Before being placed in a formwork, steel reinforcement may become rusted, because it was initially exposed to the atmosphere.
When freshly-mixed concrete is placed around this steel, the mixing water penetrates through the rust pores, where it gradually forms hydrated calcium ferrite (4.CaO. Fe2 O3 13H2 O). Moreover, this water reacts with steel and forms a thin layer of iron and calcium hydroxides, respectively [Fe(OH)2] and [Ca(OH)2].
All these products in the vicinity of steel raise the pH of concrete pore solution, up to about 13. It should be noted that when in contact with initial rust, cement hydration is disturbed and a transition zone is locally formed. Concrete is more homogeneous when far from this zone.
The concrete mixing water makes it possible to form some products, which protect the steel by passivation. More precisely, under atmospherically induced rust, reinforcement is covered with a thin protective layer of white products, containing ferrite and calcium hydroxides.
Such protection vanishes when large cracks reach reinforcements or the porosity of the concrete is large enough to allow aggressive elements to reach the steel surface.
Corrosion with rusting of reinforcement in concrete comprises two stages. In the first stage (or step), the aggressive elements, such as chloride or carbon dioxide (CO2) (Cl-), present in the surrounding medium penetrates the concrete – this is the initiation stage. The second stage is propagation which starts, when these aggressive bodies are in rather high concentrations at the reinforcement level. This corresponds to rust growth, which can break concrete cover.
These stages are described as follows:
A first stage involves the transfers of aggressive agents (mainly carbon dioxide and chloride) of water and of oxygen, inducing the corrosion initiation (de-passivation of reinforcement)
A stage of corrosion growth, leading to concrete damage, to spalling, cracks, etc. This stage starts when the contents of aggressive agents are high enough to reinforcing steel.
To describe steel corrosion in concrete, it is advisable to define, on one hand, the penetration of the aggressive agents through concrete and, on the other hand, the conditions of depassivation of reinforcement, then the dissolution rate of metal and the rust growth.
For more information, please contact the Metallurgy Unit at CARIRI at 285-5050