I thought his question was answered by the others but basically the concrete company will know how much calcium chloride to add. Cover all cold weathers pours with an insulated blanket or at least a tarp and any heat from a heater will help. You would want to heat a big area like a floor but for footings in a basement where it won't get that cold, then I wouldn't worry about it especially if it hasn't frosted there. Go ahead and cover it. A layer of straw is a good insulator also.