Shrinkage cracking

Shrinkage cracking is cracking caused by the volume reduction that concrete undergoes as it loses water or as cement hydrates, when that contraction is restrained by the element's geometry, its supports or its own stiffer parts. Plastic shrinkage cracking occurs within hours, while the material is still soft and losing surface water faster than it can bleed. Drying shrinkage develops over weeks and months as the hardened element gives up pore water to the surrounding air.

In practice

Printable mortars are more prone to shrinkage than conventional concrete, because they contain a high proportion of binder and fine material and little or no coarse aggregate to restrain the paste. The printing process adds exposure: no formwork retains moisture and every surface is open from the start. Countermeasures are the ordinary ones, applied more carefully than usual: curing immediately after printing, limiting water content, using fibres to distribute cracking into finer and more numerous cracks, and detailing movement joints rather than assuming an element can shrink freely.

Related terms

• Curing — the main control on drying shrinkage.
• Printable mortar — binder-rich by necessity, and therefore shrinkage-prone.