When looking at large-scale structures around the country, it’s apparent that engineering and design have continually advanced to the point where almost any architectural goal can be met. From skyscrapers to stadiums, practically any conceivable building can be erected within an adequate timeframe. The sheer size and logistics of some projects can present their share of challenges to contractors, but what’s more interesting is how smaller details, or outside forces, occasionally throw a monkey wrench into an otherwise perfect plan.

Project stipulations

In September 2009, the U.S. Army Corps of Engineers awarded Clark Construction Group, Bethesda, Md., a $229 million contract to build the U.S. Army Medical Research Institute of Chemical Defense (USAMRICD) replacement facility—a 526,000-square-foot complex located in the Aberdeen Proving Grounds in Harford County, Md. Considering the massive scope of the job, including site preparation, building shell construction, fit-out of interior laboratory space, and utility installation, Clark Construction had numerous priorities to account for while planning the LEED Silver certified project as a whole.

It’s understandable that one of those initial priorities was not guessing how cold winter temperatures more than a year later might impact one very specific aspect of the job. But when the task at hand was concrete curing and a colder-than-average Maryland winter had indeed struck, figuring out how to work around the cold quickly became a more important matter.

“It was a tough winter,” says Patrick Dugan, Clark Construction’s superintendent on the USAMRICD project. “There were stretches where even during the day the high temperature was only 20°F. We had to pour concrete for 45 equipment housekeeping pads—90% of which required inertia bases—but the cold weather obviously presented an obstacle to that.”

The parameters put forth by the Army Corps of Engineers to construct the housekeeping pads called for the concrete to hold a temperature of at least 50°F for the first three days after the pour, and then at least 40°F for an additional four days. Cold temperatures created a challenge in achieving the specified temperatures, but even more important was the prep work stipulation that went along with the concrete pour requirements.

“The key was that we couldn’t pour unless everything was 40°F prior to placement,” says Dugan. “We were on an elevated deck with the air temperature at 20°F and had to find a way to heat the substrates, rebar, formwork, and the slab we were pouring against before we could pour concrete.”

Delaying the project until warmer weather arrived wasn’t an option under the job schedule. The idea of enclosing the entire building and heating it with air wasn’t seriously considered either, as it would have required the crew to stop ongoing work on the structure’s outer facade—thereby passing the buck on a potential delay from one area to another.

Finding a solution

In contemplating what alternative solutions might be available to keep everybody on task, Dugan recalled reading about electric blankets that masons were using to heat sand piles. That info was passed along to the company’s purchasing agent, who then connected with Colony Hardware Supply, an equipment dealer based in New Haven, Conn. Colony carries an extensive line of equipment, including concrete heating and curing blankets manufactured by Powerblanket. It appeared that Clark had found its solution to the cold-weather concreting dilemma.

After learning what Clark was looking to accomplish at the USAMRICD jobsite, Colony directly consulted with the engineering and sales teams at Powerblanket to determine the appropriate type and quantity of products that would be required. Colony recommended 15 Powerblanket Multi-Duty MD1123 curing blankets.

Running on 120 volts, each blanket covers an 11x23-foot area. With the dimensions of the housekeeping pads ranging from 3x5 to 10x30 feet, the blankets were large enough that most of the pads could be heated with a single blanket. For the largest pads, two blankets were required.

“We tried to use every blanket during each session’s setup,” says Dugan. “Between preheating, pouring, and then heating the concrete while it cured, the process was to take about a week each time, so it was important to keep the number of sessions to a minimum.”

To maximize efficiency, Dugan and his crew worked out a scenario that would enable them to complete construction of the housekeeping pads in the least amount of time possible. After plotting out a logical arrangement for the curing blankets, as well as placement of the boxes and extension cords supplying the electrical power, Clark determined that the 45 pads could be completed in four pouring sessions.

Putting the plan into action

With the plan in place, the crew put the blankets out to begin preheating. For each pad location, the Army Corps of Engineers would check to verify that the initial temperature specification of at least 40°F had been achieved for the formwork, rebar, and existing slab edges. Once that was verified, the concrete pour could commence, with some pads being 3 1/2 inches thick and others 6 inches.

Although concrete produces its own heat through hydration, the outside air temperature at the time dictated continued use of the curing blankets to assist in the hydration process. Therefore, once the concrete was in place, the blankets were once again positioned; this time to provide heat directly into the fresh concrete to help sustain the temperatures specified by the Corps. The blankets created an ideal environment for each slab to cure.

“We never missed a temperature test before a pour or throughout the curing process during our four rounds of pours,” says Dugan. “We poured pads on an elevated slab-on-metal deck, and certainly the cold air below could have caused the concrete to lose temperature. But the blankets provided ample, sustained heat and just did a great job in allowing us to achieve our targets.”

One last consideration for the contractor was the removal of the blankets after each weeklong concrete cure. The Army Corps of Engineers had stipulated that concrete could not be “shocked,” in this case meaning that there could only be a 20°F temperature difference between the concrete and the air once the blankets were removed and the newly formed concrete exposed.

“In a situation where we attained 70°F underneath a blanket and it was 30 degrees outside, we could not have taken the blanket off immediately,” says Dugan. “In that case, we simply left the blankets in place, but unplugged them. The properties of the blankets are such that they continued to retain some heat in the concrete, but the temperature would gradually drop down. It turned out to be a perfect setup.”

Additional benefits

Dugan also cited Colony Supply’s customer support as a key contribution to the project’s success. “Colony was very helpful in getting us going with the blankets in short order,” adds Dugan. “And in addition to their functionality, the blankets were pretty versatile and easy to use. We could spread them over multiple pads. They have a 6-inch flap that we used to help shape around the edges of the formwork. It really was the solution we needed.”

The versatility of the heating blankets also has come in handy on other random jobsite tasks, including curing a concrete slab on top of an elevator shaft and heating a roadway curb. Considering the full project is among the largest Clark has ever tackled, even smaller benefits like these have been more than welcome in keeping the job on schedule.

With completion of the USAMRICD replacement facility slated for summer 2013—meaning there is still one more winter to navigate. For now, Dugan and his crew are happy they were able to meet a critical challenge at a stage when the job really heated up.

Mike Crummy is a technical writer with The Promersberger Co. in Fargo, N.D.