All That's Trucking

For greener roads, make them stiffer

June 6, 2012

When the tires roll over a roadway, the maximum pavement deflection is just behind the path of travel. It's like continually driving up a slope (exaggerated in this illustration).Think about how much more energy it takes you to walk on a sandy beach than on a firm floor. That's essentially what's happening when cars and trucks drive on asphalt roads, say researchers, and if those roads could be made stiffer, the nation could save a lot of fuel and greenhouse gas emissions.

A study by MIT civil engineers found stiffening the nation's pavements could cut fuel use by 3%, the equivalent of 273 million barrels of crude oil, or $15 billion, per year. As a result, CO2 emissions would fall by 46.5 million metric tons per year (more than Oregon emits from burning fossil fuels annually).

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.

The researchers compare it to walking on beach sand: 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.

We all know asphalt is essentially soft; the rutting seen at busy intersections is proof of that.

Stiffer pavements 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. This would decrease deflection and reduce that footprint.

A recent science blog reports that one study found nanoclay additions (nanoparticles of layered mineral silicates) improve the shear resistance of asphalt mixtures, and believes the additions may also improve the stability of future asphalt pavements.

Of course, proponents of concrete say this means road builders should be turning to concrete, rather than asphalt: "Can Building Concrete Roads Safe 50 Million Tons of C02?"

"Better pavement design over a lifetime would save much more money in fuel costs than the initial cost of improvements," MIT says in a statement. "And the state departments of transportation would save money while reducing their environmental footprint over time, because the roads won't deteriorate as quickly."

The MIT researchers say the study 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 U.S. road network.

Read more: MIT News: Civil engineers find savings where the rubber meets the road