We are concerned that the cold weather (20-30 degrees) has made pouring concrete for a basement slab very difficult. our contractor says ha can mix anti-freeze in the concrete to keep it from freezing up. Is this a common practice and are there any concerns with this approach?

Many contractors use calcium in the concrete mix as an accelerator/antifreeze. The problem is that this will deteriorate any steel rebar or reinforcing wire. It may be OK if there is no steel in the slab and you use fibers added at the concrete plant as reinforcement for the slab. In general most engineers do allow the use of antifreeze additives in concrete. A better approach is to use concrete blankets or polyethylene and straw to cover the slab when it is finished to protect it from freezing for at least 48 hours. Concrete generates it's own heat when curing. If you insulate it from the cold it will not freeze. (This is the method I normally use.)

Glenn

It's not terribly hard to protect concrete from freezing with air entrainment and calcium and blankets. The more important concern is that the soils under it are not allowed to freeze before pouring. The moisture in the ground will give the soil a crystalline expansion and cretee poured over it will be on an unstable base, leading to cracks and settling unevenly