Corrosion of steel reinforcement is known to affect the residual bearing capacity of the structure. The loss of metal supposes a decrease of the sectional area of the reinforcing bar, either homogeneously distributed or concentrated in some zones. The morphology of the corroded area is a critical issue because while the uniform loss is influencing the steel strength, when it is localized also the ductility is reduced due the local stress concentration. There are previous publications showing the impact of corrosion on some mechanical aspects of the steel as on the ductility, the strength or the elastic modulus, but still we lack of a comprehensive framework of all types of corrosion. In this work a classification is made of the types of damages for better assessing their consequences. Further, through application of basic classical mechanics are given expressions to calculate the “corrosion factor” related to the yield strength, the ultimate strength, the elastic modulus and the elongation (ductility). Finally, through fracture mechanics and an energetic approach are given the expressions for the critical stress intensity factor and the “apparent” residual section “loss” of the corroded steel bars.