Every builder, at some stage in their careers, has had to meet with a building owner and explain why a concrete floor has cracked. And it is always a tough discussion. Nobody wants the concrete to crack – not the owner, the builder, the placer or the concrete company – but it does happen.
Plastic settlement cracks
As the name suggests, this cracking occurs before the concrete has set (i.e. while it is still plastic – usually within 10 minutes and 3 hours of placing) and results from settlement of the concrete. It occurs if there is excessive concrete bleed. The cracks usually appear over the reinforcement, as the concrete settles. To prevent plastic settlement cracks the concrete should be designed to reduce the amount of bleeding. If the cracks occur very early, the concrete can be revibrated.
Plastic shrinkage cracks
These diagonal, random cracks also occur while the concrete is plastic – within 30 minutes and 6 hours of placing. They occur when strong winds and high temperatures combine to evaporate the bleed water faster than it can rise to the surface. Therefore, plastic shrinkage cracking is more likely to occur in concrete that has low bleed (in contrast to plastic settlement cracking above). To prevent plastic shrinkage cracking, anti-evaporation agents such as SikaFilm should be used during finishing, especially if there are high winds or it is a hot day, and curing should begin as soon as possible. Wind breaks can also be installed to keep the wind off the fresh concrete.
These unsightly surface cracks are a result of over trowelling and poor curing.
Drying shrinkage cracks
As concrete dries out over time it shrinks. This needs to be allowed for in the design, with properly positioned and detailed control joints. The amount of concrete shrinkage can be reduced by using superplasticisers to create a concrete with low water content that is still workable. In addition, by beginning curing of the concrete as early as possible (and if wet curing, by curing for a full 7 days) the concrete can be allowed to harden and gain strength before it starts to shrink, reducing the risk of cracking.