Depth of Carbonation
In a normal good quality reinforced concrete, the steel reinforcement is chemically protected from corrosion by the alkaline nature of the concrete. This alkalinity causes the formation of a passive oxide layer around the steel reinforcement. Concrete will react with atmospheric carbon dioxide (and sulphur dioxide) to cause gradual neutralisation of the alkalinity from the surface inwards, a process known as carbonation. The rate at which this occurs is a function of concrete quality, in particular the cement content, the water/cement ratio and the compaction. It is generally accepted that the rate of the carbonation reaction is inversely proportional to the square root of the age of the structure. If the depth of carbonation is taken in mm and the age in years then the constant of proportionality for a good quality concrete is approximately unity.
depth of carbonation mm = (Age yrs)0.5
On this basis, even with a cover of only 10mm, steel reinforcement should be safe for up to 100 years. In practice, however, carbonation often occurs rather faster either because of porous concrete or to microcracking in the concrete providing secondary paths to the steel other than by normal diffusion processes. For this reason the advice given in Table 3.4 of BS8110, for example, is rather more stringent in recognition that concrete in practice is often less than perfect.
The depth of carbonation can be measured on a freshly exposed section of the concrete (such as a core) by spraying with an indicator spray such as phenolphthalein. This turns pink when the concrete is alkaline (above pH 9.2) but remains colourless where the concrete is carbonated, usually as a more or less even zone extending from the surface. The depth of carbonation can then be measured. Phenolphthalein is the accepted indicator for this purpose but it should be noted that the pH at which the colour changes may be lower than that at which passivity has been lost. The test is described in BRE Information Sheet IP6/81, "Carbonation of Concrete Made with Dense Natural Aggregates". It should also be noted that carbonation around microcracks and along diffusion paths in poorly compacted concrete or reconstituted stone may not be readily revealed by the phenolphthalein spray.
Petrography can reveal carbonation of this kind and is recommended.