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Carbonation of concrete

Concrete structure renowned for their durability. Reinforced concrete is used widely over the globe to build bridges, tunnels, roads, and many other types of structures. The durability of concrete can be reduced by the ingress of aggressive chemicals such as chloride, freeze and thaw cycles and the carbonation of concrete. 

Concrete is alkaline. The Ph of concrete at of pouring approaches 13. Concrete alkalinity provides a protection of steel against corrosion. Concrete forms a thin passive layer that minimizes the rate of steel corrosion. The concrete passive layer reduces the corrosion of steel to 1 µm per year. Steel without concrete passive layer protection will undergo corrosion of 1000 times higher. Corrosion of reinforcement steel can severely affect the structure. Corrosion of steel can result in cracks and spalling of concrete.

Figure 1

Carbonation of concrete is a significant defect that cause reinforcement to corrode. Figure no: 2 showing the stages of the carbonation process. Carbon dioxide (Co2) is natural gas, and Sulphur dioxide combined with the moisture from the surrounding environment will react with calcium hydroxide in concrete producing calcium carbonate and calcium sulfate. This chemical reaction will reduce concrete alkalinity. The layer of concrete affected by this reaction known as a carbonated layer. Once the depth of this layer reach’s the reinforcement steel. The concrete passive layer will be eliminated, and the reinforcement will be without protection against corrosion. The ingress of oxygen and water will cause reinforcement to corrode at a rapid rate. The carbonation process will reduce the alkalinity of concrete. Concrete Ph around 13 before the occurrence of carbonation. The carbonation process will lower the concrete Ph to 8, which results in the loss of a concrete passive layer. The losing of passivity usually occurs at Ph around 11.

Figure 2

Carbonation is a slow process for high-quality concrete. It is estimated to be around 1 mm per year. The rate of carbonation depends on different factors such as:
  • The relative humidity: Highest concrete carbonation occurs for relative humidity between 50 to 75%. Below 25%, the rate of carbonation is prolonged. Relative humidity higher than 75% will prevent the Co2 penetration.
  •  Concrete permeability: concrete with low permeability will experience a prolonged carbonation reaction.
  • Grade of concrete
  •  Protection of concrete: protection of concrete from the surrounding environment by waterproofing or coating will reduce the carbonation reaction rate. 
  •  Depth of cover: Carbonation-induced corrosion often occurs on concrete with low cover. 


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