Concrete carbonation: how it corrodes reinforcement and how it is assessed

01What carbonation is

Fresh concrete is highly alkaline, and that alkalinity keeps a thin passive film on the reinforcement that protects it from corrosion. Carbon dioxide from the air diffuses into the concrete and reacts with calcium hydroxide to form calcium carbonate, lowering the alkalinity. The carbonation front moves inward over the years; when it reaches the steel, the passive film is lost and the steel can corrode if moisture and oxygen are present.

02What speeds it up

• Low cover to the reinforcement.
• Porous concrete: a high water–cement ratio, or poor compaction or curing.
• Cracks that open a path to the steel.
• Moderate humidity speeds carbonation; wetting and drying cycles feed corrosion.
• CO₂-rich environments, such as enclosed car parks.

03Signs on site

• Rust stains on the surface.
• Cracks along the line of the bars.
• Spalling: pieces breaking off over corroded steel.
• A hollow sound when tapped — delamination.

04How it is assessed

1. A visual survey and hammer tapping to map the damage.
2. Cover measured with a covermeter over representative areas.
3. Carbonation depth: a fresh break or core is sprayed with phenolphthalein; carbonated concrete stays colourless and alkaline concrete turns pink (BS EN 14630).
4. Chloride testing where salts may be involved — a different corrosion mechanism with a different repair.
5. Comparing the carbonation depth with the cover shows where steel is at risk now and where it will be.

05Carbonation or chlorides?

Both lead to corrosion, but chlorides — from salty soils and groundwater, contaminated aggregate or sea air — break down the passive film even in alkaline concrete, often with local pitting. The assessment separates the two because the repair principles differ. BS EN 1504-9 sets out the principles and methods of repair and protection.

06Repair and protection

• Concrete repairRemove damaged and carbonated concrete around corroding bars, clean and protect the steel, and rebuild with a repair mortar of the right BS EN 1504-3 class.
• Anti-carbonation coatingsCoatings that limit CO₂ diffusion while letting water vapour out, over sound or repaired concrete (surface protection in BS EN 1504-2).
• Crack treatmentCracks sealed or injected so they stop feeding moisture and CO₂ to the steel.
• Records and follow-upA record of what was found and repaired, to plan the next inspection.