A focus on the carbonation mechanism in reinforced concrete structures
A focus on the carbonation mechanism in reinforced concrete structures
Leonardo Todisco (Universidad Politécnica de Madrid)
Carbonation is the result of the carbon dioxide CO2 existing in the atmosphere reacting with the calcium oxide in concrete Ca(OH)2 to form calcium carbonate CaCO3.
Ca(OH)2 + CO2 → CaCO3 + H2O
Carbonation starts from the concrete surface and gradually penetrates deeper into the concrete. The speed of this penetration depends on the permeability of the material, which is somehow proportional to the water-cement ratio. The carbonation can take up to several or many years to achieve the reinforcement. The speed of the carbonation can be increased because of other damages to the concrete that allow CO2 to be more readily absorbed.
The concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel reinforcement is passivated and is protected from corrosion. Carbonation decreases the pH of concrete to around 9. At this value the protective oxide layer surrounding the reinforcing steel breaks down and reinforcement corrosion starts. It is worth mentioning that carbonation does not entail any risk if no steel reinforcement is used in the concrete.
Carbonation is a slow phenomenon in very dry or saturated concretes; therefore, the optimal condition is when there is sufficient moisture for the reaction but not enough to act as a barrier. A big attention should be done to areas with high moisture concentration, as well as zones with alternation of humidity-dryness.
Once the carbonation front reaches the steel reinforcement it can cause corrosion, which can lead to different levels of risk depending on various factors.
A simple test can be employed to determine the depth of carbonation penetration; it is based on the use of phenolphthalein solution. This solution is applied to the concrete. If the indicator turns purple, the pH is above 8.6. Where the solution remains colorless, the pH of the concrete is below 8.6, suggesting carbonation.
Just a final curious fact: carbonation is basically a reversal of the chemical process that occurs when making the cement used in concrete which is responsible for most of concrete’s embodied CO2. Over the lifecycle of concrete, carbonation results in the reabsorption of part of the CO2 emitted when making cement, significantly reducing the whole-life CO2 footprint of the concrete for which it is used.
