You may hear a sigh of relief coming from many residential contractors that the bitter cold of the 2007-2008 winter is gone. Many, however, have come to prefer these site conditions for placing quality concrete foundations. They have been a part of four years of successful operation using the Cold Weather Research Report, produced from 2002 to 2004 by the Concrete Foundations Association (CFA), as a basis for proving that cold, and even frozen, concrete can be the best concrete.
Cold weather concrete work does require different approaches and special considerations depending on the severity of the cold. Armed with the information generated through research specific to residential foundations, contractors have learned that concrete management is the key to long-term success.
Cold weather, as defined by ACI 116R, is "a period in which for more than three successive days the average daily outdoor temperature drops below 40° F."
CFA research was conducted to determine the impact of that very definition on residential concrete foundation walls as well as the impact to walls that freeze with standard protection applied. In the end, this research considered 44 different concrete mixes with a range of strengths and admixtures representing economical to aggressive designs. More than 800 cylinders were cast and tested in chilled and frozen conditions and later, more than a dozen walls were cast to put the cylinder data to the test. These walls were cored for strength, petrographic, and freeze/thaw studies. The results of this extensive three-year test program can be summarized by these statements:
Use an appropriate mix. The CFA offers 44 standard mix designs, but it's wise for local ready-mix suppliers to do their own maturity testing of their own batches, so that differences in cement or aggregates can be taken into account.
Be prepared to cover walls with blankets. If the ambient temperature is above 30° F, you don't need to blanket anything. (Water in fresh concrete freezes at a temperature slightly below the freezing point of pure water.) Between 20° F and 30° F, covering all or part of the wall is a judgment call, based on knowledge of the mix's behavior. However, covering the whole wall can't hurt, and it's good insurance.
Investigate using concrete maturity for your mix designs. The most success in this research, and for the contractors that have used the data since, has come from using inexpensive maturity systems to validate the performance of a mix at varying temperatures. This procedure monitors concrete temperature and predicts when the critical strength gain has been attained prior to freezing.
Do not add water to the mix onsite. Maturity testing data is only valid for the mix as designed, and additional water will change the mix properties. If a contractor needs to use extra water onsite, the maturity curves should be recreated first, using the actual water-cement ratio the concrete will have when placed.
Monitor concrete delivery temperatures. Concrete should be placed at a minimum temperature of 60° F. At the end of a cold day, ready-mix plants may have a hard time keeping the water and aggregate hot enough—sometimes their boilers just can't keep up. And a five degree difference in delivery temperature can be the difference between freezing or not freezing that concrete overnight. Finally, a frozen wall is not a bad wall—it may be your best wall
The critical step is making sure early-age strength of at least 500 psi has been attained. Maturity is the best way to determine this based on variability in the ambient temperature and the mix design. Once a wall reaches 500 psi, one freeze cycle will only stabilize a wall until curing temperatures are reached and strength gain resumes.