The term cold joint is used on site for two different things. Both come down to two parts sitting against each other without being genuinely connected.
1. The dry joint in masonry, tiling and boarding
Here a cold joint is a cross joint in which bricks, blocks, boards or panels are set dry against one another, with no filling. It is also called a dry joint or a butt joint.
- Advantage — it is quicker, and some find it looks cleaner, for instance with large format tiles or with cladding panels where the pattern needs to run through.
- As a movement joint — a cold joint of 4 to 5 mm with no further treatment is sometimes used as a movement joint. Structurally that is the weaker solution: water can get in, and the joint edges chafe and chip as the parts move. A proper movement joint of around 10 mm with a flexible filler performs better.
| Cold joint of 4 to 5 mm, no treatment | Weaker solution: water can get in, and the joint edges chafe and chip as the parts move |
|---|---|
| Movement joint of around 10 mm with flexible filler | Performs better |
2. The cold joint in concrete
In concrete work, a cold joint is an unintended construction joint: fresh concrete is placed against concrete that has already hardened too far to knit with it. The concrete does not bond there, and a continuous plane of weakness is formed.
The consequences are concrete in every sense:
- Leakage — in basement walls and water-retaining structures, the cold joint is almost always where water comes through.
- Reduced capacity — the joint transfers no tension and very little shear.
- Visible in the face — in fair-faced concrete the line remains as a colour difference or a ridge.
How to prevent it
- Keep pouring within the open time. The next load has to go onto the previous one before it stiffens. Plan deliveries accordingly, and allow for warm weather, which shortens setting time considerably.
- Vibrate through the layer. When compacting, let the poker sink some way into the underlying, still plastic layer.
- Plan a proper construction joint where continuous pouring is not possible: clean and roughen the face, install a waterbar, injection hose or hydrophilic strip, and run the reinforcement through.
- Choose the joint position deliberately — in zones of low shear, not mid-span.
Related terms
- Cross joint
- Joint
- Construction joint
- Movement joint
- Injection hose
- Concrete mix
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