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Boosting Fuel Economy Where The Rubber Meets the Road

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Illustration courtesy of MIT/Mehdi Akbarian
Pavement deflection produces the effect of a car rolling uphill, hurting fuel economy.
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Fuel economy has been largely the province of automakers and government initiatives, such as Corporate Average Fuel Economy standards. However, scientists at the Massachusetts Institute of Technology are looking at ways to boost fuel efficiency and cut greenhouse- gas emissions by building better roads.

Pavement design can play an important role in fuel economy, researchers say. "The understanding has always been that roughness matters more than deflection" when it comes to fuel usage, noted Mehdi Akbarian, a doctoral candidate who spoke last month at an open house for MIT's Concrete Sustainability Hub (CSH) in Cambridge. "But we are showing that it depends on the scenario—the location of the pavement, the [road] design."

CSH, a research group funded by cement and concrete producers, is looking to reduce concrete's environmental impact, increase its performance and improve its production processes. Led by CSH Director Franz-Josef Ulm, scientists analyzed the Federal Highway Administration's Long-Term Pavement Performance (LTPP) data to determine how pavement roughness and deflection affect fuel consumption and how those properties interact with factors such as top-layer thickness and substrate.

Roughness generally accounts for more fuel use, but researchers found that deflection still wastes 180 million gallons each year on the nation's interstate system. By comparison, roughness accounts for about 555 million gallons of fuel waste. Because interstates make up only about 2% of the nation's roads, they represent just a fraction of the total picture.

By switching to stiffer pavements, highway authorities could boost vehicle fuel economy by up to 3%, MIT says. Concrete is naturally more rigid than asphalt, but that doesn't necessarily mean that all highways need be paved with concrete. MIT has developed a model to help engineers optimize pavement designs.

Although the LTPP data show no significant overall difference in roughness between asphalt and concrete, there is variation according to the geographical location of the road, MIT found.

"Concrete becomes rougher in colder temperatures," says Akbarian. "It's more sensitive to freezing and thawing than asphalt. The opposite is true for asphalt: Higher temperature affects asphalt's roughness more than with concrete." Hence concrete could be a better road material for warmer areas, researchers say. Scientists next want to compile data on state roads, and they're looking to state transportation agencies for that data.

Life-cycle costs are another area that MIT is studying. Arash Noshadravan, a postdoctoral associate at MIT, is examining how large a role factors such as traffic volume, climate and soil play into long-term road costs. He is working on a case study of an urban interstate highway in Missouri that is subject to freeze-thaw conditions. Noshadravan requested maintenance data from road maintainers and expects to release the results soon. CSH also has examined how forecasting fluctuations in materials prices could improve decisions compared to "no change" forecasts that assume constant prices.

CSH is now ready to bring some of its data to the states, according to its executive director, Hamlin Jennings. The data is in what he calls the "bread board" state—that is, it's in a usable format, but better software tools could vastly improve its usefulness. CSH researchers are working with people in other MIT departments to improve data usability.

Meanwhile, research has been continuing in the building sector. MIT is looking at optimizing passive thermal mass and energy efficiency, which depends partly on climate. Whether a building is in a warm or cold climate, "you get the biggest bang for the buck from concrete walls in the off-season," said Randa Ghattas, research associate. Upcoming projects for buildings include prioritizing data collection and developing algorithms to determine how the costs of construction for various materials change as mass production becomes more common.

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