How do I determine when the temperature will be safe to pour concrete without additives?
Since you're in Gerogia you might not have this readily available, but in Minnesota we use hydronic heaters and insulated blankets to both thaw frozen ground if needed and to help concrete cure for a few days in below freezing weather. A hydronic heater is a a portable pump that forces heated water through flexible hoses laid over the concrete.

To answer your question, no you won't need to heat curing concrete if the temperature stays above freezing. Just let it cure a few extra days, or wait the full 28 days if you're extremely concerned.

Concrete that cures at a lower temperature is actually stronger after 28 days than concrete that cures at a higher temperature (assuming it doesn't freeze!) Source: Role of Concrete Curing