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Mar 8, 2008 9:34 am US/Central
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Construction Of I-35W Bridge A Challenging Project
(WCCO)
More than 100,000 people commute over the Mississippi River every day -- nearly all of them are driving on Cemstone concrete.
For almost a century, that company has been providing concrete for thousands of roads and bridges in Minnesota, and the new I-35W bridge is Cemstone's most challenging project yet.
They call it the ballet, but with twisting and turning of a different kind. What at first seems an unlikely analogy, oddly fits as cement trucks rush in and out of Cemstone's North Minneapolis plant on cue.
"There's one on the way back, there's one unloading, there's one on the job and there's one on the way to the job," points out John Dickey, the Vice President of Metro Operations for Cemstone.
Each truck carries 20 tons of concreted from the plant to the 35W bridge site. Since the bridge will need 90,000 tons of concrete, that means 4,500 trips back and forth for the workers.
"From the time the cement and the water come in contact with each other, you have 60 minutes to unload it," said Dickey.
The tempo of the project is the biggest challenge.
"Everybody's under extreme duress all the time," he said.
To finish the bridge by the end of the year, it's being designed and built in states.
"There's so mach engineering changes on a minute to minute basis, it's real difficult for us to plan anything," said Dickey.
The requirements for the concrete mixes can change quickly, and the Cemstone plant has to be ready.
When the trucks get to the bridge site, samples of the loads are examined for quality before they're poured.
Dickey said, "There's a lot of inspections, more so than ever, because this is such a high profile project."
Representatives from Mn/DOT, Flatiron, Cemstone and an independent testing company all check the concrete before it goes in.
What is being used to build the bridge is called high performance concrete. It is made of granite from St. Cloud and 3/8-inch material from Grey Cloud Island up the Mississippi and of industrial waste.
Cemstone Vice President of Engineering Kevin MacDonald said, "We're reducing about six to seven hundred pounds of CO2 emissions per cubic yard, which is about three and a half tons per truckload of CO2 which doesn't have to be put into the environment."
The post industrial waste adds to the concrete's strength and durability.
"One of the structural redundancies that gets put in is that the materials are considerably stronger than they are designed to be," said MacDonald.
He estimates the strength of this concrete at double what the design requires. All the mixes are above and beyond what would typically meet MnDOT standards.
"Crushing strengths in the structure are much higher than the design requires, which in turn are much higher than would ever be imposed on the structure," said MacDonald.
The 35W concrete mixes are significantly lower in cement than that in other bridges, and if you don't know the difference between cement and concrete, think about it this way.
Cement is to concrete what flour is to a cake. Like flour, cement is a powder. When it is mixed with the other ingredients -- aggregate rock and water -- it will harden through a chemical process. Like flour, it helps hold everything together.
The decision to go with a concrete bridge instead of a steel truss bridge is typical of bridge design today.
A steel truss bridge, which is what carried 35W traffic before, loses tensile strength over time. Concrete, on the other hand, can get stronger indefinitely. That's because any un-reacted cement present will continue to react as long as water -- whether humidity or precipitation -- is present.
"This is probably one of the most historic projects we've ever been involved in," said Thor Becken, CEO of Cemstone.
Becken's great grandfather started Cemstone back in 1927.
When the final span is cast toward the end of the year, it will be a proud moment for this company with deep Minnesota roots.
Becken said, "We're here just to provide a product and make sure that the new bridge gets built in a fashion that has the highest quality product for MnDOT and the State of Minnesota."
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