Although it may be tempting to pour concrete on frozen subgrade to save time on a project, this can lead to major issues down the road for a few primary reasons.
Some soils are susceptible to expansion when they freeze. Thus, when this frozen soil thaws it will cause settlement to occur. Settlement is one of the main causes of concrete cracking.
2.) Temperature Differences – Potential Concrete Freezing
Concrete is a warm material and generates it own heat in the first 24 hours after placement. If the concrete is poured over a frozen subgrade, the bottom of the concrete runs the risk of freezing from contact with the frozen subgrade. When concrete freezes early on during the curing period, it is permanently damaged and is unable to reach its designed strength.