The Potomac River, nicknamed the “Nation’s River,” has many times been at the center of struggle for the U.S. From the siege of Harper’s Ferry in 1859 to hosting several epic battles of the American Civil War to being declared “a national disgrace” by President Lyndon Johnson in the 1960s due to its severely polluted condition, it is an understatement to say the Potomac River has not been calm throughout history.
Fortunately, the civil unrest has long since passed and major efforts have been made to reduce pollution and restore the river’s beauty, but there is a new struggle involving the Potomac that has reared its ugly head in the past 30 years—trying to cross it.
Fortunately for area commuters, a decision was made by local government agencies and state and federal DOTs in the mid-1990s 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, but the construction itself would not be without its own unique struggles.
Just trying to keep up
The largest portion of the bridge is being constructed by Potomac Constructors LLC, a three-way joint venture between Edward Kraemer & Sons Inc., headquartered in Plain, Wis., American Bridge Co. and the Trumbull Corp., both based in Pittsburgh.
Though one of several contractors working on the project, Potomac Constructors LLC is 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 mid-2008, there was a lot riding on the interim milestone, which ensured that the first bridge be open for traffic on Memorial Day 2006.
“You have to understand that there are 14 contractors that all had to hit that midpoint deadline at the same time,” Hirschmugl said. “In some ways there was more emphasis placed on this initial deadline since its completion freed up those 14 contractors to continue with the next stage of the job. If one contractor was off schedule, it could have adversely affected other contractors’ schedules.”
And it was a scheduling issue that quickly threw a wrinkle into the process.
“We had originally scheduled all of our bridge deck concrete pours [on the first bridge] 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.”
Hydronic hype
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.
“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 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 a concrete bridge deck curing on a job in Salt Lake City,” Hirschmugl said. “This was for the reconstruction of 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, 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. Ground Heaters were used extensively to keep concrete pouring through the winter months.”
Besides having experience with hydronic heaters on other jobs, Hirschmugl also had used hydronic heat for cold-weather bridge deck curing earlier on the Woodrow Wilson Bridge project, but for an entirely unexpected reason: cooling concrete.
“We first used [it] on this project in the summer of 2004,” Hirschmugl said. “We had several mass concrete pours that needed to be completed and required a method to keep the core cool, while avoiding a major temperature differential between the inside and outside of the slab.”
Mass concrete is basically any concrete pour that is greater than 6 ft thick at its thinnest dimension. When cement hydrates, the chemical reaction generates heat. In the case of mass concrete, the mass prevents the heat at the core from escaping. The temperature of the core must be monitored and controlled, otherwise too much heat can lead to cracking due to expansion.
“With mass concrete pours, the idea is to moderate the temperature to cool down the core while slightly warming the outside to reduce the temperature differential,” Hirschmugl said. “The goal is to allow the entire placement to cool gradually and evenly so you end up with a solid block of concrete that doesn’t have any cracks.”
For this application, Potomac Constructors worked with representatives from Ground Heaters Inc. to develop an appropriate system to address the mass concrete issue. Potomac engineered a radiator system constructed of polyethylene pipe that was tied directly into the concrete placement. The heaters were then connected to the network of pipes, essentially building an internal cooling system into the blocks of concrete being cast.
“We used the same glycol solution that is typically found with hydronic heaters, a heat exchanger and the unit’s thermostat controls in order to moderate the temperature so we could cool down the core, while keeping the temperature warm on the outside,” Hirschmugl said. “Our goal is not necessarily cooling it down or heating it up. It’s a question of keeping the entire block of concrete at essentially the same temperature, and we had good success with it.”
Though it would not be the major use for hydronic heat on the job, it was this familiarity through use for mass concrete pours and previous job experiences that encouraged Potomac Constructors to utilize 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.
Even with the state’s acceptance and manufacturer support, the task ahead was a daunting one.
“Basically, we had to pour 13,000 yd of concrete—just on the bridge deck,” Hirschmugl said. “Picture 10 football fields, end to end, 3,300 ft long and 120 ft wide. That’s what we had to pour by Easter.”
Blanketing the area
Potomac Constructors started placing bridge deck concrete the week before Thanksgiving 2005. The standard DOT specifications indicate that when concreting in cold weather, the temperature of the concrete must be maintained between 50° and 100°F 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 E3000 Ground Heater models and three semi-truckloads of Ground Heaters’ Red Wave insulation blankets, Potomac Constructors developed a process to pour each slab as efficiently and effectively as possible.
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°-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° at the time of placement—was delivered to the deck using a 500-ft-long conveyor system. Each placement of concrete took between six and eight hours, depending on the rate of delivery.
“The slabs varied between 10 and 20 in. thick,” Hirschmugl said. “A typical placement would be 500 cu yd 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 [hydronic heaters] was 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 ft of hose. Typically, we would space out our heaters to handle about 10,000 sq ft of concrete slab per unit, looping the hoses 2 ft apart on the concrete. We would then cover the hoses with two layers of insulation blankets, followed by 16-ft sections of 4 x 4 timber to prevent the blankets from blowing off.”
The 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.”
Beyond their performance, the insulation blankets also pleased Hirschmugl with their overall capabilities.
“First of all, each roll is 6 ft wide by 125 ft long, which offers a lot more coverage than conventional blankets,” Hirschmugl said. “Secondly, even with this extra coverage, they’re light enough to be carried and placed by one person. This saved a lot of time and effort in preparation.”
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° for a proper cure,” Hirschmugl said. “Therefore, though the [hydronic heat] offers 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°, to achieve an optimum curing temperature zone of 60°. On the sixth or seventh day of the cure, we may have adjusted the output up to 170° 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 used temperature sensors embedded 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, most importantly, 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.
Staying warm
Using the 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 hydronic heat was used to facilitate concrete bridge deck curing 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.
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 is 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% efficient as far as getting BTUs out of the fuel, whereas using standard torpedo heaters and tarps is about 15% 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, the company was glad to spend it.
“For one, it saved us three to four months of what would have been unbearable financial penalties,” Hirschmugl said. “It may have cost a half-million, but it ended up saving us millions.”
