Concrete expansion joints explained

What are concrete expansion joints and why are they important? You’ll find everything you need to know in our easy-to-use guide.

What is a concrete expansion joint?

A concrete expansion joint – or control joint – is a gap which allows the concrete to expand and contract as/when the temperature changes. It forms a break between the concrete and other parts of a structure to allow movement without causing stress, which can lead to cracking. They should be used in large concrete slabs such as foundations and concrete driveways.

Why do you need concrete expansion joints?

All concrete will shrink slightly as it dries and, when it’s set, will expand or contract depending on the ambient temperature. To prevent cracks from forming, concrete expansion joints should be incorporated to allow for movement, particularly in slabs with a surface area exceeding 6m2.

Concrete expansion joints are particularly important where there have been consecutive concrete pours and are also useful when laying concrete within an area bordered by walls or buildings or if objects such as manhole covers need to be incorporated. If several construction joints are needed, it would be advisable for them to be designed and specified by a structural engineer. 

Installing concrete expansion joints

You can install concrete expansion joints before or after the concrete is laid. In the first instance, a flexible material is inserted along the length of the joint before the concrete is poured [link to how to lay a concrete slab]. Alternatively, once the concrete has set, grooves can be cut in the concrete, this will control where the concrete will crack leaving a neat saw cut at the surface and allow joint materials to be added where required, but care must be taken to ensure the correct depth is achieved (see below).

Tips for placing concrete expansion joints

  1. Place joints around 30 times the slab thickness apart. So, for a slab which is 100mm thick, the joints should be placed around 3,000mm (3 metres) apart.
  2. Make sure joints are cut deep enough: they need to be at least a quarter of the thickness of the slab. For a 100mm slab, cut the joints at least 25mm deep. 
  3. If you’re cutting joints after the concrete has been poured, don’t leave it too long. Concrete might crack if the joints aren’t cut within 12 hours after finishing.
  4. Placing joints under walls will mean they won’t be seen.
  5. Joints are most effective when the aspect ratio of the slab is kept to 1:1, for example 5m x 5m if the slab is a narrower than it is long it is possible to increase this to a maximum aspect ratio of 1.5 for example 2m x 3m, don’t leave the placing of concrete expansion joints to chance and enlist the services of a structural engineer if several are required. 

Find out more

Planning concrete expansion joints correctly is a complicated business and one that should be given careful consideration. Creating the right gaps, at the right spacing and filling them with the correct material is vital to protect the integrity of the concrete slab. If in doubt, call on the services of an expert structural engineer.

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