عنوان مقاله [English]
In this research, an experimental study on the strengthening of RC connections using FRP sheets has been undertaken. The innovation of this research is in using narrow grooves for concrete surface preparation in the joint panel zone and the ductile zone of the beam, in order to increase adherence between FRP sheets and the concrete surface, and in preventing their debonding. For this purpose, four RC connections were made and tested under constant axial load and increasing cyclic loading. The first connection, as the standard reference specimen, had closed tie spacing in the ductile regions of the beam, column and panel zone. The second connection, as a weak reference specimen, did not have these conditions in all regions. Two other connections, which had properties like the weak reference specimen, were strengthened using two different strengthening patterns with FRP sheets. In the first strengthened connection, an ordinary surface preparation method was performed for installing FRP sheets, and the surface grooving method was used for the second strengthened connection. According to the results, without regarding the closed tie spacing in the weak specimen, ultimate load and ductility were decreased by 25\% and 17\%, respectively, compared to the standard specimen. Shear failure and concrete crushing were prevented in the ductile regions of the beam and panel zone in both strengthened connections. Also, it was observed that in the strengthened connection with a grooving pattern, early debonding of FRP sheets was prevented, and sheets reached their ultimate tensile strength. This specimen had desirable ductility and bearing capacity, similar to the standard specimen. In the final stages of loading in the strengthened specimen with an ordinary surface preparation method, the maximum strain of the U-shaped sheets reached 0.49\%; close to the recommended amount by the design code for the allowed strain of FRP sheets equal to 0.4\%, although half the ultimate rupture strain of sheets. Also, some regions in the borders of the sheets were debonded and by increasing strains, progressive debonding was seen in these regions. But, in the strengthened specimen with the surface grooving method, even no slight debonding was observed, and the maximum strain of sheets was 0.97\%; close to the ultimate rupture strain amount and twice that of the design code amount.