The European Standard for concrete EN 206 and its complementary British Standard BS 8500 requires that concrete must be supplied at a minimum temperature of 5°C at THE POINT OF DISCHARGE.
Careful consideration must be taken in respect of maintaining this temperature on site via insulation. This is not just by ensuring that adequate insulation material is available, but also by having extra labour to place, finish and insulate in as shorter time as possible.
Once placed; fresh/young concrete that is allowed to cool to freezing point or below will be permanently damaged and unfit for purpose. It must be noted that the inclusion of an air-entraining agent will not protect the concrete in its fresh condition, only when the concrete has hardened.
There are many methods of protecting the concrete such as a) straw blankets with tarpaulins, b) using timber formwork rather than steel, c) space heaters, d) several layers of hessian, e) quilted mattresses wrapped in plastic sheeting and f) ensuring that the concrete is not placed onto a frozen sub-base or against formwork or reinforcement which is covered with snow or ice.
Strength Development
Under no circumstances should concrete be allowed to fall below 5°C until it has reached a compressive cube strength of 5N/mm2. (Hanson may be able to provide assistance for you in measuring the strength).
At a temperature of 5°C, a C16/20 (or Gen3) concrete using CEMI will typically take 3 days and a C25/30, 2 days to achieve this strength. Furthermore, these concretes will only reach between 30-35% of their compressive strength at 7 days compared to 65-70% where the concrete temperature is 20°C.
Striking Times
Striking times of any formwork etc. will be greatly increased during periods of cold weather. This will depend upon the type of concrete used and how low the ambient temperature falls and for how long. Below is a comparison between the striking times for temperatures of 5 and 20°C:
Structure
5°C
20°C

Beams
3 days
1 day

Walls
3 days
1 day

Columns
3 days
1 day

Slabs
7 days
4 days


Pasted from <http://www.molevalleyfarmers.com/mvf/info/tools-hardware-and-maintenance/Cold_Weather_Concreting>

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