Early planning for potential cold weather conditions is an easy, inexpensive first step.  If you have a low cement content in your mix design, due to fly ash or other admixtures, consider submitting an alternate mix that utilizes more cement, to reduce curing time.  If you can submit a mix specifically for cold temperatures, you may also consider accelerating admixtures or Type III high-early cement.  There are new admixtures being developed every year, so talk to your suppliers to keep abreast of the latest advances. 

 

Whatever your approach, the emphasis should be on minimizing the duration that the concrete needs to be protected from freezing.  In addition, addressing material issues early provides assurance to the engineer or architect that you’re taking a proactive approach toward potential obstacles.

 

Onsite, as temperatures decrease, it’s a good idea to protect your subgrade from both freezing and the accumulation of ice and snow.  Even during acceptable ambient temperatures, frozen soils present a problem.  Very cold or frozen subgrades will slow the set of your concrete, and, in some cases, the thaw that is brought on by the heat of the concrete will introduce additional water to the mix, altering your water/cement ratio.  In addition, soils that have frozen after compaction will be less stable upon thawing, creating potential instabilities that can result in cracking or settling problems down the road. 

 

Often, a precaution as simple as poly sheeting, spread over the subgrade, will offer protection from freezing.  As conditions worsen, you may consider insulated blankets, or even spreading straw over the poly, as additional insulation.  In extreme conditions, there are heated blankets and other systems that can warm or even thaw frozen sites. 

 

Check back next time, when I’ll talk about pour day, as well as precautions and protection that can be provided in the hours, days and weeks following placement.