Several experiments techniques were used to study the rust formed on steel rebars embedded in concrete during 65 years. It was found that rust was composed of two layers: an inner one composed mainly of magnetite and Ca-rich magnetite firmly adhered to the substrate, and an outer one composed of α- and γ-iron oxyhydroxides, final steel corrosion products. The mechanical properties of steel and the characteristics of the sorrounding concrete were also investigated in order to establish the possible cause of corrosion initiation.