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Special care must be taken when concrete is cast in winter. If the water in the young concrete is allowed to freeze, the concrete will be damaged to such an extent that it is unfit for use. Even if the temperature does not drop below zero, the concrete will not gain strength as quickly as it would during the warmer months.
The now withdrawn design code BS 8110 required that the temperature of the concrete should not fall below 5C at any point until the strength has reached 5MPa. The workmanship clauses are now in BS EN 13670 Execution of concrete rather than in the design code and follow the same requirement. Surfaces that will be in contact with the newly placed concrete, including sub-grade materials, need to be a few degrees above freezing (2C). Reinforcing bars (and other embedded metal) must be free of ice and snow but otherwise there is generally no restriction on their temperature prior to casting. (The American Code for cold weather concreting ACI 306R suggests that embedded steel greater than about 25mm diameter should have a temperature of at least -12OC.) Water curing should not be applied in conditions where freezing of the concrete is anticipated.
Thus certain precautions have to be taken when concreting in cold weather. When planning to cast concrete, information on the likely temperature can be obtained from the Meteorological Office.
Some suppliers of ready-mixed concrete are able to supply heated concrete, which can be supplied to site at temperatures above 10oC even in the coldest weather. The concrete should be transported and placed as quickly as possible to avoid heat loss. All plant should be protected against frost, with smaller items being kept under cover when not in use.
BS EN 206:2013 + A1:2016 Clause 5.2.9 Concrete temperature says the ready-mixed concrete as delivered should not be below 5C at the time of delivery.
Concrete should not be placed against any formwork or reinforcement covered in ice or snow. Once the setting process starts, the heat of hydration will help to maintain the temperature of the concrete. Timber formwork usually offers sufficient insulation. However, steel formwork is a poor insulator and insulating material should be fixed to the back of the forms. Once cast, exposed concrete surfaces should be covered with insulating material.
It should be noted that the reduced temperature will lead to a slower rate of gain of strength of the concrete. Hence curing and striking times will have to be extended. This is particularly effects concretes with cements replacement materials e.g. ggbs and fly ash.
Acknowledgement: The Concrete Society
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