Concrete tensile microcracking due to a mechanical loading enhances the diffusion of aggressive agents from concrete cover to the reinforcements, which leads to the beginning of the propagation period of steel corrosion. The purpose of this work is to quantify the effect of concrete microcracking on CO2 penetration. Concrete carbonation was studied on two 13-year-old reinforced concrete beams subjected to atmospheric carbonation. Indeed, these beams were exposed to the climate of the south-west of France but in a covered place and were always stored in a loaded state. Because the microcracking network is quite impossible to characterise, the tensile stress in the reinforcements was chosen as the main parameter of the microcracking state. This investigation results on a proposed model whose accuracy predicts the increase of the carbonation depth of the concrete in relation to the tensile stress in rebar.