Pouring concrete when the temperatures are below ideal may require an elevated level of protection during the early curing stage.  If the air temperature is going to be right around the freezing mark within 48 hours after pouring the concrete, simply covering the concrete with plastic would keep some of the ground heat from escaping.  In colder temperatures, the use of curing blankets, or insulated blankets, does a great job of trapping this heat from the ground.  If frost must be removed by mechanical means, these blankets are one of the best ways to keep the heat where it’s needed before and after the pour.