It’s been bone chillingly cold recently, which brings about multiple feelings on my part. First, an increased affinity for parking myself in front of the fireplace, to bask in the warmth of a wood fire. Second, a keen sense of gratitude that I am no longer in the construction business trying to build things in the freezing cold.

Cold weather has a variety of effects on building materials. Stacks of lumber can stay frozen together even if daytime temperatures rise above freezing. Handling steel will result in numb fingers and hands, not to mention trying to manipulate nuts and bolts with Wookie paw sized gloves. But, nothing is as affected as much as concrete.

Concrete is one of the most widely used building materials. You’ll find it on almost every construction project, from that tiny home around the corner to a mega sized warehouse or chicken plant. Cold weather has such an impact on concrete that ACI thought necessary to publish an entire guide on the subject. ACI 306R is a short document outlining the appropriate steps to ensure the best possible outcome when utilizing a chemical reaction to take bulk materials and turn them into a stone like material with the shape and finish of your choosing.

The ideal temp for curing concrete is 70 degrees F. If the temp drops to 30 degrees, the curing time will be over three times longer for the same mix. Drop even further to 20 degrees and the concrete won’t cure at all. Oh it will set up….from freezing! Then when it thaws you’ll have a mess rather than that fabulous porch slab you hoped for.

ACI 306R defines cold weather as any period when the average daily temp falls below 40 degrees F for more than three days. Given the recent weather, central Virginia meets this criteria about now. What does that mean for the design engineer? Not much if we have our specs right and equip ourselves with some knowledge on the subject. Most concrete contractors haven’t heard of ACI 306R much less have ever read it. We should be prepared to answer the question, “Anything special I should do with the concrete this time of year?” Answer: “Admixtures, tenting, blankets and hot coffee for the finishers should suffice”

ACI 306R is available online. If you’ve never read through it, take a minute sometime to look it up and brief through it. It’ll give you the warm and fuzzies.

Troy Rudolph


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