When the tires of a car or truck roll over a roadway, the maximum pavement deflection is just behind the path of travel. This has the effect of making the vehicle’s tires roll continuously up a slight slope (exaggerated in this illustration), increasing the vehicle’s fuel consumption. Image: Mehdi Akbarian |
A
new study by civil engineers at Massachusetts Institute of Technology (MIT)
shows that using stiffer pavements on the nation’s roads could reduce vehicle
fuel consumption by as much as 3%—a savings that could add up to 273 million
barrels of crude oil per year, or $15.6 billion at today’s oil prices. This
would result in an accompanying annual decrease in carbon dioxide emissions of
46.5 million metric tons.
The
study, released in a recent peer-reviewed report, is the first to use
mathematical modeling rather than roadway experiments to look at the effect of
pavement deflection on vehicle fuel consumption across the entire United States
road network. A paper on this work has also been accepted for publication later
this year in the Transportation Research Record.
By
modeling the physical forces at work when a rubber tire rolls over pavement,
the study’s authors, Professor Franz-Josef Ulm and PhD student Mehdi Akbarian,
conclude that because of the way energy is dissipated, the maximum deflection
of the load is behind the path of travel. This has the effect of making the
tires on the vehicle drive continuously up a slight slope, which increases 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% increase in aggregate fuel consumption leaves a substantial environmental
footprint. Stiffer pavements—which 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
thickness or composition of the sublayers of the road—would decrease deflection
and reduce that footprint.
“This
work is literally where the rubber meets the road,” says Ulm, the George Macomber Professor in the
Department of Civil and Environmental Engineering. “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. 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.”
The
new study defines the key parameters involved in analyzing the structural
(thickness) and material (stiffness and type of subgrade) properties of
pavements. The mathematical model is therefore based on the actual mechanical
behavior of pavements under load. To obtain their results, Ulm and Akbarian fed their model data on
5,643 representative sections of the nation’s roadways taken from Federal
Highway Administration data sets. These data include information on the surface
and subsurface materials of pavements and the soils beneath, as well as the
number, 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 say that up to 80% 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.
“We’re
wasting fuel unnecessarily because pavement design has been based solely on
minimizing initial costs more than performance—how well the pavement holds up—when
it should also take into account the environmental footprint of pavements based
on variations in external conditions,” Akbarian says. “We can now include
environmental impacts, pavement performance and—eventually—a cost model to
optimize pavement design and obtain the lowest cost and lowest environmental
impact with the best structural performance.”
The
researchers say the initial cost outlay for better pavements would quickly pay
for itself not just in fuel efficiency and decreased carbon dioxide emissions,
but also in reduced maintenance costs.
“There’s
a misconception that if you want to go green you have to spend more money, but
that’s not necessarily true,” Akbarian says. “Better pavement design over a
lifetime would save much more money in fuel costs than the initial cost of
improvements. And the state departments of transportation would save money
while reducing their environmental footprint over time, because the roads won’t
deteriorate as quickly.”
This
research was conducted as part of the Concrete Sustainability Hub at MIT, which
is sponsored by the Portland Cement Association and the Ready Mixed Concrete
Research & Education Foundation with the goal of improving the environmental
footprint of that industry.
“This
work is not about asphalt versus concrete,” Ulm says. “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.”
