Reinforcement corrosion for inland reinforced concrete structures often is attributed to ‘carbonation’. This is based on the claim that the entry of atmospheric carbon dioxide lowers the pH of the concrete sufficiently for corrosion of steel to initiate in the presence of moisture. Mostly only the depth of permeation of carbon dioxide is considered, usually in laboratory observations often in solutions rather than in actual concretes. Direct evidence of significant reinforcement corrosion is seldom reported. Herein detailed observations are reported for carbonation depths, concrete pH and reinforcement corrosion for several reinforced concrete columns taken from different elevations and locations after 60 years continuous atmospheric exposure in a temperate climatic zone. Despite being boldly exposed, without protection of any kind, none showed evidence of reinforcement corrosion. All showed carbonation only some 10-15 mm into the concrete matrix and reduction of concrete pH progressively from the interior to the outer surfaces. These observations are interpreted using modern corrosion science. In summary, reinforcement corrosion initiation and progression can occur only with the gradual, long-term, loss by leaching of concrete alkalis and thus loss of concrete pH. In contrast, carbonates as formed by carbon dioxide are actually protective through blocking concrete pores and thereby slowing the rate of alkali leaching. A simple model for predicting the onset of reinforcement corrosion is proposed. The present observations indicate that provided the concrete matrix is of high quality, carbonation can act as a sink for atmospheric carbon dioxide without serious risk of reinforcement corrosion. Such absorption of atmospheric carbon dioxide is beneficial for reducing green house gas in the atmosphere. It is concluded that the often-quoted fear of reinforcement corrosion through 'carbonation' is based on poor experimental representations of actual concretes and their interpretation and is mis-placed.
Reference:
Melchers R.E., Richardson, P.J. (2023) Carbonation, neutralization and reinforcement corrosion for concrete in long-term atmospheric exposures, Corrosion, 79(4)395-404.