Placing Decorative Concrete in Cold Weather
Even when it's cold outside you can place decorative concrete successfully—if you heed these precautions
Concrete doesn't like the cold—but it is still possible to get it to behave if you understand what's going on.
First: what is cold to concrete and why is it a problem? ACI Committee 306, Cold Weather Concreting, (see also 306.1-90 Standard Specification for Cold Weather Concreting) for many years defined cold weather in a complicated way, with three consecutive days and average daily temperatures. But the new version of the committee's report (due out in early 2008), defines cold weather simply as any time "the air temperature has fallen to, or is expected to fall below, 40°F."
Speaking of ACI Committee 306's report, this is really the best reference for cold weather work, although it is a little formal and difficult to read. Nonetheless, I encourage you to get a copy, since it is likely that sooner or later you will run into a job that specifies ACI 306 as the requirements for cold weather work.