Cover around reinforcement
The reinforced concrete structures are designed and built for a 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 and 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 design of traditional concrete, in particular its cement dosage and its water-cement content ratio, depends on the environment to which this material is exposed.

• dry                                                         --- 1 --- 280 --- 0,65
• wet (without freezing)                             ---2a --- 280 --- 0,60
• wet (with freezing)                                  ---2b --- 280 --- 0,55
• wet and freezing (with use of deicing salts)--- 3 ---  300--- 0,50
• sailor without freezing)                             ---4a---  300 --- 0,55
• sailor (with freezing)                                ---4b---  300  ---0,50
• chemical (slightly aggressive)                    ---5a --- 280 ---0,55
• chemical (fairly aggressive)                     --- 5b --- 300 --- 0,50
• chemical (highly aggressive)                     --- 5c  ---300--- 0,45

In very particular cases, additives can be added in the freshly-mixed concrete to improve of its workability, its mechanical properties and more rarely its protective qualities.

The concrete cover thickness around reinforcement also depends on the environment aggressiveness. But, in addition to the requirements given by the designer, it is significant to consider the implementation (reinforcements positioning, concrete mixing, curing, etc.) to estimate the durability of a reinforced concrete really in place.

Reinforcement covers
When reinforced concrete structures are exposed to a very aggressive environment, an additional protection can be considered by cover steel. The two types of the most frequent covers on steel are organic coatings and metal coatings (hot-dip galvanising, etc). These protective coatings which must adhere to steel must also ensure good a bond between reinforcement and concrete.

Coatings on concrete
Protection against corrosion can also be to improve using a coating on concrete. The purpose of coatings on concrete are to prevent reinforcement corrosion, by ensuring a proofing either water, or with gases.

In details, membranes and coatings which are not subjected to a direct action of the atmosphere, in this case, the internal moisture of the underlying concrete varies little with time, coating systems applied on visible facings. Under these coatings, the internal moisture of the concrete can fluctuate. Water can evaporate from concrete or return there as a vapour. Gas flows (oxygen, carbon dioxide) can also cross these coatings.

The effectiveness of coatings is characterised on products or concrete substrate. As a rule, the test consists in measuring the fluid (liquid or gas) flux crossing a covered specimen.

A coating on concrete can be deteriorated by disbonding, blistering or formation of wrinkles (following chart 2).

• Origins            ---Conditions             ---Too bad
• chemical attacks---Reactive components, etc.---Disbonding, Dissolution
• capillary suction overpressure in concrete---adsorption of water, cement pores unsaturated with water---Disbonding, Blistering
• solvent or vapour overpressure---Rise in temperature, cement pores unsaturated with water---Disbonding, Blistering
• osmotic processes---semi-permeable membranes, coating components soluble in water---Disbonding, Blistering, Wrinkles
• Fastening and internal stresses---temperature rise, differences in thermal expansions between coating and substrate---Disbonding, Wrinkles, Cracks
  

In practice, conventional life tests are carried out on coated concrete, to assess the performances of their protections. These tests often consist in putting specimens in a salt spray chamber and making them undergo various actions, such as temperature changes or ultraviolet rays.