Concrete, or more correctly cement-water paste, shrinks as it dries out. This is because the very fine pores present in the paste contract as water is lost from the paste and expand again if water is regained at a later stage. If shrinkage occurs while the concrete is still workable and plastic it is called plastic shrinkage. This type of shrinkage is usually restrained by reinforcement, formwork and internal friction, and concrete cracking results. The typical crack pattern is one of random, non-continuous cracks, often parallel to each other. Cracks usually form within an hour of finishing the surface of the concrete.
This type of concrete cracking is often associated with concrete slabs, but it can occur on strip footings and any other concrete where the top surface is exposed to rapid drying conditions.
Causes of plastic shrinkage cracking
Plastic shrinkage cracking occurs on concrete surfaces exposed to high surface temperatures, strong wind or low humidity. Any environmental factor which increases the rate of evaporation of bleed water from the surface of the concrete increases the probability of concrete cracking. In essence, cracking is likely if the evaporation rate exceeds the bleeding rate.
Concrete mix design also influences the probability of cracking. Mixes designed to be cohesive with low bleeding capacity are particularly prone to cracking if the environmental conditions are conducive to it. For example, cracking of pump mixes is particularly prevalent in the Western Cape in summer, as is cracking of some mixes containing very finely ground cements or extenders. Retardation of set, from whatever cause, also increases the probability of concrete cracking.
The following preventive measures will help to reduce the occurrence of concrete cracking:
- Pouring the concrete during a cooler part of the day.
- Protecting the surface of the concrete from wind and sun with shade cloth and windbreaks.
- Liberally applying a good-quality curing compound immediately after the surface is finished.
- Protecting the surface with damp sand, hessian or plastic sheet immediately after the surface is finished.
- Using fog sprays to keep the surface of the concrete damp.
- Delaying the finishing of the surface in order to close up any cracks that may have occurred.
Generally speaking, this type of cracking is not serious from a structural point of view but the durability of the concrete can be affected if the cracks penetrate through the slab and the steel is exposed, in a potentially corrosive environment.
Remedial measures for repairing cracked concrete are normally restricted to filling the cracks with ‘cementitious’ slurry or, in some cases, with a low viscosity epoxy resin.