A contraction joint is a partial cut into a building element that allows shrinkage and expansion to take place. By weakening the section locally, you force the material to crack exactly there — neatly under the saw cut, where nobody sees it — instead of somewhere random in the slab.
So the joint itself does crack; that is the point. It is also called a control joint or an induced crack joint.
Contraction joint or expansion joint?
The two are often confused:
- Contraction joint — a partial cut. The concrete runs through underneath and can still transfer load. Its job is to control shrinkage.
- Expansion joint — a complete separation through the full section, including substrate or reinforcement. Its job is to decouple two elements entirely, for example at a change of foundation or along a long facade.
Forming them in concrete and screeds
- Cut depth — the common rule of thumb is at least a quarter of the slab or screed thickness; only then is the weakening enough to pull the crack to the joint.
- Timing. This is the mistake made most often. Cutting has to happen as soon as the concrete is hard enough not to ravel, but before shrinkage stress builds too far — in practice a window of roughly half a day to a day after placing, depending on temperature, wind and mix. Cut too late and the slab has already cracked elsewhere.
- Panel size — keep panels as square as possible. A long thin panel cracks down its length. A common rule of thumb is a panel length of around 24 to 30 times the slab thickness, with a length-to-width ratio no greater than about 1.5.
- In the sensible places. At doorways, re-entrant corners, columns and openings — that is where stress concentrates anyway.
- Filling. On a trafficked floor (garage, warehouse) the joint is filled with a hard joint filler once shrinkage has taken place, so the arrises do not break down.
Related terms
- Shrinkage
- Shrinkage crack
- Expansion joint
- Screed
- Shrinkage mesh
- Concrete floor
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