Oct 112013

Cold Weather and Concrete Foundations


A freshly placed concrete slab still wet on the top surface

The surrounding local temperature has a big effect on the setting time of concrete. The steps to take during cold weather are outlined below:

 Chemical Reaction

Hydration is the chemical reaction when cement and water are combined. This produces a strong binding medium for the aggregates (sand and stone) in concrete.
The hydration reaction is exothermic, and results in the generation of heat. The reaction and the amount of heat liberated determines how long the concrete will take to set.
This all depends on the composition and quality of the cement, as well as the surrounding conditions, especially temperature.

 What Effect Does Temperature Have

In colder temperatures, concrete is very likely to take a longer time to set because the hydration rate is slower. In these cases, the following points should be considered:
The timing of finishing the concrete, for instance power-floating of floor slabs, will be critical. When it is cold, the concrete will bleed (water will rise to the surface as the sand, stone and cement settle down) for a longer time. So, it will take longer for the concrete to harden before power-floating and other finishing operations can begin.
Cutting joints in slabs will also be affected due to a slower rate of strengthening, it may be necessary to postpone joint cutting to prevent stones from being worked loose from the concrete.
Removing shuttering should also be postponed.
Experience has shown that slower setting has no effect on the structural integrity of the concrete. In fact, a higher compressive strength is finally achieved from the extended hydration of the cement.

 Ways to improve early strength of concrete

To reduce or eradicate problems with the lack of early strength concrete in cold weather, consider following these tips:
Use of a higher grade concrete is highly recommended
Because of their slower setting and strength gain, the use of high percentages of extenders such as blast-furnace slag is not recommended
Insulating the structure or the concrete surface
Increasing the curing time prior to removal of formwork
The use of an accelerated admixture, where permissible


  3 Responses to “Concrete in Cold Weather”

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  1. […] Fresh concrete needs to be protected from drying out too quickly. Gently spray water over the foundation as soon as the concrete has set. In hot dry areas the concrete can be covered with plastic sheeting to stop the concrete drying too quickly. Remove the plastic and spray water on to the concrete every so often to prevent it drying too quickly. When concrete looses moisture too quickly either by evaporation or absorption by the surrounding soil, it shrinks and if the concrete is not malleable, it will crack. See our article on drying in cold weather. […]

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