Hydronic Heating Used for Cold-Weather Curing of the Woodrow Wilson Bridge

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When its original construction was completed in 1961, the Woodrow Wilson Bridge carried the I-95/I-495 Capital Beltway over the Potomac at Alexandria, Va. This six-lane structure was designed to handle 75,000 vehicles per day over the next 20 years, but it surpassed that maximum traffic flow 12 years early. Several maintenance projects have been performed over the years to counter some of the advanced wear caused by excessive traffic, but these actions provided just an ineffective bandage as average daily traffic numbers continued to skyrocket.

Today, the bridge carries more than 200,000 vehicles per day. And it has held the undesirable distinction as the third worst area of traffic gridlock in the nation, as rated by AAA.

Fortunately for area commuters, a decision was made by local government agencies and state and federal DOTs in the mid-90s to build a new 12-lane, twin-span bridge to replace the ailing Woodrow Wilson Bridge. At a price of $2.4 billion, the new Potomac River Bridge would eventually solve traffic woes for hundreds of thousands of daily commuters.

Preparations began in October of 2000 with the initial dredging of the river. This was followed by a second contract, completed in June of 2003, that involved the construction of the majority of foundations for the bridge as well as site grading and bulkhead improvements.

The next stage included the actual construction of the bridge spans. The plan was to first build a new six-lane bridge adjacent to the existing Woodrow Wilson Bridge. When that bridge was completed, traffic was redirected to the new structure and the old bridge demolished, making room for the construction of a second six-lane bridge. Once finished, the two new identical bridges doubled the driving surface of the previous structure, with one bridge feeding into Maryland and the other into Virginia.

The largest portion of the bridge was constructed by Potomac Constructors, LLC, a three-way joint venture between Edward Kraemer & Sons, Inc., headquartered in Plain, Wis., American Bridge Company and the Trumbull Corporation, both based in Pittsburgh.

Potomac Constructors, LLC was responsible for one of the more unique, and challenging, portions of the job. “The portion of work that we’re doing is particularly unique in that it includes the heart of the project – building the portions of bridge over the river.” said Ken Hirschmugl, project director for Potomac Constructors, LLC.

Though the entire twin-span bridge structure is scheduled to open for traffic in 2008, there was a lot riding on the interim milestone, which ensured that the first bridge be open for traffic on Memorial Day of 2006.

“We had originally scheduled all of our bridge-deck concrete pours to occur during the late summer and fall of 2005,” Hirschmugl said. “Due to some unforeseen scheduling issues, this portion of the job was pushed into the winter of 2005 and 2006, which meant we had some decisions to make.”

The options were limited. With a $50,000 per-day penalty facing any contractor that didn’t make the schedule, waiting for warm weather just wasn’t feasible. Of course, there was the traditional way of cold-weather curing for a concrete bridge deck – tarping the structure and heating from the underside – but this was not a particularly inviting solution.

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“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,” Hirschmugl said.

Based on his previous experiences, Hirschmugl suggested using Wacker Neuson Ground Heaters hydronic-heating solutions as the primary component to allow for a proper cold-weather concrete curing environment.

“I first encountered Ground Heaters being used for concrete bridge-deck curing on a job in Salt Lake City,” Hirschmugl said. “This was for the reconstruction of Interstate I-15, which was a design-build project requiring reconstruction of over 20 miles of interstate highway to be built in time for the 2002 Winter Olympics. It was a joint venture between several companies, and altogether they built over 120 bridge decks in just four years. They did concrete every month of the year, every day of the week, and every hour of the day. With such a large amount of work and a tight schedule, there was a need for ingenious solutions to problems. And Ground Heaters were used extensively to keep concrete pouring through the winter months.”

The drive to use hydronic heat for cold-weather bridge-deck curing was further encouraged by Hirschmugl’s personnel. “I had hired a couple of crews who had been working on the Legacy Park project,” he said. “When they came to work for me, they brought their Ground Heaters with them. Like any other business, you carry your tricks of the trade with you. So they were the ones who really opened my eyes to the possibilities.”

Besides having past experience with hydronic heaters on other jobs, Hirschmugl had also used Wacker Neuson heaters earlier on the Woodrow Wilson Bridge project, but for an entirely unexpected reason: cooling concrete. (click here to read more about how the Wacker Neuson E3000 was used to “cool” mass concrete pours on this project)

Though “cooling” concrete would not be the major use for the E3000s on this job, it was this familiarity through use for mass concrete pours and previous job experiences that encouraged Potomac Constructors to use hydronic heat technologies to counter its winter concreting concerns. It would not be cheap. But from an efficiency and effectiveness standpoint, the company felt it was the best solution. Getting everyone on-board with this decision, though, would take a bit of an effort.

“The Maryland State Highway Administration was not very familiar with the technology, so it did take some introductory meetings to get them up to speed,” Hirschmugl said. “Fortunately, the people from (Wacker Neuson) Ground Heaters were very supportive with their engineering efforts to put together a proposal that allowed us to clearly present this process to the state, not to mention their several visits to our job site to make sure the equipment was performing smoothly.”

Even with the state’s acceptance and Wacker Neuson’s support, the task ahead was a daunting one. “Basically, we had to pour 13,000 yards of concrete – just on the bridge deck,” Hirschmugl said. “Picture 10 football fields, end-to-end, 3,300 feet long and 120 feet wide. That’s what we had to pour by Easter.”

Potomac Constructors started bridge-deck concrete pours the week before Thanksgiving of 2005. The standard DOT specifications indicate that when concreting in cold weather, the temperature of the concrete must be maintained between 50 and 100 degrees Fahrenheit for no less than seven days to prevent freezing and assure structural integrity. Following this guideline, the hydronic heaters were mobilized almost immediately.

