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Shrinkage-reducing admixtures

Shrinkage-reducing admixtures added during batching can significantly decrease both the early and long-term drying shrinkage. This is achieved by treating the cause of drying shrinkage within the capillaries and pores of the cement paste. Such admixtures should not be confused with shrinkage-compensating materials that are normally added at more than 5% by mass of cement to create an expansive reaction within the cement paste to counteract the effects of drying shrinkage.

Shrinkage-reducing admixtures can be used where shrinkage cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable. Where new concrete is used to strengthen or repair structures, shrinkage-reducing admixtures can lower the risk of cracking in what can be a highly restrained environment.

The workability of concrete containing shrinkage-reducing admixtures will not be adversely affected. However, the volume of liquid admixture must be included as part of the mixing water. These admixtures can sometimes reduce both early and late strength development. At recommended dosages, a maximum strength loss of 12 to 15% at 28 days could be expected.

Shrinkage-reducing admixtures do not adversely affect long-term durability and may improve it by reducing cracks and joints that can cause failure in floors and other structures.

Note: Not all admixture types are covered by EN 934. BS 8443:2005 Specification for establishing the suitability of special purpose concrete admixtures,  covers the following; underwater concrete admixtures, shrinkage reducing admixtures, corrosion inhibiting admixtures, pumping aids, segregation reducing admixtures, foaming admixtures, semi-dry concrete admixtures.

Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete