytical results showed that this developed method was just helpful to understand the behavior of strengthened beams in elastic region.
