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Good Question: Does A Rock Coating Really Help Streets?

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MINNEAPOLIS (WCCO) — It’s is the time of year many of us are arriving home to see a sign reading, “LOOSE GRAVEL.” The summertime dump of rock on our residential and busy streets is an irritation to many, but what’s going on with all those rocks?

“There’s actually a lot of good happening here,” said Mike Kennedy, Director of Public Works for the city of Minneapolis.

The process is called chip seal-coating. It’s chips of aggregate that are forming a new surface for the road.

“It’s one of those things you hate while it’s happening, but it does a lot of good for your streets,” said Kennedy. “This is pothole prevention work.”

City crews put down a layer of hot liquid asphalt first.

“This seals up the pavement and keeps water out. Water is the enemy. Water is what freezes up and causes the potholes,” said Kennedy.

That’s followed by another truck, dumping an even layer of rock onto the hot asphalt. The asphalt seals the road, and acts as a glue for the rocks.

“It’s specially engineered to be the perfect shape and size. It’s a crush granite we use,” said Kennedy.

Those rocks become the new surface for the road.

“It’s a new surface, a very thin surface,” he said.

Then three heavy machines with rollers try to pack down the aggregate, before cars do the real work. By driving on the rock for several days, the rock gets crushed into a new surface. After a week, a street sweeper comes back and cleans up the extra.

“It can be done several times. Typically, you’ll build a street, and than maybe one, two, or three times you can put a seal-coat on it. Then you’ll need to do something more aggressive,” said Kennedy, like grinding off the top layer and put down a new pavement.

“On an arterial, or a downtown, heavy-trafficked street, a good 7 to 10 years it extends that life cycle. On a residential street, it might give more than that,” he said.

So, if you do it several times, it can extend a road with a 20-year life into a road with a 50-year life.

The cost is the main benefit. According to Kennedy, it costs about $60,000 to do a mile of seal-coated pavement.

“To reconstruct the street would be like 60 times as much,” he said. “It’s really very cheap.”

Cities and suburbs throughout Minnesota do the same process, because it’s a cost-effective form of maintenance for the roads.

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