This section of roadway is actually a bridge. About 150,000 cars and trucks traveling in both directions passed over Interstate 90 near Interstate 405 each day in 2016, the state says.

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When is a roadway in bad enough condition to warrant repair?

Recently Tim Shore has grown increasingly concerned about a patch of concrete on his commute between Seattle and Renton heading eastbound on Interstate 90 near Factoria, so he wrote to Traffic Lab.

“For several years now, I’ve noticed that the [concrete] roadway of I-90 near I-405 has been deteriorating to the point where the rebar structure is exposed to the elements. Does the state have any plans on fixing it any time soon?” Shore asked.

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Not in the immediate future, said Nicole Daniels, a spokeswoman for the Washington State Department of Transportation (WSDOT). The agency is aware of the issue, but more urgent projects are ahead of it.

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“If it was unsafe, they wouldn’t leave it,” Daniels said. “WSDOT knows it’s there and is monitoring it.”

This section of roadway is actually a bridge. About 150,000 cars and trucks traveling in both directions passed over I-90 near Interstate 405 each day in 2016, according to WSDOT’s most recent Annual Traffic Report.

This week’s Q&A explains how bridges deteriorate and how the state responds.

Of the 3,312 bridges that WSDOT is responsible for statewide, 163 are considered structurally deficient. However, the section of I-90 roadway Shore referenced is not on the most recent list of structurally deficient bridges WSDOT compiled.

Typically, a structurally deficient bridge means that one or more of the bridge’s components requires either repair or preservation work. Parts of the bridge may be badly deteriorated, and it may have weight restrictions.

DeWayne Wilson, a bridge asset-management engineer with WSDOT, said the state is moving away from referring to bridges as structurally deficient and instead categorizing them as “poor” to match the Federal Highway Administration’s new ratings system.

Even still, Wilson said a bridge in poor condition is safe for travel.

What causes deterioration

Most of WSDOT’s bridges have reinforced concrete in the road deck, said Kamran M. Nemati, an associate professor at the University of Washington.

However, Nemati said, the reinforcing steel and other embedded metals can corrode and lead to concrete deterioration. When steel corrodes, the resulting rust takes up more room than the steel. This expansion can eventually cause the concrete to crack.

Premature corrosion of steel reinforcement is primarily caused by chloride ions, he said, when the presence of oxygen and moisture combine with the ions — like in fog or mist — to sustain a reaction.

“The most significant and common sources of chlorides are marine environments and from de-icing salts applied to road surfaces during cold weather,” he said.

While a certain amount of corrosion can be expected over time, Nemati said using stainless-steel reinforcement, electrochemical techniques or high-performance concrete are “very effective” methods in controlling the amount of corrosion that happens.

How WSDOT reviews bridges

WSDOT crews regularly repair spalled areas — the term used to refer to concrete cracked from too much moisture — but these repairs are temporary and typically last one to three years.

If the area in need of repair is greater than 2 percent of the entire road deck, the bridge is added to the list of future projects and classified as structurally deficient.

When prioritizing bridge-repair needs, Wilson said WSDOT considers the severity of the damage, the importance of the route and the risks involved in delaying repairs. The resurfacing of Interstate 5 now underway in Seattle is an example of a high-priority project.

As of June 2017, WSDOT had a statewide bridge maintenance backlog of 1,589 repairs, according to WSDOT data. About $275 million is available for bridge repairs for the 2017-2019 biennium, Wilson said. It’s tricky to give an average cost of repair, Wilson said, because each project is different.

WSDOT prefers to repair and rehabilitate concrete bridge decks, which can extend their service life by 25 to 30 years, rather than replacing the entire deck. Rehabilitating decks costs about $80 per square foot, while replacing the deck costs $300 per square foot.

However, according to the analysis, if the cost of rehabilitation is more than 60 percent of the cost of replacement, the agency will replace the bridge.

Got a question?

Do you have a question about transportation for Traffic Lab? We’d like to try to answer it. Send your questions to trafficlab@seattletimes.com, and we may feature them in an upcoming column.