Warm concrete and warm fingers are the secret to wintertime bridge constructionby Dan Olson, Minnesota Public Radio
The big hurryup to get the new I-35W bridge built over the Mississippi River in Minneapolis means working through a Minnesota winter. Even with our recent history of warmer than normal winters it can still get pretty darn cold in January and February. Not to worry, the bridge builders say. The secret is keeping the fingers, the water and the sand warm, according to the engineers.
Minneapolis, Minn. — Anyone who has wandered outside without their gloves on a cold winter day understands what Peter Sanderson is describing. Sanderson directs the I-35W bridge construction for Flatiron and Manson, the bridge building companies.
Cold fingers are clumsy and slow. The 35W bridge builders are on a tight deadline - they want the bridge done a year from December. The workers need nimble fingers because they'll be building precision wooden forms and placing thousands of feet of reinforcing steel, Sanderson says.
"You have to be efficient at it, you have to place bars one after another and you have to place them accurately. They're placed at regular intervals and you can't vary from them," he says.
To keep the digits warm, crews will enclose bridge sections where work is being done.
"We've allowed for what we call heating and hoarding...building a lot of tents over the area and blowing the heat in such that people that work in reasonable conditions right through the winter," he says.
Peter Sanderson is a civil engineer with 37 years experience. He's tall and rangy and looks a little like a younger version of the actor Christopher Plummer. A native of Britain, Sanderson grew up in the Middle East following his father, also an engineer, on various construction projects.
Besides fingers concrete needs to be kept warm in cold weather construction, too. There'll be a lots of concrete poured around the reinforcing steel to form the foundation, the footings, the piers and then the bridge deck.
Concrete that freezes before it cures fails from thermal shock, Sanderson says.
"The water in the mix can freeze before it reacts with the cement powder and turns itself into concrete. You end up with just a pile of gravel or sand that's of no use to you," he says.
The solution is to keep the water, the cement mix and the sand or aggregate warm, Sanderson says.
"You either simply put steam spears into it and blow steam directly into the aggregate piles, or you can put pipes through underneath it and send steam or heat through it and that will percolate upwards."
That works fine for concrete poured for foundations, footings and piers. They can be swathed in big insulating blankets. The blankets keep the heat in and allow the concrete to cure properly.
The I-35W bridge deck is another matter. It'll be concrete as well, but bridge decks are wide and flat - not easy to insulate in cold weather. The solution is building pieces of the bridge deck at an enclosed location that's kept warm and then hauling the pieces to the site, according to Sanderson.
"The main span over the river will all be built in segments in 200 ton segments and the segments will be floated in from between 10 and 20 river miles downstream."
The building of a new I-35W bridge in Minneapolis promises to be a sidewalk superintendent's dream come true. There'll be drilling, rigging, pouring, lifting and as many as 400 workers - most of them from Minnesota - swarming around the project.
He says there'll be two, 10 hour shifts a day, six days a week. A typical wage will be $50 an hour with plenty of opportunity for overtime.
Sanderson says dates are being set aside for the public to visit the site and view construction progress.
- Morning Edition, 10/11/2007, 7:50 a.m.