Is there a special mix of concrete that I have to use and is there an additive that I must add to prevent the concrete from freezing?
No mix is freeze-proof - and concrete has to be protected from cold weather (not just freezing temperatures) during the curing process,regardless of the mix, in order for it to obtain the ultimate "design" strengths.
Various admixtures may be used to improve "workability" or accelerate curing during a cold weather placement / finishing process. Talk to your local ready-mix supplier about what you're trying to accopmplish and they can make appropriate reccomendations. Also see http://www.askthebuilder.com/printer...eather.shtmlIf and http://www.gogenchem.com/calcium/Ind05.pdf