I experience cold weather concrete for 5 out of 12 months a year. Being from Maine our cold weather concreting season is from late October until sometime in March.
Having to make a living pouring concrete floors during Maine winters since the 1980's, I've learned a lot about what happens to fresh concrete if it's exposed to cold and freezing temperatures.
One thing I know for sure, if you let newly poured concrete freeze, you will have issues with the surface scaling, pop-outs, cracking, and overall weaker concrete.
We covered the slab above with insulating blankets, a layer of hay, and a tarp over everything after we were done troweling. The air temperatures for the next four days were all below freezing but the slab temperature under the blankets four days after the pour was 70 degrees F.
I've had concrete floors where the entire surface has peeled off (top 1/8") because the general contractor refused to protect and insulate the concrete after we troweled it and the temperature dropped into the 20's overnight.
FIRST LET'S STATE THE OBVIOUS
NEVER pour on frozen ground, Never! Sorry, just wanted to make that clear.
In cold weather, especially temperatures below freezing, the concrete mix should be batched with warm or hot water.
Generally speaking, water temps between 120 - 140 degrees F will give you a concrete temperature around 60 degrees after it's mixed in the concrete truck.
I'm speaking from my own experience actually checking the temperature of concrete mixtures when the concrete truck shows up on my jobsite.
Forms, rebar, and embedded bond-outs should all be clear of snow and ice before pouring or you'll have voids in those places.
After the concrete is placed, building enclosures, portable heaters, insulated forms and blankets should be ready to maintain the concrete temperature.
If the air temperatures are in the 30's F or below and the ready-mix company isn't using warm/hot water to batch the concrete, I would seriously think twice about pouring concrete that day.
Once the dry ingredients (cement, sand, aggregate) are mixed with water a chemical process takes place between the cement and water where they form a paste.
The heat generated from this process is called heat of hydration.
The amount of heat generated is affected by:
Heat of hydration is very important when pouring concrete in the winter.
Often the heat from hydration along with covering the concrete with curing or insulating blankets is enough to cure the concrete and keep it from freezing.
In most cases, exterior cold weather concreting should include an air-entraining admixture.
Entrained air is most important when concrete is placed in freezing weather.
Why, because air entrainment provides concrete the capacity to absorb stresses due to ice formation within the mix.
Here's my simple explanation of air-entrainment in concrete!
Picture what Dawn dish detergent does when you put it in the water to wash dishes. It bubbles up like crazy, right.
Those bubbles are kind of what air-entrainment is in concrete.
Thousands of microscopic air bubbles in the mix that allow concrete to absorb water and give the water room to expand when it freezes.
Since there's voids for the water to expand, there's usually little or no damage done to the concrete.
You know there's a pretty good chance new concrete placed in the winter will see some rain, snow, or ice. Without any air entrainment, if the concrete freezes after being saturated with some form of water, it will lose strength.
The results might not be seen until the concrete thaws, but will likely
lead to some form of scaling or pitting on the surface.
Whether your pouring concrete footings, floors, or slabs, the ground can't be frozen.
You'll have to protect the sub-grade with insulating blankets, hay, or tent the area and heat it before the pour.
A cold sub-grade will suck the heat right out of the concrete, retarding the set time dramatically.
If you tent and heat the area with a direct-fired heater like a kerosene torpedo heater, you must vent the heater to the outside air. This type of heater produces CO2 (carbon dioxide) that will combine with calcium hydroxide on the surface of fresh concrete to form a weak layer of calcium carbonate that interferes with cement hydration.
In short, just make sure fresh air is coming in through the back of the heater.
Plan, well in advance, on how you're going to protect the concrete after the pour. Have everything you'll need, blankets, hay, tarps, heaters on site and ready as soon as you need to cover the concrete.
The sooner you protect the concrete from freezing the more heat from hydration you'll save.
When pouring on some type of decking it's a good idea to enclose the area below the pour and heat the underside of the metal or wood decking.
Heat the underside area at least a day in advance to warm the decking and melt any frost, snow, or ice on the deck above.
After the concrete is finish troweled, lay some insulating blankets on top of the slab to keep it from freezing. Make sure to weigh the blankets down so the wind doesn't blow them off, 2x4's or rebar works good.
Take a look at my 9 things to consider before pouring concrete in cold weather.
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That's us Pumping a slab!