Working In Winter

Story by Mike Larson, Editor | September 28, 2010

There's no doubt about it, winter weather puts a damper on construction activity throughout the Upper Midwest, including Wisconsin and Upper Michigan.

Cold, wind, ice, and snow make some types of work, such as grading and road paving, virtually impossible.

Other types of work that can continue, like pile driving, steel erection and building construction, proceed much more slowly than when it's warm.

With concrete being essential to almost every project, one of the key challenges in winter construction is how to work with concrete in a season when the temperature is often less than half of the 60 to 80 degrees ideal for concrete work.

Western Builder recently talked with four companies working on winter projects to find out how they succeed in the cold and snow, and in particular how they work with concrete in winter.

The companies: Figg Bridge Inspection, Inc., Milwaukee, Wis.; Kapur & Associates, Inc., Milwaukee, Wis.; Consolidated Construction Co., Inc., Appleton, Wis.; and Roach Concrete, Inc., Helenville, Wis.

Figg is providing on-site engineering and inspection for the Highway 151 by-pass near Fond du Lac. Kapur is providing on-site engineering and inspection for the Highway 10 bridge in Two Rivers. Consolidated Construction is building a new hotel in Green Bay. And Roach is using tilt-up techniques to build a new facility for the U.S. Army Corps of Engineers in Neenah.

Snow, ice and cold make concrete work difficult, but the construction and concrete industries have developed some effective techniques that enable them to keep concrete projects moving, although at a slower pace than in warm weather.

The major concern among contractors working with concrete in the winter is temperature.

Though tough, durable and long-lived once it is completely hardened, concrete is finicky about temperature while it is poured and curing.

A common rule in the industry is that the concrete itself cannot be below 40 degrees Fahrenheit when it is poured or curing. And its optimum temperature is between 60 degrees and 80 degrees.

Making concrete work properly in the cold begins with making sure the air temperature is warm enough to make a pour.

Contractors seem to agree that if the air temperature at the pouring site is below about 20 degrees, things get very difficult, and if the temperature dips below zero, pouring is virtually impossible.

So what techniques have contractors and concrete suppliers come up with that enable them to work in temperatures that are colder than the inside of a refrigerator?

The answers fall into three general classes: adding heat, retaining heat, and using concrete additives or special mixes.

Heat can be added at the concrete mixing site, the concrete pouring site, or both, to help create the temperature needed for a successful pour.

Concrete producers add heat at the mixing site by using hot water when mixing the concrete. Sometimes they also heat the aggregate before adding it to the mix.

At the pouring site, forms are heated and heat is retained by creating some type of enclosure.

Additives include accelerators that help the concrete set up more quickly and plasticizers that help it flow better at low temperatures.

By-Pass Project Keeps Moving

Mike Hahn, project engineer with Figg Bridge, Milwaukee, is the on-site inspector for construction of the Highway 151 by-pass near Fond du Lac, Wis.

The project includes building six pre-stressed girder bridges to carry the four-lane highway over the Fond du Lac River and adjoining wetlands. Construction of the concrete piers and abutments is continuing through the winter.

Hahn says that one of the considerations that is different between summer and winter work is an even greater emphasis on safety because of the challenges posed by snow, ice and reduced worker mobility because of extra layers of clothing.

He also notes that contractors have to be careful not to use frozen fill because when it thaws it settles excessively.

Although welding can continue in winter, says Hahn, it is important to be sure the welding area is dry and protected, and that the metal heats according to specifications.

Regarding concrete, Hahn says that the project uses the typical techniques of mixing concrete with hot water and pre-heating the forms before pouring, as well as insulating with blankets after the pour.

The one technique not used on the project is adding accelerators. "The Wisconsin Department of Transportation doesn't allow accelerators, so we make sure none are used," he says.

Hahn explains that the chemical reaction that cures concrete actually gives off heat. With a mass of concrete the size of a pier or abutment, the curing of the concrete gives off enough heat to keep itself at 60 degrees to 80 degrees throughout its initial curing phase (about four to seven days). After that, the forms can be removed and the concrete will continue to cure to its full strength in 28 days.

Hahn says that on the Fond du Lac by-pass project, electronic temperature sensors embedded in the concrete report its temperature so the contractor and inspectors can be sure it is curing properly.

