What is Concrete Carbonation ?

Carbonation in concrete is the process whereby carbon dioxide (in moist air) reacts with the lime and other alkaline materials in the concrete and creates calcium carbonate.

This is the most common cause for loss of alkalinity in concrete, which is important as a high level of alkalinity is required to prevent corrosion of the re-inforced steel inside the concrete.

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Concrete Carbonation Facts

The Process of Concrete Carbonation

When this process takes place the pH of the concrete, which is normally 11-11.5 falls; at a pH level below 10 the steel’s thin layer of surface passivation dissolves and corrosion can take place. It proceeds from the outer surface towards the inside of the concrete but the rate of penetration depends on a number of factors. These include:

  • Atmospheric humidity
  • Concrete permeability

The rate of penetration slows down naturally as the carbonation proceeds deeper into the concrete. Surface blemishes such as blow holes, honey-combing or cracking will enable carbonation to proceed at a greater rate than on smooth concrete. The optimum condition for rapid concrete carbonation to occur is at a humidity level of 50-75%. Below 50% moist films do not form; above 70% there is too much water. This prevents penetration of the carbon dioxide – it would be difficult to have concrete carbonation in fully immersed concrete.

How is Concrete Carbonation Tested ?

Concrete Carbonation is tested with the straightforward use of a chemical indicator; the most commonly used indicator is a solution of phenolphthalein in alcohol and/or water. Phenolphthalein solution applied to fresh strongly alkaline concrete will turn pink. If the alkalinity has been lost the concrete will not turn  pink. In practice it is essential that the phenolphthalein solution is applied to freshly exposed concrete.  This can be carried out in 2 ways, either:

  1. By breaking a piece of concrete from the main area and spraying the underlying surface immediately.
  2. By drilling a hole into the concrete either at a given depth or in small increments.

The hole must be flushed out with de-ionised water immediately after the drilling to avoid contamination; this is then followed straight after with a spray of phenolphthalein. There should be a clear mark indicating the difference between pink and ‘natural’ concrete colour.  The degree of carbonation can then be measured in millimetres.

Corrosion Damage

Most problems with corrosion of steel in concrete are not due to loss of steel but the growth of the oxide. This leads to cracking and spalling of the concrete cover.

Structural collapses of reinforced concrete structures due to corrosion are rare. Concrete damage would usually have to be well advanced before a reinforced concrete structure is at risk.

Particular problems arise when the corrosion product is black rust, which occurs in low oxygen situations, in very damp concrete where chlorides are present and in pre-stressed, post-tensioned structures where corrosion is difficult to detect as the tendons are enclosed in ducts. Tendon failure can be catastrophic as tendons are loaded to 50% or more of their ultimate tensile strength and modest section loss leads to failure under load.

How Deep can Concrete Carbonation go ?

It has been found that the depth of carbonation is approximately proportional to the square root of the time. DEPTH = K x √TIME  (Where K is a constant).

Concrete Carbonation Testing


Depth of carbonation will be found to vary on different parts of a concrete structure.  Normally it will be less on areas exposed to heavy rain than on external but sheltered faces. It is essential to test all elevations and situations on a concrete building surface in order to correlate a range of results and thereafter take the appropriate action.

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