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There are two main problems with placing concrete in cold weather:
Concrete can freeze before it gains strength which breaks up the matrix
Concrete sets more slowly when it is cold—very slow below 10°C; below 4°C the hydration reaction basically stops and the concrete doesn't gain strength
But these are concrete temperatures not air temperatures. So when it is cold, we need to protect the concrete until it can handle the cold on its own. The general rule is that once the concrete has gained strength to about 3N/mm2 then it's OK.
Steps You Can Take
To help concrete reach 3N/mm2 strength, there are two things that can be done in cold weather:
Change the concrete mix to get it to set more quickly
Remove the WRA from the mix to speed up the setting
Increase the strength, meaning 3N/mm2 will be achieved faster
Add accelerator to speed up the setting of the concrete
Protect the concrete from the cold
Leave formwork in place longer
Cover concrete slabs with insulating material
The severity of weather determines generally which precautions should be taken.
Cold Temperatures Below 5oC But No Frost
Where the ambient temperature is low, but does not drop below freezing, there is minimal risk of damage to the concrete. It will just take longer to gain strength. Formwork should therefore be left in place for longer until sufficient strength in the concrete is achieved. Finishing of the concrete will take longer as the concrete will take longer to set.
Minor Frost at Night
Fresh concrete must be prevented from freezing, so it should be protected as soon as practical. Typical protection includes frost blankets or insulated formwork. Timber formwork itself often offers sufficient insulation for minor frost. The exposed surface should be cover with insulating material. Slabs would be covered with frost blankets creating and insulating air space over the concrete.
Severe Frost Day and Night
Where the temperatures do not rise above 3oC and continue to rise, concrete work should not take place unless precautions can be taken to ensure the concrete temperature is kept above freezing.
See The Concrete Society website for further information: www.concrete.org.uk
Please contact the RTU technical team on 028 9085 1441 and press 2 for sales for any more advice