Carbonation resistance of fly ash concrete

Based on systematic analysis, evaluation and synthesis of a 30 000 strong data matrix generated from 213 studies from 33 countries published since 1968, this paper details the extent of research that has been undertaken and discusses the effect of fly ash (FA) on the carbonation and carbonation-induced corrosion of concrete. It is shown that FA as a cement component, such as those adopted in BS EN 197-1:2011, increases the carbonation rate of concrete, both when concrete is designed in terms of equal water/cement ratio or strength, though with the latter to a lesser extent. This increase in carbonation has also been confirmed for in-service concrete. The net effect of FA content on the carbonation of concrete is dependent upon the combination of mix design, curing and exposure related parameters. FA in concrete is also shown to increase the corrosion of reinforcement, which can only be overcome by increasing the cover to reinforcement, concrete strength, or a combination of the two, beyond those specified in standards such Eurocode 2, BS EN 206-1:2013 and BS 8500. Contrary to the commonly held view, this study shows that the relative rate of carbonation of FA concrete with reference to corresponding Portland cement concrete remains similar under accelerated and natural carbon dioxide exposures.