Two common types of concrete cracking are plastic shrinkage cracking and plastic settlement cracking.
Both types of cracking are related to bleeding of the plastic concrete, but in different ways. This tip covers the causes of plastic settlement cracking, prevention of cracking and remedial measures.
Plastic Shrinkage Cracking
Typical plastic shrinkage cracking on a suspended slab
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. 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 wind breaks
- 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.
Plastic Settlement Cracking
After concrete has been compacted, but before it sets, the solid phase of the mix tends to settle and displace water upwards. This is known as bleeding. The amount of bleeding is a function of the cohesiveness or “stickiness” of the mix. The use of single sized sands, such as dune sands, can cause severe bleeding. The results of severe bleeding include bleed channels in the paste, voids under reinforcement and coarse aggregate particles, sand streaking of off-shutter surfaces and plastic settlement cracking.
Settlement cracks occur when the settlement of the concrete is restrained, either by the formwork, the reinforcement or, sometimes, void formers. The picture below is typical of plastic settlement cracking in a column where the settlement had been restrained by the stirrups.
When the cracks are caused by restraint by reinforcement, the crack pattern invariably follows the underlying steel and the cracks extend to the steel. In addition the concrete settles away from the steel which impairs bond. This is obviously undesirable, especially in severe environmental exposure conditions.
Preventive measures include redesigning the concrete mix to increase cohesion and the use of re-vibration of the concrete.
Cohesion can be increased by blending in finer sand, or sand containing more fines, or by using an air-entraining agent, or by using a finely divided extender.
Alternatively it is often possible to re-vibrate the concrete before it sets in order to close up any cracks.
In cases where potential loss of durability is important, remedial measures consist of injecting cracks with low viscosity epoxy resins.