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NEWS SCOPE RESEARCH MIT engineers model rigid pavements' potential for $15 billion in fuel savings The new study defines the key parame- A new Massachusetts Institute of Technol- ogy study shows that using stiffer road pavements could reduce vehicle fuel con- sumption by as much as 3 percent—a sav- ings that could add up to 273 million barrels of crude oil per year, or $15.6 billion at cur- rent prices. This would result in an accom- panying annual decrease in carbon dioxide emissions of 46.5 million metric tons. The peer-reviewed Model Based Pavement- Vehicle Interaction Simulation for Life Cycle Assessment of Pavements is the first study 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 en- tire U.S. road network. Research supporting the findings was conducted as part of the Concrete Sustainability Hub at MIT, which is sponsored by the Portland Cement Associa- tion and the Ready Mixed Concrete Research & Education Foundation. By modeling the physical forces at work when a rubber tire rolls over pavement, study authors—MIT Civil and Environmen- tal Engineering Professor Franz-Josef Ulm and PhD student Mehdi Akbaria—conclude that because of the way energy is dissi- pated, 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 under- foot: 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 in- crease in aggregate fuel consumption leaves a substantial environmental foot- print. Stiffer pavements, which can be achieved by improving the material prop- erties or increasing the thickness of the as- phalt layers, switching to a concrete layer or asphalt-concrete composite structures, or changing the thickness or composition of the sublayers of the road, decrease de- flection and reduce that footprint. "This work is literally where the rubber meets the road," says Ulm. "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 sce- narios, and the findings sometimes differed by an order of magnitude. Where do you find identical roadways on the same soils 22 | JULY 2012 under the same conditions? You can't. You get side effects. The empirical approach doesn't work. So we used statistical analy- sis to avoid those side effects." SCHEMATIC: Figure 4-2, Model-Based PVI report ters involved in analyzing the structural (thickness) and material (stiffness and type of subgrade) properties of pave- Data sets from the FHWA's Long-Term Pavement Performance program were part of the MIT report's pavement-vehicle interaction modeling. Among tools LTPP engineers deploy is the Falling Weight De- flectometer (FWD), measuring pavement deflection response to a load of known magnitude and indi- cating the slab's structural capacity, material properties and expected service life. Upon application of a load, the FWD measures deflections at various distances from the loading point with corresponding time from the loading. The D0–D6 labels here show the progression in deflection. CHART: Figure 5-1, Model-Based PVI report Within their extensive modeling of engineering data, report authors chart maximum pavement deflec- tion yunder load and characteristic wavelength (width)Ls. Perpetual soil damping and vehicle velocity, they conclude, create a distance lag that locates the maximum pavement deflection behind the direction of travel—hence wheels moving uphill. WWW.CONCRETEPRODUCTS.COM