They’re unique to the new Interstate 74 bridge, the Y-shaped piers that will be the foundation of the Mississippi River span.
As such, Iowa Department of Transportation officials say, their construction is a bit out of the ordinary.
That was evident Friday, as about a dozen people gathered in downtown Bettendorf to witness construction workers pour 136,000 pounds of concrete into a “mock pier,” built for the purpose of testing the procedures and materials to be used for the real piers.
The Iowa DOT doesn't usually conduct this kind of test, officials say, but this project is different.
“It’s such a unique pier that before we do 60-plus of them, we want to make sure we’re doing it properly,” said Cedric Wilkinson, a project manager for the Iowa DOT, who was on site Friday.
The piers feature reinforced steel that, to form the Y shape, cross one another. It’s a design conjured especially for the I-74 bridge and chosen as part of a wide-ranging process several years ago. But because the shape, particularly the intersecting steel, is so unusual, it led builders to want to conduct Friday’s test.
“We want to make sure all this concrete flows through the rebar,” Wilkinson said.
There are 15 sets of piers — one each for the westbound and eastbound spans — to be built in downtown Bettendorf for the bridge, with another 10 sets to be constructed in Moline from the river to 7th Avenue.
Fifteen sets of piers will be built in the river, which will require their own testing.
Friday’s endeavor will cost $64,000, according to the Iowa DOT.
That’s about 2/10ths of 1 percent of the $30.1 million contract awarded to Civil Constructors of Freeport, Illinois, which covers the bridge as it crosses downtown Bettendorf.
“It’s a good insurance policy, in my opinion,” said Danielle Mulholland, the I-74 project manager with the DOT.
The entire I-74 project is estimated at $1.2 billion.
Six truckloads of concrete were being brought in Friday for the pour. And a gaggle of quality control and project inspectors were on hand to monitor the event.
The forms around the piers will be kept in place for five to seven days, after which inspectors will look for gaps. In addition, there will be tests on internal temperature to ensure the concrete doesn’t set too quickly.
There also will be tests to ensure the form is withstanding the upward pressure put on by the concrete's immense weight.
Eventually, the real piers also will have a cap that will provide the base for the steel and the bridge decking. The concrete for that cap also will have to undergo tests, Wilkinson said.
Under a nearly perfect blue sky Friday, loads of concrete were lifted by crane to the top of the pier, as workers guided it to pour down what was called a tremie, a tube that controlled the flow. Officials said they would pour about 12 cubic yards per hour.
Construction of the new I-74 bridge is expected to take 3½ years, and officials say it is being designed to last 100 years.
The pier built for Friday’s test will have a far shorter life-span. After the tests are completed, officials say, they’re going to tear it down.