Existing concrete structures built between mid and late 1900s display increasing signs of deterioration due to adverse environmental conditions, reducing their load-carrying capacity. The existing structures are also required to carry increasing loads, which creates a need for increased capacity. For increased service life, existing dete-riorated reinforced concrete (RC) structures need strengthening. The finite element method (FEM) has been proven as an efficient tool for numerical simulations to accurately predict the non-linear response of concrete structures. Fibre reinforced polymer (FRP) has successfully been used to strengthen sound structures, but its application on damaged concrete structures still needs to be investigated. This thesis presents non-linear finite element analyses to assess the flexural behaviour of corro-sion damaged RC beams strengthened with externally bonded FRP. The modelling methods were validated against experimental results. Beams of four different cate-gories were analysed: A reference beam, a corroded but non-strengthened beam, and corroded beams strengthened with GFRP and CFRP respectively. Furthermore, the strengthened beams were modelled with different modelling choices to investigate the effectiveness of FRP sheets and FRP U-jackets. Pre-loading and corrosion-induced cracks were incorporated by reducing the tensile strength of concrete elements at the location of cracks. Average and pitting corrosion were incorporated by reducing the cross-sectional area of the reinforcement corresponding to the measured corrosion. Interface elements were used to simulate the bond between FRP and concrete. The FE analyses were able to capture same failure modes as the tests. It was found that modelling of pitting corrosion was of major importance to depict a reliable load and deformation capacity of the beams. Sufficient yielding zone near the corrosion pit was required in the finite element modelling to avoid premature failure of the pitted rebars due to high strain localization. A combination of a FRP plate at the beam soffit with inclined U-jackets at the ends of the FRP plate provided sufficient flex-ural strengthening; thus, intermediate U-jackets were not necessary for the studied beam geometry and corrosion damages. However, with a GFRP sheet at the beam soffit, both inclined and intermediate U-jackets with sufficient interfacial stiffness were needed to provide full utilisation of the GFRP sheet for the studied beam ge-ometry. To further study the effectiveness of the strengthening methods, it would be necessary to study beams with varying dimensions, corrosion patterns and levels, spacing and dimensions of FRP.