Where the Rubber Meets the Road
A new recipe for asphalt could give highway engineers the tools to repave the world.
Americans are prodigious drivers. each year, the country’s wheels spin across 1.5 trillion kilometers of roadways. That’s about 5,000 km for each man, woman and child in the United States. But despite the national appetite for blacktop, pavement rarely gets a second thought. Except when it’s time to swerve around a pothole, or bump across a rut-filled intersection. Then, chances are, the irate driver will indignantly demand, “Why don’t they fix these darn roads?”
Relax, they are.
All over the country, on quiet rural lanes and congested turnpikes, highway engineers are rolling out a powerful new principle for road building known as Superpave.
Superpave is a novel way to design hot mix asphalt, the gooey-when-hot mixture of tar-colored cement and crushed stone that makes up 94 percent of U.S. pavements. “It’s an outstanding technology,” says Neil Hawks, director of special programs at the Transportation Research Board (TRB) in Washington D.C. It’s also a big change for the largely empirical art of road building. Superpave replaces “recipe” construction, based on past experience, with pavement designs rooted in a more analytical assessment of road materials’ engineering properties. Superpave’s goal: to dress the nation’s arteries in long-lasting, custom-tailored pavements.
The new system was developed in university and industry labs during an intense, one-of-a-kind research push funded by Congress in 1987. Like any laboratory technique destined for use in the real world, Superpave has taken its lumps on the way. It’s had to take on powerful industries and it has meant a steep learning curve for the local transportation departments and contractors that actually pay for, and build, new roads. In 1996, they used the system in just 95 paving jobs. But this year, Superpave has taken flight, with the number jumping to 1,339 nationwide, accounting for nearly 30 percent of the pavement laid down in the United States.
Superpaved roads don’t look any different from their empirical counterparts. But researchers predict they will last far longer, perhaps 15 years instead of 10, before needing a major maintenance job. That should save highway departments more than half a billion dollars a year, according to an estimate from the Texas Transportation Institute. More to the point for American travelers, the study figured Superpave will save another $2 billion by saving time-fewer car repairs and fewer minutes stuck behind maintenance crews.
Superpave’s origins lie in a time when disco was king, the nation’s best-selling car was an Oldsmobile, and new road projects suddenly, and mysteriously, began failing. “There had always been cases where things went wrong and people couldn’t explain it,” says Damian Kulash, president of Washington, D.C.’s Eno Transportation Foundation. “But people started to feel it was becoming much more common.” Roads were developing the signs of old age well before their time: trough-like ruts from traffic on hot days and cracks from cold weather.
There were no clear causes. Contractors were using road-building recipes that had changed little since 1872, when New York City’s Fifth Avenue welcomed the nation’s first asphalt pavements. True, traffic loads had been swelling steadily. And heavier trucks with stiffer tires were suspected of committing dastardly acts of wear and tear. Others pointed fingers at asphalt itself, a sticky residue of oil refining. The OPEC oil embargo had reshuffled supply lines, and the nature of the material had changed.
Although asphalt had been in use for thousands of years (the ancient Egyptians used it for mummification) and gravel even longer, pavement’s mechanical properties lay largely unexplored. So when the roads started failing, highway engineers didn’t know what to change. Nor were easy answers likely to emerge from the research pipeline, which had long been dry. The numbers in a gloomy 1984 report from the TRB said it all. The semiconductor industry was spending 8 percent of sales on R&D; the pharmaceutical industry 6 percent; even R&D laggards such as the food and beverage industry ranked just shy of 1 percent. The highway industry had been spending less than 0.2 percent.
The specter of infrastructure decay and the grim funding scenario spurred Congress to action. In 1987, it kicked off the Strategic Highway Research Program (SHRP), a $150 million bid to find solutions to highway engineers’ toughest troubles. Fully $50 million went to solving the pavement problem.
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.
