The corrosion of a reinforcement mainly depends on the geometrical (cover thickness) and physical (porosity) of the covering concrete. It also depends on the aggressiveness of its environment, particularly when those geometrical or physical properties do not meet the requirements.

A reinforced concrete structures is designed and built for a given service life, which is defined by the designer and is controlled by the manufacturer.  For this period of time, the structure should not be so deteriorated that it does not fulfill any more its functions.

The deterioration of reinforced concrete is mainly due to reinforcement corrosion The mechanism of this deterioration is to be reminded. Reinforcements corrode when they are in contact with a high amount of aggressive agents. This is the reason why, the prevention of reinforcement corrosion, in structures to be built, is obtained mainly by controlling the thickness and the quality of the concrete cover.

The most important parameters, which induce reinforcement corrosion, are the cover thickness and the quality of concrete, as well as the aggressiveness of the environment.

Cover thickness

Cover thickness is the distance between concrete surface and the reinforcement, which is the nearest to this surface. It is determined after taking into account the required mechanical properties. According to some regulations, its minimum value depends on the exposure class and on the type of structure (structure class)..

Most often, steel corrosion occurs when the cover thickness is too thin.

Corrosion of reinforcement under a too thin concrete cover

Concrete is characterized first by the cement (or equivalent binder) content, then by the wter-to-cement ratio, which strongly influences concrete porosity.

Concrete placing is important, as it can be the origin of local defects.

In every case, the quality of covering concrete, and particularly its physico-chemical properties (mainly compactness), is of a first importance for yielding protection. The techniques applied must continuously lead to a good quality, from concrete production to its placing.

This aggressiveness is of course an important parameter, which accelerates corrosion, when covering concrete is not satisfactory.

The European standards EN 206 EN1992-1.1 and EN1992-1.2 give the requirements concerning the environment classes, which must be taken into account for determining concrete thickness and quality.

Exposure class	XC4	XS3	XD3
	Carbonation Alternate wetting and drying	Chlorides in seawater. Tidal zones, splash areas	Chlorides not due to marine environment. Alternate wetting and drying.
Examples	Façades exposed to rains, superstructures of bridges	Bridges and buildings on seashore, marine structures.	Car parks under deicing salts or with frequent water leakagess.

Some applications of the here above standards are shown in the following table, concerning structures with a predicted lifetime of 50 years.

Special metallic reinforcements