Nowadays, fibre reinforced polymer (FRP) reinforcements are considered as appropriate strengthening solution for reinforced concrete (RC) structures worldwide. FRP materials gained its popularity due to reduced labour cost, corrosion resistance, good compliance to aesthetical and design requirements. Generally, strengthening is required for structures with deflections and crack widths exceeding design limits, therefore FRP reinforcement needs pre-stressing. Unfortunately, there is a lack of such research and the behaviour of such structures is described on the basis of research data where RC elements worked with FRP reinforcement altogether from the start of loading. The behaviour of the structure will be different in practice. For the above reasons, a crack computation model was proposed, which allows to predict the development of the crack taking into account the influence of external load action and pre-tension of the FRP reinforcement. The proposed calculation model was validated by experimental data of the beams strengthened with different FRP reinforcements, taking into account effects of FRP pre-stressing and initial stress-strain state of concrete elements.