Concrete pontoons for a new Highway 520 bridge are on their way.

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ABERDEEN — On a high tide in early April, the first six concrete pontoons for a new Highway 520 bridge will begin their voyage from Grays Harbor to Seattle.

The spring flotilla will show the public a tangible product and spectacle after 15 years of planning, nine years of added gas tax and three months of tolls.

These pontoons, actually hollow sealed barges, will be anchored to the bottom of Lake Washington, to support the road deck of what will be the world’s longest floating bridge. The floating segment of the new bridge, at 7,710 feet, is 132 feet longer than the 1963 bridge it’s replacing. The roadway will be built separately in sections, perhaps in Kenmore near the lake’s north end.

The first half-dozen main pontoons, totaling 65,000 tons of concrete, are taking shape in a huge basin west of downtown Aberdeen, site of a former log yard. So far, Kiewit and General Construction are on schedule to carry out the $367 million pontoon-fabrication contract, employing 350 construction workers, state officials say.

The entire 520 corridor from Interstate 5 to Interstate 405 will cost $4.65 billion, and the state remains $2 billion short of building fixed portions at the Seattle shore.

Workers are finishing the first five lengthwise pontoons, and one crossways pontoon to stabilize the east end of the bridge. Each of the 21 lengthwise pontoons stretches 360 feet long by 75 feet wide and 28 ½ feet tall. These concrete boxes contain a honeycomb of interior walls — 52 cells per pontoon will provide internal strength and resist sinking.

“Compartmentalizing a pontoon will make it safer, if it is hit by a boat or for any reason has a breach,” said principal engineer Dave Ziegler, of the state Department of Transportation.

To speed the pace of assembly, the internal cell walls are built beforehand just outside the basin, then lowered into the pontoons by crane.

Under a blue sky Friday, ironworkers tied cell walls together by adding steel reinforcing bar at the corners, where other crews shot concrete from a hose into these new joints.

When these six pontoons are done, workers will open a pair of gates, allowing harbor water to surge into the basin, some nine feet below sea level, until the pit fills to a depth of 19 feet. Mesh screens will protect salmon from getting sucked in. Once the basin fills, cranes will lift away part of the basin wall facing the harbor, so tugboats can tow the pontoons out.

The operation must be accomplished in six hours, while the tide is high.

Pontoons will be towed past the harbor bar to the Pacific, then around Cape Flattery toward Puget Sound, finally squeezing past the Ballard Locks and Montlake Cut. They will be promptly anchored into permanent locations on Lake Washington just north of the old 520 bridge.

The bridge will be further buoyed by 54 smaller supplemental pontoons, most made in Tacoma, and fastened to the sides like ears. If rail transit is added someday, more would be attached.

The pontoons will be cinched together by rubber gaskets, concrete grout, and high-tension steel bands — to make one gargantuan 1 ½-mile barge.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom.