What is Carbonation?

Carbonation is an electrochemical reaction between carbon dioxide, moisture and the cement in concrete structures, where, athmospheric carbon dioxide (CO₂) reacts with cement hydration products like calcium hydroxide, forming carbonic acid (H₂CO₃) and subsequently calcium carbonate (CaCO₃) or calcite.

This reaction reduces the concrete's pH, leading to de-passivation of embedded steel reinforcement and its corrosion, compromising durability. In concrete with a pH of 12-13, about 7,000 to 8,000 ppm of chlorides are required to start reinforcement corrosion, however, when carbonation drops the alkalinity to between 10-11 only 100 ppm is required.

While in the beginning carbonation fills the concrete pores and reduces permeability, over time micro cracks and shrinkage will occur which reduces durability, creating vulnerability to deterioration mechanisms based on the ingress of materials into the concrete such as External Sulphate Attack or Biological contamination; contributing to cracking and spalling.

Mitigation strategies include proper mix design, adequate reinforcement cover, and regular maintenance.

Figure 1. Microscopic view of carbonation discolouration of concrete

What causes Concrete carbonation of concrete structures?

Carbonation in concrete structures is caused CO₂ from the atmosphere infiltrating the concrete and reacting with calcium hydroxide, the primary hydration product of Portland cement.

This process commonly occurs in urban areas with high levels of atmospheric CO₂ emissions, as well as polluted industrial zones, as high humidity and elevated CO₂ concentrations accelerate carbonation rates.

Concretes with high porosity and cement combinations which produce large amounts of calcium hydroxide during hydration are particularly vulnerable. Replacement of Portland cement with high proportions of supplementary cementitious materials (SCM’s) like GGBS or Fly Ash allows the creation of low porosity concretes with extremely low calcium hydroxide production that are extremely resistant to carbonation.

Figure 2. Carbonation based corrosion of reinforced concrete

What are the signs of Carbonation of concrete structures?

Loss of alkalinity.

How can I identify Carbonation in concrete structures?

Visual survey	Non-destructive testing	Destructive testing
Visual examination	Rebound hammer Water permeability Impact echo Cover depth (Cover meter)	Sampling Carbonation depth Portlandite content Capillary absorption Water Porosity Permeability coefficient

How can I prevent Carbonation in concrete structures?

To prevent carbonation in concrete structures, several measures can be implemented. Firstly, using concretes with low porosity and cement combinations that produce minimal calcium hydroxide during hydration is essential.

This can be achieved by maintaining low water to cement ratios and replacing Portland cement with high proportions of SCM’s like Ground Granulated Blast Furnace Slag (GGBS) or Fly Ash. These materials facilitate the creation of low porosity concretes with reduced calcium hydroxide production, making them highly resistant to carbonation. Geopolymer concretes offer a totally calcium hydroxide free solution that can eliminate any chance of carbonation.

Figure 3. Carbonation depth shown by phenolphthalein solution

Furthermore, ensuring adequate cover depth of reinforcement relative to the environmental exposure conditions is crucial for preventing corrosion initiation. Alternatively, applying surface protection systems such as concrete coatings or impregnation can provide an additional layer of defence.

By implementing these preventive measures, including proper mix design, material selection, and surface protection, the risk of carbonation-induced deterioration in concrete structures can be effectively mitigated, ensuring their long-term durability and performance.

Figure 4. Carbonation depth schematic and image of tested slab

How can I repair the damage from Carbonation on concrete structures?

If corrosion induced by carbonation is seen or feared, it is of primary importance to improve the concrete condition and of decrease the susceptibility of concrete to carbonation by:

Re-alkalization.

Cathodic protection. 

Corrosion inhibitors. 

Concrete Impregnation. 

Concrete Coating.

or in extreme conditions by Concrete replacement.