Self healing asphalt contains small steelwool fibers that can be heated with induction energy to seal microcracks, extending the service life of the road.
Courtesy Delft University of Technology Self healing asphalt contains small steelwool fibers that can be heated with induction energy to seal microcracks, extending the service life of the road.

‘Tis the season of the pothole—and a myriad of other problems to do with road infrastructure. Winter brings out the worst in pavement: Once water penetrates surface cracks, it expands as ice in freezing temperatures, weakening and displacing the surrounding asphalt. After thawing, the affected area remains a void for repeated water infiltration and freeze-thaw damage, eventually leading to larger cracks and potholes. According to the American Society of Civil Engineers’ (ASCE's) "2017 Infrastructure Report Card," U.S. roadways are suffering from these effects—so badly, in fact, the ASCE gave roadways a D-rating. “One out of every five miles of highway pavement is in poor condition and our roads have a significant and increasing backlog of rehabilitation needs,” the report reads. “After years of decline, traffic fatalities increased by 7 percent from 2014 to 2015, with 35,092 people dying on America’s roads.”

As a clear threat to public safely, these statistics beg the question—why aren’t roads being adequately repaired? The ASCE discloses that the federal government has been chronically underfunding highway maintenance, with a backlog today of $836 billion required for satisfactory upkeep. Thanks to the new GOP tax bill, an even more impoverished U.S. Treasury will likely exacerbate this funding backlog.

Material scientists are stepping in to help. A collection of new technologies promises to replace conventional bitumen with self-healing asphalt. For example, scientists at the Delft University of Technology in the Netherlands have created such a material by infusing asphalt with electrically conductive fibers and fillers in the configuration of closed-loop circuits. When a current is introduced near a crack, the heat generated within the circuits melts the bitumen and seals it. Similarly, researchers from ETH Zurich and the Swiss research organization Empa developed an approach in which they inject iron oxide nanoparticles into surface cracks. Once this patch material is exposed to an alternating magnetic field, the surrounding asphalt softens and heals itself—a process that only requires a few seconds per patch.

Scientists at the University of Minnesota Duluth mix magnetite-rich iron ore—found in the nearby Mesabi Iron Range—into bitumen to create a modified material that can be repaired with a special vehicle. This new compound consists of 1- to 2-percent magnetite and crushed asphalt from recycled pavement and roof shingles. Once the researchers find and patch an existing pothole with this mixture, they seal it by heating it with a microwave unit attached to a custom-built repair truck. An added benefit is the use of recycled materials in lieu of virgin material binders.

Healable bitumen
Courtesy EMPA Healable bitumen

In the report, the ASCE also highlights the need for different approaches to addressing water intrusion, including the “greater use of permeable paving materials to reduce storm runoff.” Such materials welcome water penetration, allowing it to filter through surface paving to recharge the ground below. Because pervious paving is designed with intentional voids that allow water to pass freely through the material and not linger, cold season deterioration from freeze-thaw effects is theoretically reduced. Such paving also provides the additional advantage of replenishing groundwater in the vicinity of the roadbed, thus relieving pressure on overtaxed stormwater systems. “There is a growing ‘urban surface evolution’ that is supporting the shift from gray infrastructure to green infrastructure,” writes the National Center for Sustainable Transportation in a 2015 study. “The increased use of permeable pavements is part of this movement given the many environmental, socioeconomic, and human health benefits these pavements provide, such as reduction in roadway noise, runoff, and urban heat island effect as well as improved water quality.”

With more investment, freezing can be prevented altogether. In parts of Western Japan that receive significant winter snowfall, shosetsu (snow-melting) pipes located in the crown of the roadbed deliver warm water to the street surface. Roads that are particularly important to keep clear—such as those near train stations—are heated internally with circulating hot water. Other strategies involve the placement of river channels adjacent to roadways so that fast-moving river water will carry perimeter snow away.

Smart roads can also limit surface snow and ice accumulation. Sandpoint, Idaho–based Solar Roadways offers a tile-based paving system that can be installed on top of existing road surfaces. Each hexagonal module consists of a 44-watt solar panel with integrated LED lights and heating elements protected below a high-strength tempered glass surface. (Watch a live video feed of a pilot Solar Roadways installation here.)

Another approach involves the use of conductive concrete, which—like the self-healing asphalt described above—requires the introduction of electricity to charge the road surface. In the case of a technology developed by researchers at the University of Nebraska–Lincoln, an entire road can be transformed into a heating element with 48 volts of electricity, preventing snow accumulation altogether.

Such capabilities are impressive, but at what cost? Unfortunately, conductive concrete is more than twice the price of typical roadbed construction, at $300 per cubic yard versus $120, respectively. Other smart and/or self-heated roadways are also expensive. Which brings us back to the original predicament: How can road surface integrity be ensured in light of perpetually insufficient maintenance budgets?

A promising solution might be to combine the surgical use of the material innovations outlined above with predictive technologies. For example, Bob Bennett, Kansas City, Mo.’s chief innovation officer, has developed a system of sensors and computer algorithms that target pothole-susceptible zones in his city. Working with Chicago-based technology firm Xaqt, Bennett installed pressure sensors within the roadbed of a heavily used 51-block area. The sensors, in addition to traffic light–mounted cameras, report real-time activity on the streets. This information is combined with an extensive collection of weather, construction, and repair history data to deliver a pothole prediction platform with 85 percent accuracy.

One of the ASCE’s recommendations is to consider a “road diet,” an approach that transforms existing roadways by “reducing the number of lanes and adding safety features.” Not only has such a practice proven to be safer for pedestrians and bicyclists, but it also may be fiscally necessary, given that there is currently more road than we can maintain.