it's not that it needs to be heated, curing concrete creates heat on its own. The issue is that while concrete cures, it needs to be kept wet. The water used to keep it wet needs to be kept from freezing. That's why you temper a space that is receiving a pour. Admixtures help too. but yea, they pour in pretty cold temps in MPLS, they can do it in Chicago too. I have worked a lot of construction in my life and from personal experience you can pour concrete in some really crappy weather conditions, including very cold. It probably needs to be covered to trap in the heat while curing also.