Sulfate attack is one of the most widespread causes of concrete deterioration and can develop in many types of structures both above and below the ground. There are several types of sulfate attack, the most frequently encountered of which is surface sulfate attack whereby a concrete surface is exposed to high levels of sulfate, usually derived from mobile groundwater. Other types of sulfate attack include internal sulfate attack, an example of which would be delayed ettringite formation (DEF).
Sulfate attack in concrete exposed to externally derived sulfates generally causes concrete deterioration due to the expansive growth of ettringite or thaumasite within a concrete surface, often leading to the formation of surface-parallel cracks infilled with sulfate reaction products. Ettringite is formed by chemical reactions between moisture containing sulfates and the calcium and aluminium present in the cement hydrate compounds in cement paste. Thaumasite forms in situations where there is a source of both sulfate and carbonate. Low temperatures of <15oC tend to promote thaumasite formation. The carbonate required for thaumasite formation can be externally derived from groundwater containing carbonate ions or from carbonate-rich aggregates such as limestone.
Delayed ettringite formation, a form of internal sulfate attack occurs in concrete cured at elevated temperatures in excess of about 60-65oC and can develop both in precast concrete and also in large volume concrete pours where the heat generated during cement hydration is sufficient to raise the heat of hydration to high levels. High curing temperatures in concrete lead to the formation of a meta-stable mono-sulfate compound and subsequent exposure of high temperature-cured concrete to moisture causes the meta-stable mono-sulfate to convert to ettringite, a process that often causes disruptive expansion.
Sandberg has substantial experience in investigating sulfate attack in many types of concrete structures, including foundation structures and marine structures. Sandberg was extensively involved in the investigation of severe thaumasite sulfate attack in foundation concrete on the M5 motorway and contributed to the drafting of the Report of the Thaumasite Expert Group.
Sandberg has vast experience in concrete investigation and our UKAS accredited laboratories can provide a wide range of services to assess concrete condition.
Sandberg offers a comprehensive package of site investigation and laboratory techniques to appraise this newly appreciated mechanism. Laboratory examination includes petrographic and other analytical techniques, including scanning electron microscopy (SEM). X-ray microanalysis and X-ray diffraction (XRD) facilities are also available where necessary.
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