Bits of deck concrete are sporadically falling off the Interstate 5 Ship Canal Bridge, leading the state to close a shoreline park on each side as a safety precaution.

Public lands on the south shore were closed last week, and a fencing contractor cordoned off a small park on the north shore on Tuesday.

The bridge remains safe for traffic, the Washington State Department of Transportation says. But the eroding concrete, along with potholes and rusting beams, displays yet another symptom of the state’s historical underinvestment in one of the West Coast’s marquee bridges.

The bridge was built in 1962 and carries 170,000 vehicles a day. Deck replacements are scheduled between 2023 and 2028, after the state allocated money from a mid-2010s gasoline-tax increase.

For years, small fragments of less than 2 inches have dropped from the deck, said spokesperson Tom Pearce.

WSDOT decided to add fencing after a repair crew this fall — whose tasks include scraping away material — reported enough loose concrete to pose a hazard, he said. The agency doesn’t keep statistics on reports of fallen pieces.

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“The bridge is aging. We want to make sure the public is safe,” Pearce said.

A full bridge inspection in July found continuing concrete loss, known as spalling, that’s been documented since at least 2013. Pearce said the surface joints between roadway decks are a critical area, where concrete has eroded from the ends.

Besides pavement and deck-edge erosion, inspectors found “delamination” throughout the bridge’s underside, where concrete flakes away and exposes steel rebar. Those situations “are an ongoing repair,” the 2021 inspection says.

Fences will probably stay a couple years, depending on how and when bridge re-decking proceeds, Pearce said.

One stretch of chain-link fence obstructs 300 feet of South Passage Point Park in the Eastlake neighborhood. Fuhrman Avenue East and sidewalks remain open.

On the other side, new fencing blocks North Passage Point Park. Both sites next to Lake Union are state-owned but managed by the city parks department.

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Fragments are falling more often, and some have damaged parked cars around Pocock Rowing Center at the south shore, according to administrative director Patricia Finney. “The closing of the parks is impactful, but the safety of our people is more important,” she said in an email.

In 2018, WSDOT performed urgently needed repairs to fill a deck hole where concrete crumbled away. Though deck erosion doesn’t immediately threaten the structure, it does allow rain and grit to ruin steel beams over time, as happened on the Aurora Bridge two years ago.

WSDOT plans to re-deck the I-5 crossing for about $44 million, from 2023 to 2028. Other surface rebuilds are planned from Yesler Way to North 117th Street, requiring significant lane closures this decade, Pearce said.

The Eastlake Community Association hasn’t heard reports of anyone hit by falling pieces, said president Detra Segar. She said neighbors can stroll to the lakeshore at Good Turn Park, which the community has been cleaning and gardening lately. There are about 35 parks encircling Lake Union.

Seattle City Councilmember Alex Pedersen, whose district includes the bridge, called falling bits “very concerning,” and said he was informed Tuesday about the park fencing. “I encouraged WSDOT to expedite their repair process underneath their I-5 bridge,” and asked city transportation staff to monitor the situation, he said.