Concrete slabs and pavements need to be the right thickness for optimal strength and durability. If the slabs are too thin, they can begin to curl. This is the upward or downward bending of the edges of the slab due to differences in moisture between the top and bottom surfaces of the concrete.
This curling effect can lift the edges or the middle of the slab, leaving an unsupported portion of the concrete which can then crack and break when weight is applied. Slab edges can chip off when they curl up, or entire slabs can crack in half when the edges curl down. In most cases, curling can be seen in the early stages of curing, but the effect can also happen over longer periods of time.
Why does curling take place in concrete slabs?
As mentioned, concrete curling is mainly caused by changes in moisture and temperature on either side of the slab due to drying shrinkage. This problem is also more noticeable in thin slabs with no internal reinforcement. When one surface of the slab changes dimensions, the entire slab will curl on the shorter side – either lifting the edges up or pushing them down.
Curling of a slab soon after placement is caused by the rapid drying of one surface and poor curing. Excessive bleeding or too much surface moisture caused by poor curing procedures can cause drying shrinkage to occur on either the top or bottom surface of the slab. The dryness of the underlying substrate can also affect curing and curling.
Temperature differences between the top and bottom surfaces also affect curling. If the top surface is exposed to direct sunlight and high ambient temperatures, it could dry faster and shrink than the cooler, damp bottom surface. Alternatively, on a cold night, the bottom surface can contract when in contact with warm soil, causing the slab to curl downwards and lift the middle section off the ground.
How to minimise the curling of concrete slabs
The following building practices will help to reduce the chances of slab curling, especially in varying humidity and temperature conditions:
- Use the lowest practical water volume in the concrete mix.
- Use the largest practical size of aggregate possible to reduce the effects of drying shrinkage, especially with slabs.
- Avoid high cement content in the concrete mix.
- Try to avoid excessive bleeding by wetting the underlying soil before pouring the concrete.
- Cure the concrete thoroughly. Pay attention to joints and edges.
- Use a joint spacing of no more than 24 times the thickness of the slab.
- Increase the thickness of the slab if possible, especially at the edges.
- Internal reinforcements need to be placed properly at adequate intervals within the slab.
These tips will help to minimise the chances of a concrete slab curling. Moisture and temperature differentials are the main cause of this problem, but there are numerous other factors that need to be considered by contractors. The thickness of the slab also affects the chances of it curling, so contractors should avoid placing thin concrete layers.
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