Plastic shrinkage cracks sometimes occur on the surface of freshly mixed concrete soon after it has been placed, while it is being finished or shortly thereafter. These cracks which appear mostly on horizontal surfaces can be substantially eliminated if preventive measures are taken.
CSA Standard A23.1 requires that when the rate of evaporation exceeds 1 kg/m²/hr, precautionary measures such as windscreens are required around all sides of concrete elements.
One or more of the following precautions listed can minimize the occurrence of plastic shrinkage cracking
- Dampen the subgrade and fog forms prior to placing concrete
- Erect temporary windbreaks to reduce wind velocity over the concrete surface.
- Erect temporary sunshades to reduce concrete surface temperatures.
- Protect the concrete with temporary coverings, such as polyethylene sheeting, during any appreciable delay between placing and finishing.
- Fog the slab immediately after placing and before finishing, and take care to prevent the accumulation of water that may reduce the quality of the cement paste on the slab surface.
- Add plastic fibres to the concrete mixture to help reduce plastic shrinkage crack formation.