Compared to other winters in recent years, the 2008 winter in the United States has been more severe, with colder temperatures and higher snow amounts, making this season more fun to be a child and more difficult to be a grown-up building structural concrete buildings. Placing concrete under winter conditions is difficult even under the best circumstances for labor, construction schedules, and the cost of construction. Winter conditions increase the level of risk for everyone, and maintaining good concrete conditions is a challenge.
Two general contractors who self-perform their own concrete work were interviewed for this article: McHugh Construction, based in Chicago and the builders of Trump Tower in Chicago, and Mortenson Construction in Minneapolis, builders of the new Minnesota Twins Ballpark. The two constructions are very different, resulting in unique approaches to placing concrete.
On the Trump Tower, at the date of publication, McHugh finished placing concrete on the 63rd floor, 838 feet above ground. Weather conditions at that elevation can be very different compared to ground level. For instance, if there are light snow flurries on the ground, there can be blizzard conditions on the 63rd floor; work on the ground proceeds at a normal pace while work ceases on the top deck. Sometimes ice buildups on the tower cranes prevent work from continuing too. When snow falls on deck forms, it's impossible to shovel it when steel reinforcement is in place. You can't push it over the edge of the building either because people could be hurt down below. Because you can't place concrete on top of snow or ice, Mark Wilson, a finisher foreman for McHugh, says that sometimes they can remove a deck form panel and push snow to the floor below. However, they usually have to melt the snow by heating the forms from below and passing flamethrowers over the deck from above.
Structural floor areas at the current levels are approximately 14,000 square feet, requiring 400-cubic-yard concrete placements. Delivered ready mix temperatures are approximately 60° F and have a nonchloride accelerator (NCA) added so that carpenters can start forming operations for the next floor level approximately four hours after placement. Concrete is pumped from the ground in one lift at the rate of 90 cubic yards per hour. Workers enclose the deck forms on the floor below with tarps and heat the area to keep the concrete warm. Wind screens on top of the deck help to reduce wind chill temperatures, protect workers, and reduce surface drying of the concrete. Wilson adds that in winter weather they strike off the concrete and bull float it, with no additional finishing performed. Because cold weather conditions mean dry weather as well, one person on the crew sprinkles water mixed with an antifreeze compound after bull floating to help prevent moisture loss from the surface.
The Minnesota Twins Ballpark
Dave Mansell, a general superintendent for Mortenson Construction, is responsible for the concrete work for the new Twins baseball stadium. The 40,000-seat stadium has a concrete instead of a structural steel frame, which he notes is a little unusual for stadiums. They currently are placing about 600 cubic yards of concrete per week on the 51,000-cubic-yard project.
All types of concrete work are going on at the present time. At one end of the project, workers are installing foundations while decks and columns are being cast at other locations. At the other end of the project third-level decks are going up.
Dressing up for winter concreting
Sun, wind, temperature, and snow are the forces of nature to prepare for. Sunshine can be very warming, even on a cold day. Bob Siegel, a project engineer for McHugh, says that wind is the real killer for them. On the upper levels where concrete is placed, wind gusts up to 65 mph. “For safety's sake, we stop work when there are gusts of 40 mph,” he says. Because skin can freeze when wind chill temperatures reach -20° F, Mortenson Construction stops work when ambient temperatures reach -10° F. In Minnesota labor unions often set the low temperature limits that their membership will be allowed to work, says Mansell. For iron workers, the limit is -10° F and for carpenters the limit is -20° F. He adds that Minnesota construction workers perhaps are more experienced with cold weather than most other parts of the country.
Mortenson Construction takes additional precautions for its workers by maintaining jobsite warming shacks for workers and providing glove drying stations. Workers also are encouraged to bring two pairs of boots to work along with extra dry socks. Mansell adds that many workers also bring hand-warming packets that generate heat when you activate the chemistry; they can put their hands in their pockets whenever they need.
Wilson offers the following tips on how to stay warm.
- Wear layers of clothing.
- Cover your neck carefully. Blood vessels are near the surface and wind can push cold air around your body beneath your clothes.
- Drink plenty of water. You lose a lot of moisture in cold weather.
- Don't let your hands or feet get too cold. In colder temperatures, he advises mittens instead of gloves.
