The guide

Concreting in cold weather

Cold is the costliest weather for a pour. Below the placing minimum the mix barely gains strength, and if a fresh pour freezes in its first day the damage is permanent. Here is what the temperature has to do, and how to keep a winter pour safe.

The limits at a glance

Placing minimum: ~5°C and falling (concrete & screed)
Mortar & render: ~3°C and falling
First 24 hours: must not freeze (permanent damage)

What temperature is too cold to pour concrete?

The working rule is don't place concrete or screed below about 5°C and falling. Hydration, the reaction that builds strength, slows right down as the mix cools and effectively stalls near freezing, so a slab placed into the cold sits weak for far longer than the schedule assumes.

Masonry mortar and cement render are more sensitive again. Don't lay them much below 3°C and falling, and protect the new work from frost overnight. This follows BS 8000-3 and NHBC guidance for cold-weather working.

The first 24 hours are what really matter

The real failure is freezing. Water expands as it turns to ice, and ice in a green mix breaks up the structure as it forms, so the strength never comes back. A pour that freezes in roughly its first 24 hoursusually means rip-out and re-pour. That is why CanIPour treats an overnight low at or below freezing as a no-go, and a low at or below 4°C as a caution: the air temperature at which ACI 306R-16 says cold-weather concreting precautions begin.

How to pour safely in the cold

If the job can't wait for milder weather, hold the pour above freezing until it has set: insulated curing blankets, a heated enclosure, or both. Warming the subbase before you pour helps, and a later finishing time gives the surface the warmest part of the day. Keep an eye on the forecast for the night after the pour, not just the pouring hours.

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