Today I want to continue talking about the durability of concrete, specifically about the corrosion of steel in reinforced concrete. How does it work? For what is this? How to avoid it? In this article, you will find everything you need to know about this aspect.
Electrochemical corrosion of the reinforcement
As I have already mentioned, this corrosion occurs through an electrochemical or galvanic mechanism caused by the presence of water and oxygen inside the concrete.
On the other hand, this process is influenced by different factors :
- The characteristics of concrete.
- The 'cover thickness ', ie the distance between the steel bars and the concrete surface.
- The temperature. The higher the temperature, the faster the oxidation process.
- The presence of chlorides, which accelerates the process.
Depassivation of the concrete reinforcement
Generally, the steel in concrete tends to passivate due to the alkalinity given by portlandite or CaOH2 from the hydration of the cement and its alkalis.
However, this passivation can be lost for a number of reasons:
1. CARBONATION
In carbonation, atmospheric CO2 penetrates the air-filled capillaries of the concrete and reacts with calcium hydroxide, giving rise to calcium carbonate. The pH of the concrete must be between 12'5 and 13'5 to properly protect and passivate the reinforcements inside. The presence of calcium carbonate lowers the pH of the medium that surrounds the steel bar to values close to 9, resulting in a more acidic medium than it should and that begins to cause a constant, uniform, and progressive corrosive effect of the steel.
The time required for the penetration front of the CO2 to reach the steel bars depends on the porosity of the concrete, the thickness of the coating of the steel bars, and the humidity of the air.
2. CHLORINE IONS
Chloride ions destroy the passivating layer of the steel in a specific way, giving rise to pitting corrosion that spreads in-depth, being able to section the bars and especially the prestressing wires, which are thinner and with a more delicate crystalline structure. This is why the maximum chloride content is limited to 0.4% in reinforced concrete and 0.2% in prestressed.
In a strict sense, the critical content of chloride s depends on the degree of carbonation of the concrete and the relative humidity of the environment. For corrosion to occur, there must be a significant drop in pH so that the steel remains depassivating, and this drop may be caused by the presence of acidic substances in the concrete.
Measures to avoid the corrosion of the armor
As measures to be taken against the corrosion of steel reinforcements, it is worth highlighting:
- Use good concrete with quality components, with a cement dosage above the minimum and with the maximum water/cement ratio indicated by the instructions.
- Give a suitable coating thickness to the steel bars depending on the environment in which the structure is located and respecting the minimums indicated in the instruction.
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