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Cracks caused by excessive drying from exposed surfaces Plastic shrinkage cracks can form in young concrete, within the first few hours after placing. At this time the concrete is plastic and has little strength. Water can move relatively freely and tends to move upwards towards the surface, a phenomenon known as bleeding.
Evaporation of water occurs at the surface, and is more rapid at high temperatures and/or low humidity, particularly in windy weather. If evaporation occurs at a faster rate than bleeding, there is a net loss of water from the surface layer of concrete, leading to a net reduction in volume. The surface layer of concrete tries to shrink but is restrained by underlying layers which are not subject to the same reduction in volume. The result of the restraint is that tensile stresses develop in the surface layer and cracks form.
As the concrete has very low strength at this stage, the pattern of plastic shrinkage cracks is usually random but may be influenced by the direction in which finishing operations have been carried out. The cracks tend to be 1-2 mm wide, 300-500 mm long and 20-50 mm deep, though, in some circumstances, they may extend through the full depth of a member.
The occurrence of plastic shrinkage cracks can be significantly reduced by adequate curing.
Acknowledgement: Concrete Society
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