E3000 Job Study: Colorado Hotel Cure and Thaw "The Ground Heater is excellent for curing because of its efficiency and short preparation time. We can run a Ground Heater for seven days at about a cost of $200 per day." Rick Koesel   Ground Heater "cures" winter construction woes Breckenridge, Colo. – Cold-weather construction is always challenging, no matter what the task. At 10,000’ above sea level and an average snowpack of more than 70", winter excavation and construction in Breckenridge, Colo., is virtually impossible. Challenged by the complications of excavating frozen ground and concerns of cold-weather concrete curing, many contractors shut down for the winter. For Towback Partners, a construction company located in Breckenridge, shutting down was not an option in 1997. Towback Partners had just begun work on a $9 million project in Breckenridge in October 1996. The project, estimated to last 2 ½ years, involved the construction of a 65-unit hotel/condominium complex known as The Corral. With limited time allotted for completion of the job, Rick Koesel, construction manager of Towback Partners, realized work would have to continue through the winter to remain on schedule. It was while searching for a way to facilitate winter construction that Koesel discovered the Ground Heater. Recognizing the Ground Heater’s potential to solve many construction problems on The Corral and other jobs, Koesel decided to purchase the E3000, the largest of the Ground Heater line, from Summit Equipment Rental Company in Breckenridge. "I realized that we would be running into these same construction problems in the future, so I decided in the long run, buying a Ground Heater outright would cost less than renting," Koesel said. He intended to use it not only for thawing the ground, but also to facilitate proper concrete curing. Towback Partners took possession of the E3000 in mid-January and immediately pressed it into service.   Rick Koesel Construction Manager, Towback Partners   Thawing Frozen Ground Confronted with up to 7’ of frost, Koesel first needed the Ground Heater to penetrate the frozen ground. "Before installing footers or doing any foundation work, we had to thaw the ground," Koesel said. "Normally, we’d have to wait until June to do any type of excavation work. Trying to excavate frozen ground is like digging in concrete; you can hammer away all day and only take out small chips of material." To prepare for the thaw, Koesel and his crew removed the snow layer from the foundation site. Next, they covered the area with the Ground Heater’s heat transfer hoses spread approximately 16" apart. The hoses were then covered with a layer of polyethylene film to serve as a vapor barrier (this is important because moist ground conducts heat 10 times faster than dry ground). The vapor barrier was covered with 3 layers of concrete curing blankets for thermal insulation. "The Ground Heater worked great," Koesel said. "On an area roughly 4,000 sq. ft., we were able to thaw 4’ of frost in about 3 days and were ready to excavate. Without the Ground Heater, winter excavation would be literally impossible."   . 1. The E3000 is equipped with 3,000' of heat transfer hose. It is the best selling model of the Ground Heater line. Concrete Curing Koesel’s objective was to cure the poured wall to the "safe stripping strength" required for "partial load, exposed surface." To attain this required compressive strength, the wall had to be kept at 55° F for 5 days. Achieving this level of compressive strength would then allow his crew to strip the forms and begin to load the wall with no further concern of freezing temperatures or the need to apply additional heat to protect the cure. He then made some assumptions about the relevant variable factors including: Temperature of the concrete to be delivered to the site (75° F or higher). Average ambient outdoor air temperature forecasted for the cure period (Koesel made sure sub-zero temperatures were avoided). Koesel also determined and considered pertinent, non-variable factors including: The mass of the concrete wall (315,000 lbs.). The specific heat of the concrete. The thermal efficiency of the concrete blankets to be used. The amount of "free" heat generated by the cement hydrating into concrete. The amount of heat the concrete would lose when poured into the forms. Koesel calculated that maintaining 55° F in the 315,000-lb. wall, including forms, would require nearly the entire rated heat output of the E3000 Ground Heater (300,000 BTUs/hour). Thus, he decided to operate the Ground Heater at its 180° F maximum temperature setting rather than adjust it down. He got things off to a good, warm start by using the Ground Heater to preheat the forms prior to the pour. Lastly, Koesel decided to cure the wall 7 days, rather than the minimum of 5 days, to provide a safety factor against exceptionally low temperatures. "The Ground Heater only cost about $35 per day to run, so 2 extra days is cheap insurance," Koesel explained.   . 2. Heat transfer hoses were strung on the wall forms using tie wire before pouring concrete. Executing the Cure Before pouring concrete, the Ground Heater hoses were strung horizontally in loops across both sides of the 150’-long wall form sections. The loops of hose were spaced approximately 1’ apart on the form, from top to bottom, to ensure uniform heating. The hoses were covered with concrete curing blankets and the Ground Heater preheated the forms. 5’ lifts were poured into the forms, using a long-reach pump truck. Vibrating between each lift provided proper consolidation. Once the entire 15’-high wall section was poured, concrete blankets were placed over the top of the forms and the Ground Heater continued to heat the newly poured wall. After 7 days, the cure was complete and the forms were stripped.   . 3. Concrete was poured into the forms in five-foot lifts at a time, vibrating between each lift to provide proper consolidation. "The Ground Heater is excellent for curing because of its efficiency and short preparation time," Koesel said. "Normally, we’d have to use tube-type propane heaters to cure the concrete. That involves a lot of time and money. We can run a Ground Heater for 7 days at a cost of about $200, compared with propane heaters that would cost $200 per day." (A savings of $1,200 per week!)   4. A long-reach truck was used to distribute concrete at the site. To prevent freezing during transport, concrete was delivered to the site at a temperature of 75º or higher. In addition, preparation time poses a significant difference between the 2 methods. "With the Ground Heater, our set-up time is cut in half, Koesel said. "We can set up the Ground Heater, blanket the wall and be ready to pour in about half a day. On the other hand, using propane heaters involves building tents over the walls. This is not only time consuming but inefficient as well, because you’re heating the air around the wall instead of heating the wall directly."   5. Once the entire 15'-high wall section was poured, concrete blankets were placed over the forms and the Ground Heater continued to heat the newly poured wall. Koesel plans to use the Ground Heater through May. "This is a big job, so we’ll continue to pour through spring and during the next two winters," Koesel said. "Right now we’re using the Ground Heater about 15 out of every 30 days. It has really become an essential piece of equipment for us." So far, Towback Partners has only used the Ground Heater to thaw frozen ground and cure formed concrete walls. "I can easily see using the Ground Heater to cure slabs on grade, elevated slabs, columns…literally any type of concrete placement," he said. "It really is a tremendous tool." Go to Ground Heaters Surface and Air Heater Products Page Go Back to the Job Studies Page 6. The project, estimated to last 2½ years,involves the construction of a 65-unit hotel / condominium complex know as The Corral.