Wednesday, July 20, 2016

It takes time to do the job right

By Tom Pearce

Remember in the movie "Cars" when Lightning McQueen was sentenced to repaving a section of highway before he could leave Radiator Springs? He races through the work and yeah, technically the road's paved, but it's a horrible mess. He ends up having to repave it, and learns his lesson.

That's something we want to avoid, but that's also why we have weekend-long lane reductions for our #SouthKingSlowdown project. In order to do this project right, we need full weekends and not just overnight closures.

We'll have lane reductions for a total of five weekends on southbound I-5 this summer: three this month to repave 2.7 miles in SeaTac and Des Moines and two in August to replace four expansion joints on the southbound I-5 Duwamish River Bridge.

We've talked a lot about what it takes to replace expansion joints, so you probably understand that. The bigger joints simply can't be done overnight. It takes a weekend.

But paving? Does it really take a whole weekend? In this case, yes. If you're just putting asphalt over asphalt, that can be done with overnight lane reductions. Just do some grinding, then pave.

When you're putting asphalt on top of concrete, it's a whole different story.

The section of I-5 in SeaTac and Des Moines is made of 12-foot by 15-foot concrete panels. These panels are more than 50 years old, and more than 500 are broken and need to be replaced. It'd take a couple of years to replace those during overnight lane reductions. Eventually, they'll all break and need to be replaced.

We studied the best way to repair this section and determined it would be more cost-efficient to repave it with asphalt. To do this, during weekend lane reductions our contractor crews are taking out a football-field length of concrete panels at each end of the work zone and replacing them with asphalt. This creates a smooth transition from concrete to asphalt and back again.

Once the concrete panels are removed, a roller compacts the soil on
the transition before that section is paved with asphalt.

As for the rest, you can't just put asphalt on old concrete. Old concrete panels move. It's slight, but if you put asphalt on top of those panels, they'll continue to move and crack the asphalt. So before we put down the asphalt, we're using a process called "crack and seat." Our contractor is using a machine to crack the concrete. After a 35,000-pound roller compresses, or "seats" it, we can put asphalt on top of it.

The video below is of a machine that drops a six-ton blade onto the concrete panels, cracking them. A 35,000-pound roller then compresses the roadway to create a stable base for asphalt paving.

Asphalt paving is time-consuming. We need eight inches of asphalt here, enough to support the more than 100,000 vehicles that use it each day, and it has to be paved in layers. Each layer needed to be compressed by a heavy roller for strength, then it has to cool for 30 to 60 minutes. Then we repeat the process a couple of times.

For now, we have just the first layer on the right side of the interstate. That's OK for a couple of weeks, but for the best final product, the rest of the layers need to be paved soon. Spreading out that paving over a couple months of nighttime lane reductions could diminish its durability. With the weekend lane reductions, in a couple of weeks we'll have repaved this 2.7-mile section of I-5 the right way.

We know that closing lanes on a summer weekend is tough but this is vital work that needs to happen when we have predictably dry weather, and so we appreciate your patience, understanding and cooperation. And as Lightning McQueen learned, taking the time to do the job right the first time is the only way to go.

1 comment:

Angela said...

I wish you would have started the repaving right at the onramp to I-5 South from 518. That stretch of road from entering the freeway to where you began the asphalt is almost the worst of the whole stretch. Although I do see newer concrete panels and markers on that stretch so perhaps you have another remedy for that rather than asphalt.

Hope this method of asphalt is proven for durability!