Contractors working in the northern United States and Canada have three words of advice when it comes to stamping concrete in cold weather: Wait until Spring.
But that’s not always possible. While residential customers can often be persuaded to wait for more favorable conditions, some commercial projects must get done no matter how bad the weather gets.
Considerations for winter stamping include not only temperature and humidity, but the condition of the subgrade, the cementitious content of the concrete, different set times, and freeze-thaw cycles after the job is complete.
Ian Paine, marketing director for Lafarge North America, says it’s not unusual to do flatwork in cold temperatures as early as September in Canada. He suggests covering the subgrade with an insulated tarp, or at least a straw and tarp combination, to keep the moisture out ahead of time. A cold subgrade will pull the heat out of concrete, so one of the best things to do is to use content that is more cementitious, which creates more internal heat and helps the setting process, he says. Use high-quality concrete such as Lafarge’s Artevia for decorative work, he says. “Don’t go on the cheap. Any quality issues that you have will affect the growth of the decorative concrete industry in your area and certainly your own business growth.”
Lafarge’s Weathermix can extend the construction season, he says, because it is formulated to withstand a wide range of cold weather and some subfreezing ambient temperatures. And it speeds up set times, which are seemingly interminable during cold weather.
Paine also suggests breaking up a big job. For example, split a 2,000-square-foot pour into four smaller jobs, so if one section doesn’t set properly, the entire project isn’t wrecked. “Be smart about it,” he says. “Don’t let the customer push you into doing the whole thing at once. It’s too risky.”
As with any flatwork job, knowing the weather conditions of the area is critical. “Cold weather often comes with high humidity — the Great Lakes area — or in the West, say Denver, it comes with low humidity, so you can have shrinkage cracking. Protect against that and make sure it doesn’t craze on you.”
Also, be aware of freeze-thaw cycles, Paine says. If the ground freezes and thaws during the 28-day curing time, that’s pretty tough on the concrete. “There, the only approach is to make sure you get it done as early as you can.”
Because new concrete loses heat and moisture fast in cold weather, heated insulating blankets can keep the concrete moist and the temperature above 50 degrees for proper curing. For the concrete to keep gaining strength, it needs to stay at 50 degrees or higher. According to the American Concrete Institute, concrete must be protected against early freezing until it has attained at least 500 psi.
Customer satisfaction is critically important when you’re dealing with decorative concrete and the look and quality of the finished product is weighted above the schedule, Paine says. Cold-weather conditions increase your risk. “If you do force people to wait for the correct conditions, you’re much better off in the long run,” he says.
For Dale Mizer, there was no talking Cleveland State University out of a huge stamped sidewalk project planned for the 2004 winter break, where freezing rains, snowstorms, gusty winds and subzero temperatures presented a horror show for contractors.
Mizer, a representative for the project’s decorative concrete supplier, Chas. E. Phipps Co., worked closely with the concrete subcontractor, Union Industrial Contractors. Because concrete cannot be placed on frozen subgrade, it had to be thawed first using a heated tubing system. Workers also needed to erect a movable shelter to stamp 300 square feet of sidewalk at a time. Shelters can be made simply with PVC piping and plastic sheeting — the key is to keep the weather off the project and the workers. Also, Mizer says to use a nonchloride accelerator so the concrete will not take forever to set. (Chlorides will leave white marks.)
Mizer recalls that the buildings created a wind tunnel. “I felt bad for the guys out there,” he says, “but Cleveland State University wanted it done, so it got done.” He says the workers were bundled in layers of clothes. “Morale isn’t ever as high on a 15-degree day as it is on a 75-degree day, but a cup of coffee goes a long way.”
It was 34 degrees and snowing when Mizer spoke with Concrete Decor magazine, and he was heading to a stamping project in downtown Cleveland that workers were rushing to finish. “Take every precaution to protect your project and protect your workers, and try to give the owners the best job you possibly can.”
For more information on cold-weather stamping, refer to ACI’s 306.1-90, “Standard Specification for Cold Weather Concreting.” The specification covers prep work, temperature, and protection after a pour. The guidelines apply when the air temperature is 50 degrees or less for at least 12 hours during any 24-hour period, and when the average daily air temperature is less than 40 degrees.