Bridge Project Builds Through Winter

Similar techniques are also being used in constructing the new Highway 10 bridge in Two Rivers.

Mike Miller, engineering technician for Kapur & Assoc., Inc., Milwaukee, Wis., is part of the inspection team overseeing replacement of the bridge, which will span the West Twin River.

The project requires constructing piers in the river, as well as abutments on both shores. Constructing the piers includes driving sheet piling to form a cofferdam, clamming out pieces of the old piers, placing forms, pouring new concrete piers, and removing the sheet-pile cofferdams when the new piers have reached final strength.

Each of the piers takes between 115 and 270 cubic yards of concrete.

Even working inside a cofferdam at the water's edge or in the river, heating the forms is effective.

Says Miller, "The contractor wraps the forms in blankets above the water line to help insulate them, and lowers a propane heater into the hole two to three hours before the pour. By the time the concrete arrives, the forms and space inside are up to between 60 and 70 degrees."

Miller says the pouring technique for wet areas also helps. "The cofferdam is not absolutely waterproof, so some water from the river seeps into the forms before the concrete is poured. For those applications, we use concrete that has a heavier cement ratio. Also, the contractor works the leading edge of the pour from one edge of the form to the other so it pushes any remaining river water ahead of it. The water is pumped out as the pour continues, guaranteeing a good, solid fill."

Covering the top of the forms with insulating blankets helps hold in the heat that the concrete generates as it cures. This project, too, uses electronic temperature monitoring to assure proper curing temperature.

Another special winter technique mentioned by Miller is loosening the forms and leaving them in place for awhile before completely removing them. "It helps adjust the ambient air temperature around the concrete more gradually than just stripping the forms and letting the cold air shock the concrete," he says.

Hotel Construction Continues

Consolidated Construction is the general contractor building the new, four-story, 105-room, Aloft Hotel in Green Bay. In addition to managing the project, Consolidated is self-performing the concrete and steel work.

Although the official groundbreaking came in early January, site preparation began in November, and construction is continuing through the winter.

Consolidated's vice president of field operations, Dan Holum, discussed the keys to successful concrete work in winter. "Consolidated Construction has been working with concrete in cold weather for years. On this project, we're applying the techniques we've developed from lots of experience," he said.

"The three keys are to make sure there's no frost in the pouring area, to use the proper accelerators in the concrete mix, and to cover the newly poured concrete while it cures," he said.

"Before we pour the slabs, we will bring in a ground heater that circulates heated solution through a pattern of hoses to be sure there is no frost whatsoever. After we've poured a slab, we'll lay insulating blankets and the ground heater's hoses on top of it to help keep the slab warm as it cures," said Holum.

Site superintendent Paul Ver Voort talked about other winter considerations involved with the project. "One thing we'll do differently than in summer is to build the two elevator shafts two stories at a time, rather than four stories at once."

"Winter winds make it very difficult to build a sturdy enough four-story scaffold-and-plastic enclosure for the masons to work in. We'll build two stories of shaft, then when the rest of the building has risen that high, we'll finish the final two stories," he said.

Ver Voort also said that since the building is largely wooden framed, it will be essential to use hot-air-circulating dryers to ensure all moisture is removed from the interior of the building once it has been enclosed.

Winter Won't Stop Tilt-Up

Roach Concrete is using some creative techniques for dealing with winter as it builds a new facility for the U.S. Army Corps of Engineers in Neenah.

Because the building is being built using tilt-up construction, Roach chose to construct a heated temporary casting slab on which to pour and cure wall panels before tilting them up and lifting them into place with a crane.

Says Roach General Manager Jason Norton, "Controlling temperature during the casting and curing of tilt-up wall sections was vital, so we built a hose-type ground-heater into the casting bed, surrounded the bed with a temporary enclosure, and heated it with a forced-air system. We also use a curing blanket on top of the panels to help them retain heat as they cure."

The casting bed and enclosure are large enough that Roach can cast from four to five wall panels at a time. Each panel can measure up to 20 feet wide and from 15 feet to 60 feet tall.

"The enclosure is a simple, cost-effective beam-and-poly structure that is working well and will be easy to dismantle when the job is done," he said.