A massive concrete slab is being poured for the new Mississippi River bridge.
It will be nearly as long as two football fields and 5 feet thick. It will contain 85 semitrailer-truck loads of steel rebar, wood, cable and other materials and 360 truckloads from a concrete mixer. It will weigh more than 7,000 tons.
Most likely, it will be the thickest slab of concrete ever poured in a Minnesota road project, officials say. The construction marvel, begun this week, will constitute the southern portion of the southbound lanes of the $120 million Hwy. 61 bridge being built across the Mississippi River next to downtown Hastings.
Crews started building the bridge in the fall of 2010, and with huge girders installed last month and the giant slab now coming together, it's beginning to take shape. When the 1,938-foot-long bridge is done next year, it will be the longest free-standing, above-deck arched bridge in North America, said officials at the Minnesota Department of Transportation.
The big slab is the second of four major sections of bridge deck. The 40-foot-wide hunk stretches more than 550 feet from the river's edge pier, over 2nd Street and onto Vermillion Street (Hwy 61). It will carry two-way traffic next year while the structure it's replacing, a 61-year-old steel truss bridge, is dismantled.
"We hope to have traffic on the new southbound bridge by June 2013," said Doyle Honstad, Lunda-Ames bridge manager.
Last month, crews installed girders for the first bridge deck section on the north end. River cranes 170 feet tall hoisted 45 concrete girders, each more than 100 tons, into place between north-side piers.
The third section will be the signature 545-foot steel span with graceful arches being bolted and welded together half a mile upstream. Three huge barges will float it into place by year's end, weather permitting.
Pouring for the 5-foot-thick slab will finish next week.
"I've never poured a [bridge] slab that thick," said MnDOT project manager Tom Villars. That was echoed by Honstad and bridge concrete contractor Robert Tousignant, who has poured concrete for his family business since 1973. He said the next deepest was a 4-foot-thick slab in a Fergus Falls bridge built about three years ago.
Honstad said the southbound slab is being poured on three days over two weeks to allow time to cure between pours. Each day a dozen or so mixer trucks trundle back and forth about nine hours between the bridge and a Cemstone plant about two miles away on the south side of Hastings, he said.
On Wednesday, two concrete mixer trucks were unloaded at a time on either side of the southbound bridge. The molten mix of sand, gravel and water was discharged into a bin and pumped up a flexible hose and onto the steel-rebar-laced bridge deck supported by wood and steel framing. In turn, the framing is held about 20 feet above the riverbank by crossbeams and steel posts.
Inside the deck rebar cage lie 16 white plastic pipes that will carry hundreds of thick steel cables through the length of the 550-foot segment. After the bridge deck has hardened, powerful jacks pull the cables taut, compressing the bridge concrete. The tension makes the bridge strong enough to carry millions of pounds of cars and trucks and still last 100 years, Honstad said.
"It's made so the whole thing can be full of [vehicle] axles and it is still not going to come down," Honstad said.
The southbound bridge slab will hold 3,600 cubic yards of concrete. One cubic yard weighs 2 tons, so the whole slab will weigh 7,200 tons, or more than 14 million pounds.
The fourth and final bridge section will be built after the old truss bridge is removed. A two-lane, northbound section will be poured adjacent, but angled into the southbound lanes. North and south lanes will merge on the big pier on the edge of the south shore. The unified, four-lane roadway will continue north over the 545-foot center span onto the bridge's north approach, Honstad said.
Major spring flooding delayed work for months last year; the completion date was originally pushed into 2014, but crews are now aiming for December 2013, Honstad said.
"If it don't rain between now and then, we will make it," he said.
Jim Adams 952-746-3283
Poll: Who will end up starting more games for the Vikings this season?