Influence of Fly ash on Concrete Carbonation and Rebar Corrosion

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Title: Influence of Fly ash on Concrete Carbonation and Rebar Corrosion

Author(s): C. Branca, R. Fratesi, G. Moriconi, and S. Simoncini

Publication: Special Publication

Volume: 132

Issue:

Appears on pages(s): 245-256

Keywords: calcium aluminates; calcium silicates; carbonation; corrosion; durability; fly ash; free lime; hydration; porosity; reinforcing steels; Materials Research

Date: 5/1/1992

Abstract:
It is well known that reduction of free lime in reinforced concrete by air carbon dioxide neutralization and calcium carbonate formation makes steel reinforcement more susceptible to corrosion, since the reduction of pH eliminates the passivity conditions and can promote iron corrosion. On the other hand, the addition of pozzolan also causes a reduction of free lime by producing calcium silicate and calcium aluminate hydrates. It may therefore seem that fly ash addition, which improves many aspects of concrete durability, can specifically reduce the iron protection from corrosion promoted by reduction of pH. Concretes containing different amounts of ordinary portland cement and fly ash were produced. Fly ash (an additional 20 percent by weight of cement), was used either to replace cement or as an additional ingredient without any cement reduction. Plain and reinforced specimens were manufactured. All the specimens were kept in a carbon dioxide enriched (30 percent by volume) room to accelerate the carbonation process. The carbonation depth on plain concrete specimens and electrochemical measurements on reinforced specimens were carried out. The results indicated that fly ash addition reduces the carbonation rate when used without cement reduction, whereas it accelerates the process when used to replace cement. The electrochemical measurements of electric potential and polarization resistance are not significantly affected by the presence of fly ash, especially when added without cement reduction. The visual observation of iron reinforcement indicated that fly ash addition does not substantially modify the corrosive aspects, even when it replaces cement.