A few readers contacted us recently, questioning why construction crews choose asphalt over concrete for road-resurfacing projects. One person wondered how long the new asphalt on a stretch of Interstate 5 will last.

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Asphalt is cheaper. Concrete can last twice as long.

In some areas of Interstate 5, crews are using asphalt for part of a massive resurfacing project to improve the highly trafficked freeway. In other areas, they are replacing its concrete.

A few readers contacted Traffic Lab recently, questioning why road crews choose one material over the other. Steve Corthell, of West Seattle, asked specifically about the interstate project that began in April.

“How soon are they going to have to repave I-5 because that asphalt wears out?” he wondered.

Traffic Lab is a Seattle Times project that digs into the region’s thorny transportation issues, spotlights promising approaches to easing gridlock, and helps readers find the best ways to get around. It is funded with the help of community sponsors Alaska Airlines, CenturyLink, Kemper Development Co., NHL Seattle, PEMCO Mutual Insurance Company and Seattle Children’s hospital. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

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Crews have a list of pros and cons when deciding whether to use asphalt or concrete to resurface a roadway, weighing traffic volume and expense. Concrete can cost up to three or four times more than asphalt, said Tom Pearce of the Washington State Department of Transportation (WSDOT).

On 4 miles of freeway between SeaTac’s South 216th Street and Tukwila’s South 178th Street, crews have been overlaying the aging concrete with asphalt as part of a three-year project called Revive I-5.

They will do the same thing on portions of the freeway in Seattle. The entire resurfacing effort spans 22 miles from Kent to Ravenna.

Interstate 5 through central Puget Sound is made up of hundreds of heavy, 12- by15-foot concrete panels that can shift and crack when soil underneath gives out.

On many parts of the resurfacing project, crews will replace the panels rather than paving over them.

For laying asphalt, crews use a process they call crack, seat and overlay. The concrete panels are smashed into small pieces with a massive blade and then covered with layers of asphalt.

Asphalt placed directly on top of the shifting, deteriorated panels — without breaking them up first — would quickly crack.

Sections of I-5 have not been repaved since it was built a half-century ago. That’s twice as long as its expected life span, Pearce said.

But why asphalt instead of concrete, especially on the South King County stretch?

“If we did the roadway in concrete, it would last 30 to 35 years — that’s what you can count on,” Pearce said. “It costs less upfront for asphalt, though we’ll back in 10 to 15 years to repave.”

Ultimately, he said, the decision comes down to how much money WSDOT has available when the project starts.

“Over the life of a roadway, or the life span of concrete roadway, it’s about the same cost if you do asphalt,” Pearce said, even though asphalt has to be replaced more frequently.

In general, a 1-mile lane of industry-standard Portland cement concrete pavement typically costs between $2.5 million and $3.5 million, depending on location, WSDOT says. A 1-mile lane of hot-mix asphalt, meanwhile, costs about $1.5 million.

The SeaTac-Tukwila resurfacing project is expected to cost about $31 million total.

Also, Pearce said, laying new concrete would take longer and require more lane closures than using asphalt.

“With doing concrete, it’s incredibly disruptive to drivers,” Pearce said. “That’s another cost that needs to be factored in.”

The South King County resurfacing project is expected to continue through next summer, using nighttime-weekday lane closures to get the job done. An average of 13,650 vehicles pass through the area on a midweek day, between 10 p.m. and 5 a.m., WSDOT says.

For major street work in Seattle, project leaders tend to choose concrete for busy streets and asphalt for areas with less traffic, but each project is different, Seattle DOT says.

The city usually replaces torn asphalt with concrete panels at bus stops and along popular routes, where buses cause significant wear and tear. A transit bus weighs about 15 times more than a car.

As part of the Revive I-5 campaign, crews will resurface 13 miles of freeway in Seattle, between Martin Luther King Jr. Way and Northeast Ravenna Boulevard. That work is just beginning.

By 2019, crews will have replaced 37 expansion joints that connect sections of elevated freeway, replaced hundreds of broken concrete panels, and repaved on- and offramps.

The project is expected to cost $36 million.

For live updates on delays and lane reductions, follow @wsdot or @wsdot_traffic on Twitter.

Got a question or suggestion?

Last week, we asked authorities if they have noticed more vehicles with expired car tabs now that drivers must pay Sound Transit 3’s significantly higher motor-vehicle excise tax. The week before, we shared readers’ comments about all-way-walk intersections that stop vehicle traffic in all directions at once.

If you have a question or idea for us, send it to trafficlab@seattletimes.com. We may feature it in an upcoming column.