As WSDOT begins a new round of repaving on Interstate 5, engineers are noting how long the concrete pavement has lasted.

The original design of I-5 had called for a lifespan of around 20 years before concrete slabs began crumbling completely.

More than 50 years later it's still holding up relatively well.

"I was pretty sure they were going to fail about 10 years after I left," Newt Jackson, a retired pavement and soil engineer for WSDOT, said. Jackson was working on I-5 projects in the early days back in the 1970s just after the interstate was built.

Jackson says the long life span is in the materials. Gravel from the Steilacoom Pit was used which is much tougher and durable than most materials in the U.S. That, paired with Seattle's mild climate have helped I-5 last all this time.

We're amazed at how long it's outlived predictions," he said.

"Today it's a little bit of a guessing game, but from my own personal observations, they are starting to deteriorate much more rapidly now. So the time is approaching."

WSDOT has already repaved much of I-5 in South King County in the southbound lanes. Work is now happening in the northbound lanes, over the next year and a half.

WSDOT has more on a long list of closures on its blog.