I'm not sure about the USA, but up here in Canada most plants have boilers and run all year. Heated concrete starts on November 15 every year. The producer will add varying percentages of hot water to the mix (depending on aggregate and ambient temperatures).
Ground temperature should never be below freezing or concrete can freeze on contact. The recommended minimum ground temperature is 40 F (5C). Concrete will produce heat by the process of hydration, but this process is severely retarded with lowered temperatures. The concrete will quickly lose heat and freeze at the surface if subjected to freezing air temperatures at an early age. You can protect the concrete from freezing with insulated tarps, straw or heaters.
DO NOT use heaters which exhaust directly into the heat source such as many salamanders. The concrete reacts with the carbon dioxide in the exhaust and will cause dusting of the surface by a process called carbonation. Most contractors in my area use indirect fired heaters (exhausted outdoors).
Canadian standards recommend concrete be protected from temperatures of 50F (10C) or below until it reaches 2500psi (17.5Mpa), which usually occurs within three to four days.
There are a number of set accelerators and concrete anti freezes that can be used for cold weather concreting.
