With Thanksgiving just around the corner and a chill already in the air, Potomac Constructors' crew was preparing to spend the winter of 2005–2006 on a bridge deck spanning the Potomac River between Maryland and Virginia. The first phase of the new 12-lane, twin-span Woodrow Wilson Bridge had to be ready for traffic by May 2006, which meant a healthy dose of cold-weather concrete curing.
“Picture 10 football fields, end-to-end, 3300 feet long and 120 feet wide,” said Ken Hirschmugl, project director for Potomac Constructors, a joint venture between Edward Kraemer & Sons, Inc., Plain, Wis., American Bridge Co. and Trumbull Corp., both based in Pittsburgh. “That's what we had to pour by Easter.”
With a $50,000 per day penalty facing any contractor that didn't make the schedule, waiting for warm weather wasn't feasible. The team investigated the traditional cold-weather curing approach for a concrete bridge deck—tarping the structure and heating from the underside. However, the estimate for that method was several million dollars with no guarantees on performance.
Hirschmugl had another idea. Potomac Constructors had used hydronic surface heaters earlier on the Wilson Bridge project for a very different application: cooling concrete (see sidebar on page 32). That experience encouraged Hirschmugl to recommend using hydronic heat technology to solve the winter curing problem. Potomac worked with Ground Heaters Inc. to convince the Maryland State Highway Administration (MSHA) that the system, although not inexpensive, would be the most effective solution.
OVERNIGHT HEATING PROCESS
Using nine model E3000 surface heaters and three semi-truckloads of Ground Heaters' Red Wave insulation blankets, Potomac streamlined a process for placing each slab. The day before 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 warm the reinforcing steel overnight.
The following day, inspectors would check for the steel to be about 40° F with no frost on the forms or steel. Blankets and hoses were then removed and warm concrete, 60° to 70° F at the time of placement, was delivered to the deck using a 500-foot-long conveyor system. The slabs ranged from 10 to 20 inches thick, with a typical placement using 500 cubic yards of concrete. Each placement took six to eight hours.
Once finished, the fresh concrete would be covered with burlap and wetted with water to retain moisture. When the concrete hardened enough to walk across, the slab was ready for the hydronic heaters.
“Each of our Ground Heaters was equipped with a pump pack and an extra reel of hose,” Hirschmugl said. “This allowed us to double our coverage for each heater. 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 in hydronic heating applications. The insulation blankets, supplied in 6-foot-wide by 125-foot-long rolls, were light enough to be carried and placed by one person. Two layers were applied in a crisscross pattern to provide an adequate thermal barrier and to minimize potential seams between the fresh concrete and the cold atmosphere. This arrangement also provided an effective vapor barrier for curing.
Based on the ambient temperatures and the stage of the curing process, the heaters' output was adjusted to maintain the 50° to 100° F needed for a proper cure. “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°,” Hirschmugl said. “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.”