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Delayed ettringite formation (DEF)
Delayed ettringite formation (DEF) is expansion and cracking of concrete associated with the delayed formation of the mineral ettringite which is a normal product of early cement hydration. DEF is a result of high early temperatures (above 70oC – 80oC) in the concrete which prevents the normal formation of ettringite.
DEF-induced damage is not a common phenomenon in concrete. Water or moisture from an external source is required for the reaction to occur, the availability of which will affect both the rate and the extent of expansion.
DEF has caused cracking in some heat cured precast concrete components (typically railway sleepers) several years after the concrete was produced. There is also concern that cracking may have occurred in larger in-situ concrete structures resulting from the build up of heat from the heat of hydration in the early life of the structure.
The principal effects of DEF are visible displacement and cracking. DEF can also increase the risk of secondary forms of deterioration such as freeze/thaw attack or reinforcement corrosion.
DEF can be prevented by limiting the internal concrete temperature to 70oC during its very early life. This can be achieved either by direct specification, or indirectly by limiting the cement content or specifying the use of low or very low heat cement.
Itīs presence can be detected using petrography.
See, also, Building Research Establishment, Delayed ettringite formation: in-situ concrete. Information Paper IP 11/01, 2001
Acknowledgement:
The Concrete Society
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TR71 Concrete petrography
