Source: Massachusetts Institute of Technology, Cambridge
A new MIT study shows that using stiffer road pavements could reduce vehicle fuel consumption by as much as 3 percent—a savings that could add up to 273 million barrels of crude oil per year, or $15.6 billion at current prices. This would result in an accompanying annual decrease in carbon dioxide emissions of 46.5 million metric tons.
The study is the first to use mathematical modeling rather than roadway experiments to look at the effect of concrete and asphalt pavement deflection on vehicle fuel consumption across the U.S. road network. Supporting research was conducted through the Concrete Sustainability Hub at MIT, sponsored by the Ready Mixed Concrete Research & Education Foundation and Portland Cement Association.
By modeling the physical forces at work when a rubber tire rolls over pavement, MIT Civil and Environmental Engineering Professor Franz-Josef Ulm and PhD student Mehdi Akbarian conclude that because of the way energy is dissipated, a load’s maximum deflection is behind the path of travel. This has the effect of making vehicle tires drive continuously up a slight slope, increasing fuel use. The deflection under the tires is similar to that of beach sand underfoot: With each step, the foot tamps down the sand from heel to toe, requiring the pedestrian to expend more energy than when walking on a hard surface.
On the roadways, even a 1 percent increase in aggregate fuel consumption leaves a substantial environmental footprint. Stiffer pavements reduce that footprint, decrease deflection, and can be achieved by improving the material properties or increasing the thickness of the asphalt layers; switching to a concrete layer or asphalt-concrete composite structures; or, changing the road base’s thickness or composition.
“We’ve got to find ways to improve the environmental footprint of our roadway infrastructure, but previous empirical studies to determine fuel savings all looked at the impact of roughness and pavement type for a few non-conclusive scenarios, and the findings sometimes differed by an order of magnitude,” says Ulm. “Where do you find identical roadways on the same soils under the same conditions? You can’t. You get side effects. The empirical approach doesn’t work. So we used statistical analysis to avoid those side effects.”
He and Akbarian fed their model data on 5,643 representative sections of the nation’s roadways taken from Federal Highway Administration data sets. They include information on pavements’ surface and subsurface materials, plus type and weight of vehicles using the roads. The researchers also calculated and incorporated the contact area of vehicle tires with the pavement.
Ulm and Akbarian estimate that the combined effects of road roughness and deflection are responsible for an annual average extra fuel consumption of 7,000 to 9,000 gallons per lane-mile on high-volume roads (not including the most heavily traveled roads) in the 8.5 million lane-miles making up the U.S. roadway network. They find that up to 80 percent of that extra fuel consumption, in excess of the vehicles’ normal fuel use, could be reduced through improvements in the basic properties of the asphalt, concrete and other materials used to build the roads.
The researchers say the initial cost outlay for better pavements would quickly pay for itself not just in fuel efficiency and decreased CO2 emissions, but also in reduced maintenance costs. “Better pavement design over a lifetime would save much more money in fuel costs than the initial cost of improvements,” says Akbarian. “State departments of transportation would save money while reducing their environmental footprint over time, because the roads won’t deteriorate as quickly.”
“This work is not about asphalt versus concrete,” adds Ulm. “The ultimate goal is to make our nation’s infrastructure more sustainable. Our model will help make this possible by giving pavement engineers a tool for including sustainability as a design parameter, just like safety, cost and ride quality.”