Concrete can be placed in sub-freezing temperatures, provided that the contractor takes appropriate steps to keep it from freezing before it sets up. With flatwork, that's hard to do. With a footing it's pretty easy (assuming the ground is not frozen and assuming the contractor is prepared ahead of time). A wall is harder because it has more surface area. The smaller the placement, the harder it is to keep warm. We placed footings once for a parking garage with temperatures in the teens. Pulling the blankets off 3 days later they were still steaming. Of course, they were 3' thick and 8' wide. One more thing. If someone is taking cylinders, they have to either be placed under the same blankets as the work itself or taken into a lab to cure. Don't even think about putting them into one of those little insulated plywood boxes on the job site. They'll freeze before they cure and crumble to pieces. Not that I would know from direct experience. [:-taped]