Non-metallic reinforcement, commonly referred to as fiber reinforced polymer (FRP) composites, is made from various types of fibers that have widely varying mechanical properties. These composites are used to reinforce concrete structures. The most commonly utilized FRP products are externally bonded reinforcement (EBR), which comes in the form of strips, sheets, or fabrics to strengthen existing reinforced concrete structures. An important category of FRP products consists of rods with circular cross-sections and sidewalls shaped similarly to steel bars, designed for use as reinforcement in concrete structures subjected to unfavourable environmental conditions. This paper presents analyses of the behaviour of reinforced concrete beams strengthened by different techniques with external FRP reinforcement and steel reinforcement with different mechanical properties, as well as the results of analyses of the behaviour of concrete beams reinforced internally with FRP bars with different mechanical properties. An analytical approach based on cross-sectional analysis is presented to calculate the ultimate load of the beams and their deformation. For this purpose, a trilinear moment-curvature relationship model is developed to evaluate the behaviour of the beams over the entire load range. The results of the analyses indicated a very strong influence of the mechanical properties of the FRP reinforcement on the behaviour of the beams. FRP reinforcement is very effective in increasing the load carrying capacity due to its very high tensile strength. However, these benefits are severely limited by excessive deflection of the beams due to the low Young's modulus of most FRP bars.