In reinforced concrete, carbonation is a harmful chemical process that assaults the rebar and causes corrosion. Carbonation in concrete is the loss of pH that occurs when atmospheric carbon dioxide reacts with the moisture within the concrete pores and converts high-pH calcium hydroxide to calcium carbonate, which has a more neutral pH. When carbonation progresses to the depth of the steel, the protective passivating oxide layer is no longer stable. At this lower pH level, corrosion is able to begin.
For carbonation to take place, moisture must be present. Permeability also affects the tendency of concrete to carbonate. To detect carbonation, spray a 1 percent to 2 percent solution of phenolphthalein in alcohol onto the concrete. Carbonated areas of the concrete will not change color, while areas with a pH greater than 9 to 9.5 will turn bright pink.
A few options are available for repairing structures that have been diagnosed with carbonation-induced damage. Cathodic protection and re-alkalization are both options for repairing severely damaged concrete. However, these are expensive methods. Often, the most feasible option is to repair and protect the concrete. But in order to have an effective and lasting repair; the underlying causes of the corrosion must be addressed. For this reason, anti-carbonation coatings and corrosion inhibitors are often used.