Drying Shrinkage

Summary

Drying Shrinkage - All concrete shrinks as the water in the concrete evaporates to the atmosphere.

Technical Information

The prediction of drying shrinkage is complicated. Concrete floors usually lose more water from the upper surface, resulting in non-uniform shrinkage and, potentially, curling. Any steps taken to reduce shrinkage will reduce curling.

Although curing is of great importance in achieving a durable concrete floor, it does not reduce shrinkage. A floor will eventually dry and shrink by an amount that is almost independent of when that drying begins.

The main factors influencing drying shrinkage are the volume of cement paste and its water content. Cement and water contents should be as low as possible, consistent with the specified maximum free-water/cement ratio and the practicalities of placing and finishing. The maximum water/cement ratio should be 0.55. The use of water-reducing admixtures is strongly recommended.

Although the cement paste is usually the only component of concrete that undergoes significant shrinkage, some aggregates are known to have high levels of drying and shrinkage. Aggregate shrinkage should be determined according to EN 1367-4.

The combined grading of the coarse and fine aggregates should be adjusted to minimise the water demand. The largest available size of aggregate should be used, consistent with the thickness of the slab. In practice this is a nominal maximum size of 20mm in the UK.