Several hundreds of buildings made of reinforced concrete are now classified as historical in France. Their major pathology is carbonation induced corrosion. The cracking and spalling decays induced by this natural aging process cause loss of material incompatible with the deontology of historical monuments conservation. Therefore it is crucial to understand the corrosion process of embedded steel and to be able to evaluate precisely the corrosion activity on site, in order to predict and may-be to prevent such decays. Current on site non destructive tools for diagnosis operations are potential mapping or corrosion rate measurements. But, such electrochemical techniques are clearly season influenced. Moreover, corrosion rates induced by carbonation are often very low and therefore can be difficult to measure. As a consequence, in areas heavily affected by spalling phenomena, low corrosion activity is often evidenced, so that the interpretation of the results can be problematic. A last hypothesis could be that the oxide layer formed is so resistive that the conductivity necessary for the measurement is lost. In order to better understand the corrosion process of rebars embedded in carbonated concrete, and may be to improve the reliability of those diagnosis tools, it seemed interesting to try to correlate on site measurements with the identification of corrosion patterns. Therefore, on a 56 years old industrial building (Air Purifier of the Meudon Wind tunnel), several sets of potential mapping, resistivity and corrosion rate measurements were performed, respectively in winter time and in spring time, and after medium or heavy moistening. Then reinforced concrete cores were sampled both in supposed active and passive corrosion areas, in order to evaluate the carbonation depth and to identify the corrosion patterns.