Washington State DOT Shows Off 520 Pontoons

Nov 14, 2012

Washington State Department of Transportation officials gave the media a tour Wednesday of pontoons on the State Route 520 floating bridge. The pontoons are part of a massive concrete structure that will keep the new bridge afloat.

The tour was part of an effort to show progress on the $586 million project. But it was also an opportunity for DOT to try to alleviate public concern regarding structural cracks found in six of the nine pontoons.

Each pontoon is about the length of a three-car garage. One pontoon on the media tour had visible cracks and water inside. State Construction Engineer Jeff Carpenter said some pontoons have been repaired, and the work is ongoing.

Carpenter said cracks are not uncommon when working with concrete.  “All concrete is going to shrink and crack," he said. "It’s called shrinkage cracks. They’re expected. We have a mix that we’re working on to try and minimized those as much as possible. For obvious reasons, we want as few cracks as possible in any given pontoon. But when you’re pouring concrete you know there are going to be some.”

Carpenter said there are ways to fix the cracks. “If they were less than 6/1,000 of an inch we have a crystalline water proofing. It’s slathered on and penetrates the crack themselves.” Cracks thicker than a piece of paper are routed out and filled with epoxy.

Divers are inspecting the pontoons to determine if they’re still leaking. Carpenter says the DOT is confident in the work of its contractor, Kiewit-General.

But Washington Governor Chris Gregoire has asked for additional reassurance. This month she asked for an independent panel to review the pontoons' safety.

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