A freezing wind whipped off the Potomac River as the tugboat pushed the barge into place, and the first load of concrete began moving up the conveyor belt. Construction workers, bundled in layers of flannel and sweat shirts beneath their puffy life vests, nudged a large hose into place.

The hose hovered like an elephant's trunk over an 18-foot hole sucked dry at the river's deepest point. At 12:09 p.m. yesterday, concrete dropped from the conveyor belt into the hose and -- whoosh! -- spattered into the hole.

For the next 100 years, builders say, this concrete will support the draw span on the Maryland side of the new Woodrow Wilson Bridge's inner loop. It must be strong enough to withstand a hurricane, an earthquake or a ship crash, as well as the constant thrum of up to 290,000 trucks and cars rumbling over it daily. It also must keep the draw span so still that, even after years of vibrations and hundreds of openings, its teeth will line up within an inch of each other every time the span closes.

Fifty construction workers are working 12-hour shifts over 48 consecutive hours -- through two freezing days and nights -- pouring the concrete. When they finish tomorrow afternoon, they will have poured 6,200 cubic yards -- enough to fill an elementary school gymnasium from the floor to the rafters -- making this one of the largest concrete pours for any bridge project on the East Coast.

"It doesn't get much bigger than this," said Jim Ruddell, a construction manager with Potomac Crossing Consultants, which is overseeing the project.

Beneath the seemingly straightforward task lies a delicate balancing act. That's because hardening concrete generates heat. The more liquid concrete, the more heat. There is so much in this pour that, if not carefully controlled, the core temperature would rise to about 140 degrees.

The problem: The concrete hardening on the outer edges of the slab is exposed to the freezing temperatures. If workers do not keep the core and outer temperatures within 35 degrees of each other, the concrete will crack.

While Maryland and Virginia highway officials continue to watch that the $2.4 billion bridge project stays within budget, construction is humming along. The pouring of the foundations that will support the four new draw spans marks a major milestone for 135 construction workers who have spent the past 17 months driving piles and building the new bridge's massive foundations.

The concrete slab being poured will be 16 feet thick, most of it never again to be visible, except during extremely low tides. Three pyramid-shaped concrete pedestals atop the slab will protrude from the water and form the base of V-shaped piers that will form the arch design of the new bridge.

The difficulty of building a bridge mid-river is obvious: The water gets in the way. Workers spent the past two months and $3 million preparing a dry hole in the middle of the Potomac where the concrete would be poured.

Because the river is so deep in the shipping channel, the draw span foundations had to be suspended in the river -- no easy task.

Workers arrive by boat after a 15-minute ride from the shore at Oxon Hill. In addition to thick gloves and well-worn boots, most bring a change of clothes in case they get wet and a cooler of sandwiches and sodas.

They stick their coolers in small crevices throughout the work site. Some workers will pour concrete down in the hole 12 hours straight, grabbing a quick bite when they can. Their only chance to get warm comes during occasional breaks in a heated shipping container about half the size of a one-car garage. The small rooms are part of an island of barges that hold Port-A-Johns, generators, cranes and other massive machinery. The barges are connected by three-foot-wide gangplanks.

Last winter, workers drove 39 steel piles that will support the concrete foundation. In the past two months, they built a kind of large, steel bathtub and lowered it over the piles. Holes were cut in the tub's bottom for the piles to poke through. Divers then sealed the gaps between the holes and each pile with steel rings that fit like doughnuts. Workers attached the steel bathtub to the piles 18 feet below the water's surface and suspended 10 feet above the river bed.

"It's like pushing a bathtub down into the river and holding it there," said Mike Bonin, the engineer overseeing the construction.

They then poured six feet of concrete through water to form the concrete floor of the bathtub, he said. After that concrete hardened, workers spent two days pumping the river water out of the tub. Then steelworkers built what Bonin described as a massive jungle gym inside the dry hole.

These bars will reinforce the concrete. They also serve as a steel framework that the workers use to climb around the hole, so they can pour concrete below them without having to step in it. Sixteen feet above ground, they walk gingerly but confidently along steel bars half an inch wide.

After the concrete arrives by barge, workers on the lower rungs of the jungle gym guide the hose to different spots to pour the concrete. Workers on the top rungs pull electric cords for other workers below, who use machines to vibrate the concrete to ensure that it spreads evenly.

They also installed one-inch tubes that run through the jungle gym every three feet. When any of 18 temperature sensors shows part of the concrete generating too much heat, workers pump river water through the pipes to cool it. The massive slab will generate so much heat as it hardens that it will take up to three months to finish.

With this pour, the new bridge's foundations, costing a total of $125 million, will be about 70 percent finished. Next comes construction of the V-shaped piers, scheduled to begin in June.

In the construction of the new Wilson Bridge, concrete is brought from shore in the cone-shaped hopper at right and then poured into what will become the foundation.Construction crew members work inside the cofferdam that will be filled with concrete to form the foundation for the new bridge's draw span.