Experimental tests on existing RC beams strengthened in flexure and retrofitted for shear by C-FRP in presence of negative moments

The shear strength of reinforced concrete beams extracted from existing buildings often reveals insufficient transversal steel reinforcement, mainly due to design or construction defects or increased design load requirements. FRP wrapping is one of the best solutions to improve beam shear strength as the retrofitting intervention is fast and the cost is modest. Design codes provide clear indication about the retrofitting design of simply supported beams, while the case of a beam with negative moments at the end is not considered, although this is in the case of a beam in a framed structure. One of the main uncertainties lies in the effectiveness of the FRP U sheet anchorage behavior in the area of negative bending moments with cracked concrete. This may limit the shear strength of the retrofitted beam. In this study, two beams extracted from an existing building constructed in the 1930s in Rome and retrofitted by carbon fiber-reinforced polymer (C-FRP) U strips placed at beam ends, where also negative bending moments were present, and have been evaluated with experimental tests at the laboratory of the Department of Architecture of Roma Tre University. Beam steel and concrete characteristics were evaluated by means of different tests. The experimental results are discussed considering the final results in terms of maximum shear resistance in the presence of negative bending moments. Load deflections at different points along the beam, shear-C-FRP deformation along the reinforcement strips and the damage state for different load levels, are presented. The importance of avoiding possible fragile mechanisms in the sections retrofitted with FRP is clearly shown.