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Pavement’s Moonshot

Yet innovation was a concept so alien to asphalt that Kulash, who became SHRP’s executive director, despaired of finding able researchers. “There was very little going on,” he says. “We were concerned there wouldn’t be enough technical capacity.” But this was to be pavement’s moonshot. So, under the leadership of Tom Kennedy at the University of Texas, Austin, engineers from academia and industry pulled together for a six-year effort to find better ways to build pavement.

The outcome of all the research is Superpave, which stands for superior performing asphalt pavements. Superpave isn’t a single supermaterial or one key insight. Instead, it’s a new set of testing equipment and protocols for mixing up roads guaranteed to hold up to the weather and traffic of a specific location.

Take asphalt, also known as “hot oil” by the refinery workers who produce it as a byproduct of the catalytic cracking that frees gasoline and heating fuel from crude. Before Superpave, contractors from northern Maine to the Virginia line all used the same grade of asphalt. That’s because they tested only its viscosity, to see how well it would ward off rutting in summer.

But in the Northeast, pavement cracks open from the cold more often than it ruts from the heat. With Superpave, explains Connecticut Department of Transportation (DOT) materials engineer Nelio Rodrigues, asphalt now has to “jump through a hoop” by passing performance tests at both high and low temperatures. One test subjects glassy room-temperature asphalt to a machine known as the “dynamic shear rheometer.” Adapted by the SHRP researchers from a model that measures the consistency of cream cheese, the device applies a twisting force to gauge asphalt’s capacity to resist deformation. Another test asks chilled asphalt sticks laid across tiny andirons to support a weight. If they crack, the asphalt isn’t right for cold regions.

“Looking at low temperatures is a great step forward,” says April Swanson, a research chemist at Amoco’s Whiting, Ind. refinery. “Until Superpave, there was no practical method for doing it. Now we’re really measuring engineering properties.”

Under Superpave, Whiting technicians test the 40,000 barrels of asphalt produced by the plant each day, grading batches with labels such as “Performance Grade 58-28.” The label means the asphalt is rated to perform its duties from a sizzling 58 C down to -28 C. Swanson says Amoco now adds an assortment of polymers to meet the higher grades. To select the perfect asphalt for a paving job, engineers look up the temperature extremes at their construction site using National Weather Service data.

Choosing the ideal asphalt is only half the game. Engineers then need to stir in the right mix of rock, or aggregate. Before Superpave, road builders intuitively felt they had a solid grasp on what proportions of coarse and fine stone made for a good pavement. “Everyone was using what they considered to be tried and proven’ mixes,” says Kulash. “But they weren’t the same things. There was no gospel.”

Superpave combines the best existing know-how into a new scripture for “volumetric mix design.” Lengthy tables define the ideal combination of stones, air pockets and asphalt, depending on the amount of traffic expected. To make sure theory holds up in practice, engineers use another SHRP invention-the Superpave gyratory compactor-to knead a paint-can sized container of pavement under high pressure. The resulting specimens show what the road surface will look like after being compacted by rollers and the years of traffic that follow.

If the blocks fail performance exams, engineers tweak the mix design by changing the stones or adding a better asphalt. Instead of the one-size-fits-all designs of the past, says Rodrigues, with Superpave each road wears a custom-made pavement.

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