If you intend to pour concrete in an area where the temperature regularly drops below zero or freezing, then you will need to use a special concrete additive in your concrete mix to ensure that the concrete does not fail prematurely. Since concrete is porous it will absorb and retain water. If this water is still in retention in the concrete when the temperature drops below freezing for example at night, the water trapped inside the concrete will turn to ice which causes it to expand. This expansion will crack, spall or break the concrete.
There is no suitable treatment for concrete once it has already cured to protect it from freeze and thaw conditions. The only method to protect the concrete is to preemptively know that the concrete will be exposed to sub zero temperatures regularly, and thereby add a special admixture to the concrete. This special admixture is called air entrainer.
Similar to a mild detergent soap, air entrainer adds tiny bubbles into the concrete mix. These tiny bubbles stay until the concrete hardens leaving small pockets of air in the finished concrete product. These small pockets of air are what ultimately protect the concrete from the damaging effects of freeze and thaw conditions.
The small pockets in the finished concrete allow space for water to expand into when it freezes. This prevents the force of expansion from pushing outwards on the concrete itself. The amount of air entrainer admixture that you need to use in your mix is actually very small. A single teaspoon of air entrainer admixture powder would be enough to freeze protect 5 gallons of mixed concrete which is equivalent to approximately 100 pounds of finished product.
Be sure to not add too much air entrainer product to your concrete mix as this does marginally reduce the finish strength of the mix. It is also worth noting that air entrainer admixture changes the finishing properties of the concrete. The cement becomes slightly smoother and less inclined to stick to your trowel making it easier to trowel than other types of concrete.
Instead of adding air entraining admixture directly to your mix you can also source and use air entrained portland cement. This is portland cement that has already been freeze protected. This can significantly increase the consistency of your freeze protected concrete from one batch to the next. Adding air entraining admixture individually in each batch can create variance between the batches.
Part of the problem with concrete that fails in freezing temperatures is the fact that more water than required is used when mixing for most applications. Most applications require very little water in order to act as the catalyst for the concrete to harden. The amount of water required to harden concrete is similar to that of sand that is slightly damp to the touch.
Any water in excess of this amount is used solely for the purpose of placing and finishing the concrete. Wet concrete is easier to place and finish and requires less physical effort than a dry mix. The problem is that only a small amount of the water used in the mix will be consumed in the hardening process. The remainder of the water will not be used and will eventually evaporate leaving empty pockets for environmental water to collect. It is this environmental water that is likely to freeze and fracture the concrete in the future. By using air entrainer in your concrete mix then small empty pockets exist evenly throughout the entirety of the concrete and thus it will not experience undue pressure from the water expanding into ice.
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