Why does the city ‘repair’ streets by covering them in gravel?

You’re drenched in sweat, city pools are closed for lack of lifeguards and the stench of asphalt is in the air. Welcome to summer in America’s coolest city.

Austin’s hot, dry summer creates ideal conditions for hardening fresh asphalt. So the city’s Public Works Department toils through the sweltering heat to resurface as many miles of roads as possible from May through September.

That left a KUT listener wondering.

“I wanted to know why it is that we seem to repave roads by throwing out what looks like a lot of gravel or tar balls on to the road,” Abigail Norman asked our ATXplained project. “I noticed that it often seems to happen in the summertime. So I wondered: Is this a Texas thing where it’s so hot here that we just throw out little balls of tar and then it melts into the road?”

She’s not the only one curious. The city gets a lot of complaints about gravel on the road.

Across town on a blazing hot day, a crew of city workers was applying something called a seal coat on Hancock Drive in North Austin. Seal coat extends the life of a street, so the surface doesn’t have to be completely milled out and repaved.

First, a street sweeper brushed away leaves and pebbles. Workers lined the street, spaced about 75 feet apart and holding giant rakes.

“We start at 6 in the morning. We get out on the street as early as possible,” said Kenneth Picou, one of the workers holding a rake. “The last thing you want is heatstroke, man. Some people don’t recover from that.”

After the street was swept, a truck drove slowly spraying a hot, black oil that smelled like asphalt. The liquid was a combination of asphalt cement globules, water and a chemical to help them combine. Workers call it an asphalt emulsion.

“It’s an adhesive that the rock sticks to,” Picou explained.

A tanker truck spreading a wide strip of hot, black asphalt emulsion on the street. A little bit of steam or smoke can be seen rising from the freshly spread black oil-like substance. In the background, three city workers in reflective safety vests are talking to each other about something.

An asphalt distributor truck sprays a hot black asphalt emulsion on the street, the second stage in the process of applying a seal coat to Valley Oak Drive.

A few seconds later, a chip spreader followed, pouring gravel over the asphalt. Picou used his rake to make sure the gravel was spread evenly.

This was the gravel Abigail Norman was wondering about!

A close-up of a yellow vehicle spreading gravel over the freshly spread asphalt emulsion

After the asphalt emulsion is sprayed on the ground, a chip spreader drops crushed rock along the wide strip of black adhesive.

Finally, a pneumatic tire roller — kind of like a steam roller but with four tires instead of one giant steel wheel — pressed the gravel into the asphalt.

A pneumatic tire roller -- like a steam roller but with four tires instead of one giant steel wheel -- is driving on top of the gravel to press it into the asphalt emulsion. Behind the roller is an orange dump truck. This is happening on a residential street with single-family homes and towering trees.

A pneumatic tire roller pushes crushed rock into the asphalt emulsion along Valley Oak Drive.

So there we were. A street covered in gravel. Underneath the gravel is a hot asphalt glue.

This is when people start to call 311.

“That’s the main calls that we get. That there’s a lot of loose gravel. ‘Please come sweep it up,'” said Janae Spence, a division manager who oversees paving operations for the the Public Works Department.

Why do they need all that gravel?

“When a street is in a type of condition where it gets too smooth, we need to make sure that we still have that wet weather friction for cars,” Spence said, explaining the rough surfaces of the rocks increase traction. “That’s why we add the gravel.”

Janae Spence, a division manger with Austin's Department of Public Works, is speaking to reporters in front of someone's lawn on Valley Oak Drive. She's wearing a white construction hat and reflective yellow safety vest. Spence is gesturing with her right hand as she explains the process of seal coating a street.

Public Works division manager Janae Spence explains the seal coat process on Valley Oak Drive.

“In the first application, it will seem like a lot of loose gravel,” Spence said. “We come back the next day [and sweep it up]depending on how much is still there. We’ll come back again maybe in a week. And it all depends. We come back and look at it periodically.”

The street takes a long time to cure completely. After four to six weeks, the road’s surface is a lot more uniform.

A before and after image from a city presentation slide showing how seal coat changes the street's surface. The text on the slide reads, "Seal Coat. Purpose: seal cracks to keep water out; protect surface from aging." The "before" image on the left shows a street with some cracks filled in with a black sealant. The "after" image on the left shows are more darkly colored surface free of cracks.

This image from a Public Works Department presentation shows a street before and after a seal coat is applied and allowed to cure.

“I think we have listened to the public and we’ve gone to a smaller rock size that still helps it be a bit more smooth while adding enough friction,” Spence said. The city made the change about eight years ago.

“I appreciate that they’re listening to feedback and using smaller gravel,” Norman said when I explained everything I learned about the seal-coating process. “I’ve learned a lot and it is so interesting to me that it really does only happen during the summer months.”

The City of Austin plans to apply seal coat to 300 miles of roads by the end of September.

A tanker truck called an asphalt distributor vehicle trails off down the road leaving behind a wide black strip of asphalt emulsion.

A tanker truck called an asphalt distributor vehicle trails off down Valley Oak Drive leaving behind a wide black strip of asphalt emulsion.

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