Cold Weather Concrete Curing

Originally designed to take the winter frost out of the ground so contractors could get an early spring jump on their projects, ground heaters are increasingly used to allow concrete pouring through the winter.

As an example of the versatility and utility of the ground heater, Hawkins Construction

Company is using two Thawzall model 6A ground heater/concrete curing systems and two Construction Management Stations to cure concrete after bridge deck pours along US Highway 75 in Omaha. According to Hawkins' Bridge Deck Superintendent John Armstrong, the systems rented from Logan Contractors Supply are eliminating winter downtime.

"I'd never used the system until this season," said Armstrong, who has been with Hawkins for 14 years. "We just didn't pour bridges when it got to be too cold in the winter, but the Thawzall allows us to pour bridge decks any time of year."

Thawzall ground heater/concrete curing systems are fully contained trailer units which use a boiler system to heat a biodegradable, environmentally safe propylene glycol mixture. The propylene glycol mixture is then circulated through industrial hydronic hose laid out on the ground for thawing or on concrete for curing.

Armstrong's crew used the system in early February to cure concrete on a bridge deck pour on the Cummings Street Exit off Highway 75. "We poured on a Tuesday morning starting at about 11 o'clock and we finished the pour at about 4 in the afternoon," Armstrong said. "We put the hoses on it as soon as it was hard enough to walk on."

To achieve and maintain the proper temperature for curing, the crew determined that they needed to place the hoses on the deck at 18-inch centers and that required two Thawzalls for coverage of the nearly 9,000-square-foot deck. The circulation hoses were placed on top of the deck, burlap and plastic sheeting, then covered with insulated blankets.

"The Thawzall will heat to 180 degrees but we wanted the temperature between 40 and 104 degrees," Armstrong said. "We used two Construction Management Stations that have supply and return temperature gauges. We just had to regulate the temperature to stay in the range we wanted."

Armstrong said he monitored the temperature the first few hours to make sure it was correct, then just left the machines on for the five-day cure period. The crew checked on the machines and checked on the fuel, of course, but once the temperature was regulated, they let them run.

"It's very easy to operate," Armstrong noted. "Andrew (Logan Contractors Supply Sales Rep Andrew Grimit) came out the first time we used it and showed us how to set it up and use it. Hewent through the entire machine with me to show me the ins and outs and troubleshooting. They're pretty simple to operate and the take down is easy because the hoses just roll back onto the reel using the foot pedal."

According to Grimit, an increasing number of contractors in the Omaha area are turning to the Thawzall to keep the work going through cold weather.

"We have more and more contractors using it for the thawing and the curing is really taking off, too, more this year than in past years," said Grimit. "Using it on bridge decks is kind of new to Nebraska but I see that being very big for the future because it provides the contractor an opportunity to pour through the cold weather."

Logan Contractors Supply carries all three sizes of Thawzall ground heater/concrete curing units: 2M, 6A and 12.

"The systems are ideal for thawing frozen ground of course, because that is what they were originally designed for, and they are also good for curing concrete on bridge decks and concrete on grade," Grimit said. "With frozen ground, the units pull about a foot of frost out in 24 hours."