Utilizing nine of the Wacker Neuson E3000 Ground Heater and three semi-truckloads of Wacker Neuson’s Red Wave insulation blankets, Potomac Constructors developed a process to pour each slab as efficiently and effectively as possible.

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The day previous to a concrete pour, the bridge-deck crews would place the hydronic heaters’ hoses directly on top of the reinforcing steel. The hoses would then be covered by a layer of insulation blankets, and the heaters would be started, letting them warm the reinforcing steel overnight. “We were looking for the steel to be in the 40 degree plus range and to make sure there was no frost on the forms or steel,” Hirschmugl said. “That’s what the inspectors needed to see before each pour.”

With the blankets and hoses removed, warm concrete – typically 60 to 70 degrees at the time of placement – was delivered to the deck using a 500-foot-long conveyor system. Each placement of concrete took between six to eight hours, depending on the rate of delivery.

“The slabs varied between 10 and 20 inches thick,” Hirschmugl said. “A typical placement would be 500 cubic yards of concrete – or 50 truckloads.”

Once finished, the fresh concrete would be covered with burlap and wet down with water, which served to retain moisture. When the concrete hardened enough to walk across, the slab was ready for the hydronic heaters.

 “Each of our E3000s were equipped with a pump pack and an extra reel of hose,” Hirschmugl said. “This basically allowed us to double our coverage for each heater – with an extra 1,500 to 3,000 feet of hose. Typically, we would space out our heaters to handle about 10,000 square feet of concrete slab per unit, looping the hoses two feet apart on the concrete. We would then cover the hoses with two layers of insulation blankets, followed by 16-foot sections of four-by-four timber to prevent the blankets from blowing off.”

The Red Wave insulation blankets are specifically designed for use with hydronic heating applications. Two layers were used to provide an adequate thermal barrier and to minimize potential seams between the fresh concrete and the cold atmosphere. “We would apply the first layer longitudinally, in other words, along the length of the bridge, and then we would position the second layer transversely, essentially forming a cross-cross pattern,” Hirschmugl said. “This placement not only resulted in good heat retention, but also provided an excellent vapor barrier.”

Adjusting the heaters’ output varied based on the ambient temperatures and the stage of the curing process. “Again, we need the concrete to be at 50 to 100 degrees for a proper cure,” Hirschmugl said. “Therefore, though the E3000s offer the ability to generate a lot of heat, we didn’t always need it. On many pours, we would have the heaters turned down rather low, perhaps set to 100 degrees, to achieve an optimum curing temperature zone of 60 degrees. On the sixth or seventh day of the cure, we may have adjusted the output up to 170 degrees since the concrete had gone through its hydration process and was no longer generating its own heat.”

Since concrete curing is a very temperature-sensitive process, Potomac Constructors wanted to make sure it was closely monitoring temperature levels for each bridge-deck section poured. Additionally, because this was a relatively new method of bridge-deck curing and involved a very high profile project, the agencies involved were very concerned that proper temperatures were being achieved and maintained. Therefore, Potomac Constructors implemented the use of embedded temperature sensors into each concrete slab.

Roughly 14 to 16 sensors were tied into the rebar at specific locations throughout the concrete placement area. The sensors were essentially buried in concrete, only leaving the connecting wires exposed. Once the pour was complete, a data logging device was then hooked up to the embedded sensors at various times throughout the seven-day cure process to read and record internal temperature readings.

“The nice thing about using the embedded temperature sensors was that it allowed us to make sure that our concrete never went above or below the ideal curing temperatures, but also provided the state agencies with continuous documentation to prove it,” Hirschmugl said.

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Using Wacker Neuson hydronic heaters, Potomac Constructors was able to execute an average of two bridge-deck pours per week and several times achieved up to three weekly bridge-deck pours. “We were pretty much bullet proof against the cold temperatures,” Hirschmugl said. Even with the confidence that the hydronic heaters were doing their job, one person was designated to go around twice a day – once at daybreak and again at sundown – to make sure the machines were operating properly. “With each bridge-deck pour worth around $250,000, you just don’t take chances with that kind of investment.”

The E3000s were used to facilitate concrete bridge-deck curing up until early April. Following that point, 24-hour ambient temperatures were such that no additional heat was needed. Potomac Constructors’ final pour on the first bridge took place toward the end of April and is on track to make the specified traffic-flow deadline.

In the end, every pour used in conjunction with the hydronic heaters was a complete success. With this experience, Hirschmugl wonders why some contractors still use traditional methods. “A lot of people are convinced that tarping a bridge deck and heating from underneath is the only way you can cure concrete in cold weather,” he said. “But by using hydronic heat, we were able to keep everything warm from the top down.”

According to Hirschmugl, some may avoid using hydronic heat because they assume the major cost of winter concreting lies in the equipment, but in reality it’s actually the fuel. “Imagine a home furnace,” he said. “The cost of the furnace itself pales in comparison with the cost of the fuel you run through it. The same goes with heaters used for winter concrete curing. Therefore it’s ideal to choose the most efficient method. Using the traditional way, you typically spend a lot of money heating the atmosphere instead of the bridge deck, but hydronic heaters are a lot more efficient. I’d estimate that using hydronic heat is 80-percent efficient as far as getting BTUs out of the fuel, whereas using standard torpedo heaters and tarps is about 15-percent efficient. There’s just no comparison.”

Of course, no matter what method is used, cold-weather concrete curing is expensive. With this project, factoring in the equipment, insulating blankets, fuel and labor, using hydronic heat cost Potomac Constructors about a half-million dollars. But compared with the cost for tarping and heating (estimated in the several million dollar range), the company was glad to spend it.