Anti-carbonation measures

As discussed previously, there is ample evidence that normally specified cover thicknesses and qualities of ordinary structural concrete that comply with the relevant guidance in codes of practice can provide adequate protection to embedded reinforcing steel for working lives of 50 or 100 years in temperate environments where carbonation-induced corrosion is the sole concern (Hobbs and Matthews, 1998). Only in special cases therefore (e.g., where cover depths have to be restricted) should supplementary protection be necessary and, in such circumstances, anti-carbonation coatings of various types are often applied to concrete surfaces to improve their resistance to CO2 ingress (Robinson, 1987; Davies et al., 2002). Effective anti-carbonation coatings may be expected to exhibit a number of properties that enable them to exclude CO2 and water, adhere to the substrate, allow water vapour to escape and, in certain climates, resist thermal cycling, UV degradation, etc. A number of European Standards relevant to coatings for masonry and concrete have been undergoing development in recent years, two of the most significant with respect to anti- carbonation coatings being BS EN 1062-6 (2002) and BS EN 1504-2 (2004).

Several other supplementary measures against carbonation-induced corrosion are also sometimes advocated; for instance, zinc coatings, applied to reinforcing steel by hot dip galvanising, are considered effective for structures exposed to carbonation in the absence of significant chloride contamination (Treadaway et al., 1989; Bautista and Gonzalez, 1996). It is important when using galvanised steel reinforcement, however, to ensure that adequate coating thicknesses are specified in relation to the required service life, bearing in mind that, in routine processing, hot dip galvanising is said generally to result in thicknesses of at least 100 um  (Yeomans, 2002). Measurements of the corrosion rates of gal- vanised coatings in carbonated mortars exposed to certain moist environments have shown values can sometimes significantly exceed 1 m/year, particularly so when chlorides are present in the material (Gonzalez and Andrade, 1982), but it has been suggested that even 80 m thick galvanised coatings would typically be expected to last over 100 years in carbonated concrete that is free from chloride (Bertolini et al., 2004).