In reinforced concrete (RC) elements externally bonded with fibre-reinforced polymer (FRP) materials, the bond behaviour between the external FRP reinforcement and the concrete has been largely studied from experimental, analytical and numerical points of view due to its influence on design of these elements under both serviceability and ultimate conditions. Under service loads, the bond behaviour determines the tension-stiffening contribution of concrete and crack formation and propagation. This study presents a numerical methodology to analytically simulate the tensile behaviour of a RC tie strengthened with externally bonded FRP materials. The problem is simplified to a one-dimension model aimed at calculating the stabilized cracking process given the linear bond-slip laws at both the steel-concrete and FRP-concrete interfaces. The solution, in terms of strains and stresses in materials and slips at interfaces, is found through a finite differences method that allows obtaining the tensile load in the tie for a given value of increasing slip at the end. Results also include the calculation of the difference between the mean strain in both reinforcements and concrete and the crack spacing at the stabilized condition. In the paper, a parametric study is performed to evaluate the influence of the tensile area of concrete and the bond-slip law of FRP reinforcement on the cracking and deformability of the tie, by means of comparisons with experimental results available in literature.