Hydronic heating systems take the chill out of cold-weather concreting.
By Rick Zettler
As the mercury drops, many contractors in North America store concrete finishing equipment and migrate indoors to take a break from bitterly cold winter temperatures. Whether it’s due to the expense or fear that the cold will affect slab quality, most contractors try to avoid cold-weather concrete jobs. “The only time we pour concrete in the winter is when we have to,” mentions Brett Johnson, senior superintendent for Pepper Construction of Chicago, Ill.
Pepper’s and many other contractors’ resistance to pouring concrete in the winter rests in the fact that this infrastructure building block is sensitive to freezing temperatures. Contractors face three primary challenges with cold-weather concreting.
First, time to initial set is lengthened, requiring longer days for finishing crews, which could result in cost overruns. Second, freshly mixed concrete freezes at 29 degrees Fahrenheit (-2 degrees Celsius), and early freezing permanently damages concrete, making it less durable, weaker and possibly unstable. Finally, lower temperatures reduce the rate on strength gain, increasing the time before protections can be removed and possibly raising project costs.
At first glance, it may be appealing to consider not pouring concrete when the temperature drops. That is, however, until reviewing the number of days the temperature falls below freezing point.
Widespread Freezing Temperatures
The National Climate Data Center tracks daily temperatures throughout the United States and Canada. Contractors can use this information to analyze how many days per year the mercury drops below freezing within their company’s market area.
Not surprising, northern United States cities such as Anchorage, Minneapolis and Fargo fall below freezing a total of 191, 156 and 129 days, respectively, throughout the year. A little more surprising is that contractors in Reno and Cincinnati deal with more than 100 days with subfreezing temperatures at some point in the day, and contractors in Flagstaff face more than 200 days.
“Cold weather,” according to the American Concrete Institute (ACI), constitutes a period of more than three consecutive days when: a) the average daily temperature is less than 40˚ F (4˚ C); and b) the air temperature is not greater than 50˚ F (10˚ C) for more than one-half of any 24-hour period. Under ACI’s definition, “cold weather” conditions typically exist from three to six months for a vast majority of the northern two-thirds of the United States and Canada. Contractors avoiding cold weather concreting significantly limit their profit potential.
Historically, those concrete contractors braving the elements have turned to the traditional methods of air heat, antifreeze admixtures and accelerators. These tools have been used with varying degrees of success, and the expense has made some a bit cost prohibitive in certain applications. “The additional cost to the project can be as high as $1 per square foot if not more,” comments Ed Jaroszewicz, director of Climate Control for Wacker Neuson.
Successfully employed for more than a decade, portable hydronic heating systems continue to make their way into more North American job sites. These systems, like Ground Heater units from Wacker Neuson Climate Control, allow contractors to properly cure concrete at a much lower cost than traditional heating methods.
With hydronic heat, a glycol/water solution is pumped through flexible hoses, forming a closed loop system. Portable hydronic heaters serve a variety of heating purposes at the job site, from thawing frozen ground to frost prevention on forms to uniformly curing concrete in cold-weather conditions. If equipped with heat exchangers, hydronic heaters also provide clean, dry air to heat buildings.
The heater’s flexible hose is laid out in a pattern over the ground or on concrete to deliver uniform heat. Heat transfer fluids are 970 times denser than air and offer more than six times air’s specific heat. Therefore, hydronic heaters deliver extremely high BTUs at very low temperature differentials between the transfer hose and concrete. This prevents hotspots commonly experienced with air heat and provides even heat distribution to enhance the quality of the concrete.
“ACI specifications can be interpreted that concrete structures should not have temperature differentials greater than 25 degrees F (-4 degrees C) anywhere within the structure,” explains Jaroszewicz. “Hydronic heat can produce more event temperatures that generate differentials in the range of 10 degrees F (-12 degrees C).”
