Sun, wind, low humidity and warm concrete pull water off the surface quickly, the top shrinks while still plastic, and short, roughly parallel cracks open, from a few inches to a few feet long. Low-bleed mixes such as silica fume and high-SCM blends are most at risk. Windbreaks, fogging, evaporation retarder and early curing prevent it.
Wind came up at noon and we got plastic shrinkage cracking across the east end before the evap was on.
Also heard as
- plastic cracking
- plastic shrinkage cracks
Related terms
Evaporation retarder
A film-forming liquid sprayed on fresh concrete to slow moisture loss between finishing passes.
Silica fume
Ultrafine silica dust from silicon smelting that makes concrete much denser, stronger and less permeable.
Curing
Keeping fresh concrete moist and warm enough, for long enough, that the cement keeps hydrating.
Drying shrinkage
The volume loss of hardened concrete as it gives up moisture over months and years.
Crazing
A fine, shallow network of hairline cracks on a concrete surface, often in a rough hexagon pattern.