Concrete carbonation is a slow chemical process in which carbon dioxide from the air penetrates concrete and reduces its alkalinity. On its own it does little harm to the concrete; the problem is what it does to the steel reinforcement inside. By lowering the alkalinity that protects the reinforcement, carbonation sets the stage for corrosion — one of the main reasons reinforced concrete structures deteriorate with age. Understanding concrete carbonation is essential to understanding the durability of any reinforced concrete building.
This article explains the chemistry of carbonation in plain terms, why it threatens reinforcement, what controls how fast it advances, and how it is measured and managed. It is a quiet, invisible process, which is exactly why it is worth understanding before it reaches the steel.
The chemistry, in plain terms
When cement hydrates, one of the products is calcium hydroxide, which makes the concrete strongly alkaline — a pore-water pH around 13. This high alkalinity is what protects the embedded steel by maintaining a passive oxide film on its surface. It is the foundation of reinforced concrete durability, as set out in our guide to reinforced concrete.
Carbon dioxide in the atmosphere slowly diffuses into the concrete’s pore structure and reacts with the calcium hydroxide, converting it to calcium carbonate. This reaction consumes the alkalinity and lowers the pH of the affected zone. The carbonated concrete itself is actually slightly denser and harder; the issue is purely the loss of the alkaline environment that the steel depends on.
Why carbonation threatens the steel
Carbonation advances as a front, moving inward from the exposed surface over years and decades. As long as the carbonation front stays in front of the reinforcement, the steel remains in alkaline concrete and is protected. The critical moment comes when the carbonation depth reaches the depth of the reinforcement — the concrete cover. At that point the steel is no longer in an alkaline environment, the passive film becomes unstable, and corrosion can begin if moisture and oxygen are present.
This is why concrete cover is so important. Adequate cover of dense, low-permeability concrete delays the carbonation front from ever reaching the steel within the design life. Thin or porous cover lets carbonation reach the reinforcement quickly, triggering the corrosion sequence described in our explainer on rebar corrosion — cracking, staining and spalling.
What controls the rate
Carbonation does not advance at a constant speed; it slows as the front gets deeper, because the carbon dioxide has further to diffuse. The main factors that control the rate are:
- Concrete permeability: dense, low water-cement-ratio concrete resists carbon dioxide far better than porous concrete. This is the single biggest factor.
- Cover depth: more cover means the front has further to travel before it reaches the steel.
- Moisture and exposure: carbonation is fastest at moderate humidity. Very dry concrete lacks the moisture for the reaction; saturated concrete blocks carbon dioxide diffusion. Sheltered concrete exposed to air but not rain often carbonates fastest.
- Carbon dioxide concentration: urban and enclosed environments with higher carbon dioxide carbonate faster.
- Cracks: cracks provide a direct path for carbon dioxide to reach deeper into the concrete and accelerate localised carbonation.
Because permeability dominates, good concrete quality and curing at construction stage are the most powerful tools against carbonation.
How carbonation is measured
Carbonation depth is measured with a simple, reliable test: a freshly broken or cored concrete surface is sprayed with a phenolphthalein indicator solution. The still-alkaline concrete turns bright pink; the carbonated zone stays colourless. The depth of the colourless layer is the carbonation depth, which is then compared with the cover to the reinforcement.
If the carbonation depth has reached or is approaching the steel, the structure is at or near the point where corrosion can begin. This is a standard part of a durability assessment on an older structure and feeds directly into any structural engineer’s report on the condition of an ageing reinforced concrete building, alongside cover and chloride measurements.
Slowing carbonation and managing it
For new structures, the defences are designed in: low-permeability concrete, adequate and well-compacted cover, good curing, and crack control. These ensure the carbonation front does not reach the steel within the design life. Supplementary cementitious materials change carbonation behaviour and are considered as part of a balanced durability mix, since durability against carbonation and against chlorides sometimes pull in different directions.
For existing structures where carbonation is advanced, options include anti-carbonation coatings that slow further carbon dioxide ingress, and re-alkalisation techniques in serious cases. But the most effective approach remains preventing the problem at construction stage by getting the concrete quality and cover right. These measures sit within a broader approach to concrete durability rather than standing alone.
Frequently asked questions
Is concrete carbonation bad for the concrete itself?
Not directly — carbonated concrete is actually slightly denser and harder. The harm is indirect: carbonation lowers the alkalinity that protects the embedded steel reinforcement, allowing corrosion to begin once the carbonation front reaches the steel.
How fast does carbonation progress?
It varies widely and slows over time as the front deepens. The rate depends mainly on concrete permeability, cover depth, moisture conditions and carbon dioxide levels. Dense, well-cured concrete with good cover may protect the steel for the full design life; porous concrete can carbonate to the steel in a few decades.
How is carbonation depth measured?
By spraying a phenolphthalein indicator on a freshly exposed concrete surface. Alkaline concrete turns pink; carbonated concrete stays colourless. The depth of the colourless zone is the carbonation depth, compared against the cover to the reinforcement.
What is the most important factor in resisting carbonation?
Concrete permeability, set by a low water-cement ratio and good curing, combined with adequate cover. Dense, low-permeability concrete resists carbon dioxide ingress far better than porous concrete, making good concrete quality the most powerful defence.
Can carbonation be stopped in an old building?
It can be slowed with anti-carbonation coatings, and reversed in serious cases with re-alkalisation, but these are remedial measures. Preventing carbonation through good concrete and cover at construction stage is far more effective and economical than treating it later.
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Concerned about carbonation or corrosion in an ageing concrete structure? Talk to our consultancy or get in touch for a durability assessment.
