عنوان مقاله [English]
There are many concrete structures that need to be strengthened, due to such reasons as changes in codes of practice, design or construction error, damage from earthquakes or such events and/or changes in their usage. Among retrofitting systems, the use of FRP sheets in the concrete retrofitting industry is attracting more attention worldwide. Since the retrofitted concrete structures have concrete with different strengths, the behavior of FRP sheets can be different in different structures. Therefore, in this paper, the behavior of reinforced concrete beams with different concrete and bond strength, retrofitted with FRP sheets, is reported. Experimentally obtained results are compared with those obtained from ABAQUS. For related research, using five different concrete mixes, 15 reinforced concrete beams were constructed. For each concrete strength comparison, two beams were strengthened using FRP sheets and one beam was left to be tested as a control beam. The bond strengths of the FRPs used were measured using a new twist-off method. This new method is a partially destructive test that can be used under laboratory and site conditions. Preparation of concrete surfaces and the application of epoxy resin glue were undertaken so that the best adhesion could be achieved. In order to assess the FRP/concrete bond strength, partial cores were made using a diamond tipped coring drill. The depth of the partial cores was such that they penetrated about 5mm into the substrate concrete. The damaged caused by the twist-off method is very trivial and can be repaired by hand if required. The results tend to indicate that an increase in concrete strength and the bond between the FRP layer and the concrete increases the ultimate load (capacity) of the retrofitted reinforced concrete beams. Therefore, when opting for retrofitting with FRP sheets, the surface strength and the interfacial adhesion of FRP/concrete should be given enough consideration.