Through the use of hydronic heat, contractors realize predictable and reliable results, regardless of ambient temperatures. “Contractors have commented to me, ‘Every day I know my crew can pour concrete because of the Ground Heaters hydronic system,’” adds Jaroszewicz. Due to hydronic heating’s efficiency, these desired results are achieved more economically than with other heating methods.
For ground thawing prior to pouring concrete, a hydronic system will deliver rapid thawing capabilities for as low as $0.06 per cubic foot. Conversely, the expense to build an enclosure and heat the space with air may cost the contractor as much as $1.36 per cubic foot. “In addition, hydronic heat thaws frozen ground at an average rate of 1 foot per 24-hour period,” mentions Ken Cannella, product manager for Wacker Neuson Climate Control equipment.
Contractors realize similar savings for slab on grade and wall curing. Considering a four-day cost analysis, contractors pay an average of $0.16 per square foot using hydronic heat, while it will be nearly four times as expensive to use air heat.
Taming the Unexpected
While contractors try to avoid cold-weather concreting, sometimes the schedule or factors beyond their control dictate otherwise. Building delays during the season will often force contractors to pour during the winter months in an effort to meet contract deadlines.
During construction of the Woodrow Wilson Bridge, crossing the Potomac River at Alexandria, Va., concrete pours of the first deck spanning the river were scheduled for late summer. Unforeseen issues pushed back the pours to the winter, necessitating some tough decisions.
A stiff contract disincentive of $50,000 for every day that phase extended beyond Memorial Day forced Potomac Contractors, LLC to search for an effective heating solution. “We had an estimate done to tarp and heat the bridge deck, but the required investment to do this was in the several million dollar range and there were no guarantees on performance,” says Ken Hirschmugl, project director for Potomac Contractors.
Rather than this costly approach, the contractor used a hydronic heating system. This negated the need to build an enclosure and the prohibitive cost of air heat while allowing the contractor to dodge three to four months of contract disincentive payments. Over the winter, crews poured and cured 13,000 yards of concrete, averaging two to three deck pours each week to meet the contract deadline.
After the project, Hirschmugl commented on the decision to use hydronic heating. “It may have cost a half-million, but it ended up saving us millions.”
At times, it’s not scheduling delays but a very tight contract deadline that mandates cold-weather concreting. The Interstate 35 Bridge reconstruction in Minneapolis, for example, included an aggressive design-build schedule that required much of the initial work to take place during the frigid Minnesota winter.
Most of the I-35 bridge’s concrete footings and 70-foot piers were poured when daytime temperatures rarely rose above freezing and nighttime lows often fell below 0˚ F (-18 degrees C). The contractor employed four hydronic heaters to aid in concrete curing.
As the foundation and pier segments were poured, the formwork was wrapped in plywood. Bent nails held the hoses against the plywood, and everything was covered with insulating blankets. Digital temperature readouts on the hydronic heaters assured the contractor that the optimum 68˚ F (20 degrees C) temperature was achieved.
Still, for other contractors, project owners move up the completion date, forcing a pour in subfreezing temperatures. This is the story of Brett Johnson and Pepper Construction for one strip mall application earlier in the year.
With temperatures fluctuating between five and 35 degrees F (-15 and two degrees C), Pepper used an E1100 hydronic heating system to thaw 1,100 square feet of ground and then cure 20 cubic yards of concrete. In this application, the contractor used Pex tubing, tied to the rebar 18 inches apart, and poured concrete directly over the top of the tubes.
For seven days, heat from the hoses cured the concrete from inside, maintaining a temperature range of 55 to 60 degrees F (13 to 16 degrees C). On the last day, crews removed the blankets to reveal a flawless, properly cured concrete slab.
“I wouldn’t want the responsibility of concrete poured in the winter without heat,” confides Johnson. “If we consistently poured a lot of concrete in below-freezing temperatures, the E1100 is one piece of equipment that I’d want around.”
This article appeared in the October 2009 issue of all ACP magazines.