laboratory and Field Examinations of Ettringite Formation in Pavement Concrete

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Title: laboratory and Field Examinations of Ettringite Formation in Pavement Concrete

Author(s): J. Stark and K. Bollmann

Publication: Special Publication

Volume: 177

Issue:

Appears on pages(s): 183-198

Keywords: air voids; concrete pavements; durability; ettringite; expansion; moisture

Date: 1/1/1999

Abstract:
The formation of ettringite in hardened concrete is not only a problem of heat treatment. Ettringite also occurs in no heat-treated concrete, which is exposed only to normal climatic conditions. In some cases the mechanism of damage in concrete pavements correlates with this ettringite formation in the hardened concrete. Structural changes by ettringite formation were caused above all by varying moisture conditions and, as a result, by transportation of moisture and substances within the concrete structure, which also lowers the pH value of the pore solution. The primary ettringite from the paste is microcrystallin at normal pH of 13.5 to 14 in the pore liquid. Thus ettringite may dissolve in the pore liquid and recrystalize at a lower pH in larger spaces, where the capillary transportation is interrupted. This recrystallized ettringite in the air voids was stable up to 60°C. But the mechanism of this ettringite formation is supported and accelerated by higher temperatures (e.g. 60°C) because of the intensive drying. Microstuctural defects like microcracks may be created by alternating temperatures and later on filled and may be widened by ettringite crystals. In concrete pavements no indications were found for recrystallized ettringite itself to be the primary cause of crack formation. The expansion of concrete is reduced by introducing artificial air voids, because there is more available space for accumulation of ettringite. But the combined action of freezing and thawing and de-icing salt after filling the artificially entrained air voids with ettringite crystals may causedamages.