Shrinkage is the dimensional change in which a building material gets smaller, usually because it loses water. Almost every wet-applied or hygroscopic material shrinks: concrete, mortar, plaster, screeds, timber, and to a lesser extent plastics as they cool.
As long as a material can shrink freely, little happens. The problem arises when shrinkage is restrained — by reinforcement, by bond to the substrate, or because the element is held at both ends. Tensile stress then builds up, and because concrete and mortar have low tensile strength, they crack.
Shrinkage by material
- Concrete — early shrinkage as it hardens, then drying shrinkage that continues for months or years. The ultimate drying shrinkage of ordinary structural concrete is in the order of a few tenths of a millimetre per metre. More water in the mix means more shrinkage, so a low water-cement ratio is the first measure.
- Cement screed — shrinks more than structural concrete, and most strongly in the first weeks. That is why a screed must not be tiled too soon: if it is, the shrinking screed pulls the tiles loose or the joint cracks with it.
- Timber — shrinks mainly across the grain. Tangentially (along the growth rings) the movement is around twice the radial figure; along the grain it is negligible. That is why a board cups and a beam splits, but does not get shorter.
- Masonry — calcium silicate and concrete blocks shrink, while clay bricks expand slightly and permanently after firing. The two should not simply be combined in one continuous panel.
Preventing shrinkage damage
- Do not work too wet. Every surplus litre of mixing water is future shrinkage.
- Cure properly. Cover or damp down concrete and screeds so the surface does not dry faster than the core. It is the cheapest measure with the biggest effect.
- Direct the shrinkage with joints. Contraction joints (sawn or formed) decide where the material cracks.
- Use shrinkage reinforcement. Reinforcement does not prevent cracking, but spreads it over many fine cracks instead of one wide one.
- Fix timber at the right moisture content. Around 8 to 12 per cent indoors, higher outside. Let material acclimatise before fixing.
Related terms
- Concrete shrinkage
- Shrinkage crack
- Contraction joint
- Shrinkage mesh
- Screed
- Water-cement ratio
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