- Wear a face mask.
- Keep busy; your muscles generate heat.
- Don't allow yourself to get wet.
- Take breaks to warm hands and feet when they get cold.
Keeping concrete warm
“If workers can stand it, concrete can too,” says Mansell, but you have to take precautions. It must be supplied at the right temperature and maintained in that condition. Both McHugh and Mortenson order concrete with hot water added. Cemstone, Mortenson's ready-mix supplier, heats aggregates and water, and deliver 70° F ready mix to the jobsite. McHugh's supplier, Prairie Material, delivers 60° F ready mix to their jobsite with NCA to help bring about initial set within four hours after placement. Both companies heat their formwork. Boom pump trucks place the concrete on the stadium project so they insulate the boom pipes and often run exhaust from the pump truck through the line to warm the pipe before pumping concrete. They also insulate and heat column and beam forms during the night preceding placement so there is no drop in concrete temperature from the forms. After placement is complete, workers wrap the forms with insulating blankets, with gradual reduction in concrete temperature over time to avoid thermal cracking. When they place concrete decks, “freeze blankets” are kept on the deck forms until just before placement. Workers immediately cover the concrete with them afterward. If they cast slabs on ground, they tent the area and use ground heaters to remove frost beforehand, adding enough heat to the subgrade so that concrete temperatures will remain stable after placement.
How winter concreting impacts the schedule
Dave Alexander, senior vice president for McHugh Construction, says you are always rolling the dice when trying to predict the weather. “The good news is that our company has a lot of work this year. The bad news is that we have a lot of work this year.” But he notes that some of their contracts are multiple-year constructions, so hopefully the cold winter this year balances out with other years. Trump Tower, however, represents a new challenge for them because of the height of the building, which tops out at about 1200 feet aboveground and the changes in weather increases as elevation rises.
The risk that winter construction presents to project schedules is an item that is negotiated in the contract, says Alexander.
Managing the effects of winter
Managing risk due to construction expenses in wintertime is almost always the responsibility of the contractor. It's rare that owners share in this so it's important to cover items in your bid that directly impact costs. Bids for concrete placement during winter months must include additional items. Siegel offers the following list for consideration.
- Tarps and reinforced plastic to enclose areas
- Renting construction heaters and supplying propane or other fuels
- Providing concrete freeze blankets
- Labor to tent areas and maintain heaters
- Inclusion of NCA admixtures
- Increased overtime labor and lower labor productivity costs
- Increased lighting costs because of shorter days in the winter
- Melting snow and removing it from deck forms
- Wind screen protection
When contractors bid work to be performed in the winter, their best estimating tool is their past experience—the companies historical data. In addition to including the items listed above, a company's experience helps them define their productivity. It's known that wearing heavy clothing slows workers down and makes them less efficient but Mortenson Construction doesn't include that as a bid item.
Ready-mix producers are your partner in winter placements. McHugh determines how much concrete will be placed under winter conditions and then comes to an agreement on price with their ready-mix producer. If the yardage of concrete changes, then further discussion will determine if the price remains fair.
The requirements for projects are all different, so the balance between buying concrete at different delivered temperatures to the jobsite, including NCAs (which are expensive), adding heat on the jobsite, and providing windscreen protection has to be worked out to find the best financial solution. Adding wind-screen protection for high-rise construction is a more recent practice. Though these self-rising screens are expensive, Alexander says they are very worthwhile because they add safety for labor, prevent wind from blowing materials and supplies over the edge of a building, and conserve heat for both labor and concrete.
One of the new realities about construction today is the inclusion of pozzolans as replacement for some of the portland cement in mixes. Dave Pace, the vice president of strategic accounts for Cemstone, says that projects applying for LEED platinum certification require 40% replacements, but it's not unusual to see 25% replacements now. Under winter conditions this concrete almost always requires accelerating admixtures, and there are ideal concrete temperature ranges that make them the most effective.
Not pausing for winter
Structural concrete projects once came to a grinding halt during winter months but that isn't the case anymore. As long as concrete doesn't freeze and labor can stay warm enough, concrete will be placed. The industry has learned how to be productive in terms of cost and schedule. Changing technology moves it full steam ahead—pausing for winter is becoming more and more a thing of the past.
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