Keywords: retrofitting, reinforced concrete, beam, finite element, flexural, carbon fibre reinforced polymer.

Reinforced concrete (RC) structural elements may need structural strengthening for several reasons, such as design and implementation errors, time-dependent degradations, change of intended use and modifications in structural codes. In recent years, structural retrofitting using fibre reinforced polymers has increased, because of their advantages such as their high strength and durability, that they are made from a noncorrosive material, their ease of application and the negligible increase in cross-sectional dimensions and the weight of structural members. The flexural ductility of a retrofitted RC beam is mainly dependent on the failure mode, which is governed by the detailing of fibre reinforced polymer. The design flexural capacity must equal or exceed the flexural demand.

In order to explain stress distributions and failure modes, several studies, which include both testing and finite element modelling of strengthened RC beams subject to monotonic loading, have been performed. In this study, finite element plastic analyses of RC beams strengthened using carbon fibre reinforced polymers (CFRP) were performed considering flexural capacity. Three-dimensional finite element analyses (FEA) of beams with different geometrical and material properties, which are available in the literature, were conducted. The properties of the beams in the nonlinear finite element model are the same as those of the actual beams. As a result of the symmetry in the geometry of the beams and loading pattern, only half of each beam was analysed by imposing appropriate boundary conditions along the line of symmetry. FEA using the ANSYS programme were performed to determine the flexural strength of RC beams strengthened with CFRP, which a perfect bond between concrete and reinforcement, and concrete and CFRP is assumed. Observing that the experimental and numerical load-deflection curves are consistent, the strains in the CFRP obtained through FEA were compared with the predictions by ACI 440. It is observed that as the tensile reinforcement ratio increases, the strain in the CFRP decreases. Since ACI 440 equations do not consider tensile reinforcement, the same value is predicted for the beams with different tensile reinforcement ratios using the ACI 440 equations. It can be suggested that the tensile reinforcement ratio should be considered in the calculation of the ultimate strains in the fibre reinforced polymer (FRP) according to ACI 440.

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