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Concrete Stamps,
Cold Weather Stamping
Stamping concrete in cold weather requires careful preparation. Degussa Admixtures, Grace Construction Products and Fritz-Pak make accelerating admixtures specifically for cold weather. Using warm water in the mix can also be very helpful.
by John Strieder
Consider this: Some say a temperature drop of only 20 degrees Fahrenheit will double the set time of a typical slab of concrete. That’s motivation enough to make special preparations for stamping in the cold.
Freezing temperatures will slow the hydration process in a slab, extending the set time. On top of that, sunlight and rising daytime temperatures, along with wind, can cause the surface to dry faster than the colder concrete underneath, causing “surface crusting,” or splitting and cracking. The fact that stamping increases the surface area of a slab complicates things even further.
One way to get the bottom and top of a slab to set together is to draw bleed water to the surface. A pass with a wood bull float will do just that. It also won’t trap water like a finish achieved with a magnesium or metal float or trowel.
Contractors in cold-weather climates have amassed a few more tips of their own for stamping in cold weather — beyond the idea that it should be avoided whenever possible.
Brad Berg, co-owner of Architectural Enhancements Inc. in Shakopee, Minn., and a former concrete contractor, says his minimum temperature for stamping was about 40 degrees Fahrenheit. “The cold does change things,” he says. “Overlays that set up in a half hour in the summer can take all day. New pours do the same but not to the same extreme. You do not want to do it if there is a chance of freezing, and if you do you stamp at seven or eight in the morning, let the sun shine on it, then cover it with insulated blankets.”
Berg’s team also used accelerants, but not often, because they could affect the color, he says.
“One thing we would do is, with large torches, heat the ground. This brings the temperature up and traps the heat under the slab, allowing for more cure time at warmer temperatures. Blanketing the ground where you’re preparing and around the surface ahead of time also helps.
“Depending on the project, you can tent it and run heaters. It all depends on the scope of the project.”
John Buteyn, technical manager with Colorado Hardscapes Inc. in Denver, Colo., says his firm will stamp in 30-degree weather.
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