New road paving mix is strong, but spotty

Article by: LAURIE BLAKE , Star Tribune
Updated: November 25, 2011 - 10:42 PM

Blotches that formed after resurfacing jobs in Inver Grove Heights and Eagan will be fixed; hopes for the product remain high.

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A longer-lasting, more expensive blacktop experts liken to a "granite countertop on the road" just made its debut on two heavily traveled metro highways. But it's so new that an unexpected problem has already popped up.

The new material was installed this fall on Interstate Hwy. 35E in Eagan and Hwy. 52 through Inver Grove Heights, and to the dismay of the Minnesota Department of Transportation, the coarse new pavement has developed unwanted smooth surface spots in dozens of locations.

The spots look like dark blotches on the pavement. There's a concern that they might be slippery for drivers, and the department is requiring the paving company, Hardrives Inc. of Rogers, to fix them at the firm's expense.

"The structure of the road has not been compromised. This spot is a very, very thin film on the very top," said Dave Herzog, resident engineer of MnDOT's Mendota Heights construction office.

But, said Herzog: "The appearance is not good. We need to make a correction to provide a consistent pavement characteristic. I want to be sure slipperiness is not an issue."

Hardrives thinks the spots were caused by oil sticking to the cellulose fibers used to hold the mix together, said division manager Tony Kieger.

The so-called "stone matrix asphalt" material originated in Europe and was introduced in the United States in the 1990s. Wisconsin put a strip on I-94 between Madison and Milwaukee in 1993.

The pavement is made with high-quality, even-sized crushed rock that is bound together by a mixture of oil, cellulose strands and mineral dust without the sand found in traditional asphalt. The concentrated stone-on-stone contact makes it strong. A secondary benefit is that the pavement reduces tire noise and tire spray.

"When it's raining you can actually be traveling behind a semi and see," said Judie Ryan, engineering specialist in asphalt mixtures for the Wisconsin Department of Transportation's Bureau of Technical Services.

Wisconsin has been happy with the stone matrix asphalt and uses it primarily for interstates "because it stands up to higher traffic loads," Ryan said. Drivers can find segments of it just over the Minnesota border on I-94, she said. It lasts at least as long as regular asphalt and does not have the cracking problems of traditional asphalt, she said.

The spots are an indication that some of the oil in the mix has separated and risen to the top, she said. Wisconsin contractors went through a learning curve during which there were also spots to correct and they had to develop the art of compacting to avoid the spots, Ryan said. Wisconsin's remedy to eliminate slipperiness from the oil spots was to blast them with sand. Eventually the spots wore away, she said.

'Granite countertop'

Hardrives had a $6.8 million contract to resurface 4 miles of I-35E, between Hwy. 13 and Lone Oak Road in Eagan and a $1.3 million contract to resurface 3.7 miles of Hwy. 52 between I-494 and Hwy. 55 in Inver Grove Heights.

Kieger said the company learned a lot about how to mix and handle the asphalt in its first experience with it.

"We had to slow down our paving speed to lay the material down -- about half the speed we normally use for the mixes we pave with," Kieger said. "We used three different rollers on it to put it in place."

Under the spots, the surface is solid, Kieger said. "It is nothing but granite -- it's a tough mix. It's like a granite countertop on the road."

The company intends to fix any spots 10 feet long or longer that might reduce skid resistance, Kieger said.

A first attempt to remove the spots by scraping them off with the blade of a Bobcat failed, but grinding will certainly remove them, Herzog said.

Company to cover cost

Unless it proves more difficult than expected, Hardrives will cover the cost, Kieger said. "At this point we are going to be good citizens."

The spots have not soured MnDOT on the asphalt, said Dave Van Deusen, metro district materials and pavement engineer. It has performed well on smaller projects and Wisconsin's experience has shown the value of the pavement, Van Deusen said.

"The hope is that we can use these experiences to improve the process for next time."

Stone matrix asphalt costs 45 percent more than traditional asphalt -- $87 per ton versus about $60 per ton for regular asphalt, Van Deusen said. For the extra cost MnDOT expects it to stand up to traffic several years longer and will monitor its performance.

Laurie Blake • 952-746-3287