It will be years before we know whether Superpave lives up to its name, but encouraging reports are already rolling in. In New York, contractors repaved 50 kilometers of Interstate 81 across Cortland County with two-thirds original recipe and one-third Superpave. After just one winter, according to state director of technical services Paul Mack, the old-fashioned asphalt began deteriorating. “The Superpave section looks very good in comparison,” he says.
Superpave has seen its share of technical failures and real-world resistance. Several of the asphalt tests bombed in the real world, and Amoco’s Swanson says others are proving tricky to use. Although her technicians make careful measurements, she regularly haggles over the results with the hot mix plants. These firms, which supply construction crews with raw pavement, repeat the performance tests, often with different outcomes.
Road crews also face a learning curve. The new mixtures feature larger stones and are proving tough to compact, says New York State materials engineer William Brudi. In typical road work, a paving machine crew throws down a swath of piping-hot mix and gliding steel-wheeled rollers follow about 60 meters behind, ironing it down at temperatures near 93 C. But Brudi says that’s not hot enough for Superpave. Now, on a $90 million project to construct a 9-km HOV lane on the Long Island Expressway, rollers follow hot on the tail of the paving crew, compacting the mix at a blistering 150 C. “That demands a lot more coordination of the machines,” says Brudi. “The teams aren’t used to communicating, now they have to to get it right.”
Sometimes those who resist the newfangled technology have a point. At a 1993 job near Kingman, Ariz., contractors following Superpave guidelines omitted the antistripping chemical that is conventionally used to protect the road from moisture. Today, moisture cracks snake over the road’s surface. “Our feeling is that for us to blindly accept this design procedure without using our engineering knowledge is a disservice to the citizens of Arizona,” says state DOT engineer Julie Nodes.
The reaction of folks like Nodes is critical-Superpave’s fate rides on a thumbs-up from such officials because state DOTs finance the majority of the nation’s road construction. So far, most have taken up the voluntary Superpave specifications eagerly, with Indiana, Maryland and New York leading the way. Overall, 38 states now ask contractors to use the new performance graded asphalts, and nearly half are implementing the aggregate mix design.
While the states are pavement’s prime movers, Superpave’s success also depends on buy-in from the 40,000 local governments that own and operate roadways, as well as all the myriad contractors, equipment makers and quarries that serve them. “The job of implementation isn’t nearly over,” says Gary Henderson, leader of the Federal Highway Administration’s Superpave technology delivery team. Nor is it cheap: the equipment, training and follow-on studies needed to bring Superpave out of the laboratory and into the field have already cost $150 million-three times more than the original research.
Expense aside, bringing academics, contractors and suppliers together in a common purpose may be the new system’s most vital innovation. Whereas mistrust once reigned in the fractious industry, says Kulash, “the hallmark of Superpave is that it has created a constructive environment.” And that, he believes, should speed the introduction of new technology.
At several centers, engineers continue plotting ways to make road building even less of an art. For instance, at the University of Maryland, a team led by Matt Witczak is creating computer models that, when fed with traffic estimates and raw materials’ performance properties, should accurately predict how many years a pavement will last before rutting or cracking from temperature and fatigue.
However, the future of such projects was thrown into question by the “Transportation Equity Act for the 21st Century,” which President Clinton signed into law on June 9. The six-year, $217-billion spending plan increased highway construction spending by nearly 40 percent but gutted funds for federal asphalt research and implementation, re-routing much of the money to the states.
Is the transportation act kryptonite for Superpave? Not likely. Although Superpave doesn’t have as much media glitz as a new medical cure, and may not produce as many millionaires as a Silicon Valley startup, it has the key attribute of any hero that endures-a heart for the people. It’s a solution fit for all 50 states that makes pavement last in Los Angeles or Omaha, in the heat and in the wet, good times or bad. And there’s nothing that Americans like better than a long, straight stretch of blacktop-without cracks.
Become an MIT Technology Review Insider for in-depth analysis and unparalleled perspective.Subscribe today