Portable hydronic heaters were introduced about 12 years ago to the construction market as ground thawing equipment. The portable boilers heat fluid which is pumped through hose laid out on an area to be defrosted so contractors can work through frozen winter months. But industrious contractors soon learned that hydronic heaters could be used to allow concrete pouring through winter as well.
As concrete cures, it puts out heat. The warmer the concrete, the faster it cures. But when it’s frozen or very cold, concrete can stop curing altogether.
According to the American Concrete Institute (ACI), concrete placed at or below 42 degrees must have heat protection for a proper cure. So contractors don’t have many choices during the cold months of winter: protect the pour, use additives or don’t work.
There are various methods for protecting a concrete pour, including concrete insulation blankets, propane heaters with tents, etc. There’s also portable hydronic heating systems, which can help keep concrete pours at ideal curing temperatures from 65 to 75 degrees F.
Hydronic heating systems can thaw ground and cure concrete in a fraction of the time of traditional direct- or indirect-fired gas heaters. Not only does this help contractors to more accurately quote and plan their jobs, it also reduces downtime for crews and equipment.
“To put it simply, hydronic heaters help contractors take the unknown factor of weather out of the picture,” says Ken Cannella with Ground Heaters.
How it works
There are a few different hydronic heating systems available on the market today. While each system is unique, the concept is generally the same and pretty simple.
The unit’s boiler heats a propylene glycol mixture, which is like a food-grade anti-freeze that effectively conducts heat. The warm fluid is pumped through long loops of heater hose, which are generally laid out on 12- to 24-inch centers on the area to be heated.
The hoses are covered with a vapor barrier, which helps prevent moisture from escaping the thaw area. Insulating blankets are laid over the vapor barrier. The layers keep the energy in the ground, changing the ice to water and accelerating the thaw.
Air heaters generally only direct about 15 percent of their heat into the ground, but hydronic heaters conduct 93 percent of their heat into the ground. Of course the percentage varies with the number of blankets used.
The goal of each heater during concrete curing is as simple as the technology: to keep the pour at an ideal, constant temperature so the concrete cures properly. The heater can also be used initially to heat the ground so the concrete can be poured on a warm base.
Setting up a heater system varies depending on the type of pour. For example, when pouring a slab, the ground should be preheated. Fluid in the system is heated to about 180 deg. F. which should raise the ground temperature to 85 to 90 deg. Depending on soil conditions, most hydronic heaters can remove frost from the ground at a rate of one foot per day for the first three feet, than 1/2 foot per day thereafter.
After removing the heating system and blankets, the concrete is poured.
When the concrete is set, the vapor barrier, hydronic hose (with 24-in. spacing) and insulation blanket are placed. Reduce the heating-system temperature to 120 deg. F and control to keep the outgoing temperature of the glycol mixture in a 65- to 75-deg. range. Maintain the heat until the concrete has reached its desired strength. It can takes three to seven days, depending on ambient temperature.
Thickness of the pour doesn’t matter. Concrete generates its own heat when curing, so the hydronic heater just protects the concrete from freezing.
Different-sized heaters vary in their capacities for curing concrete, with ranges from 1,100 sq. ft. to 50,000 sq. ft.
Spotlight: Heating and Concrete Curing Equipment
Hydronic heaters can help cure tilt-up concrete construction, slab-on grade, poured walls, columns and elevated placements, such as bridge decks and multi-story buildings. They can also heat rebar before pouring.