When using concrete as a material in construction, it’s important to have an understanding of its intricacies so you can anticipate potential problems in the future, such as carbonation, which is associated with shrinkage and the corrosion of the steel rebar used in concrete to carry tensile loads and increase overall strength.
Carbonation happens when CO2 dissolves in the concrete pore fluid and reacts with lime and other alkaline materials contained within the concrete itself, creating calcium carbonate.
This is, in fact, the leading cause of a loss of alkalinity in concrete, important because you need a high level of alkalinity in order to prevent the corrosion of reinforced steel.
When carbonation happens, the pH of the concrete falls and when it falls below ten (from a typical level of 11-11.5), the thin layer of surface passivation on the steel dissolves, allowing for corrosion. This works from outside in, with the rate of penetration dependent on atmospheric humidity and the permeability of the concrete.
The deeper the carbonation goes into the concrete, the slower the rate of penetration, but you will likely start to see signs on the surface such as cracking, honeycombing or blowholes, which can speed carbonation up.
Concrete testing for carbonation involves the use of a chemical indicator such as a solution of phenolphthalein in either water or alcohol. This is applied to concrete and if it turns pink, alkalinity has been lost.
Interestingly, most of the problems you see associated with corrosion of steel in concrete are down to the level of oxide in the material, which results in cracking and spalling. If you’re concerned about a structure and would like to arrange for a test, get in touch with us today.
Find out more about carbonated concrete in the Science Direct journal.