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The map shows the potential reductions in overall GHG emissions from the transportation sector, state by state, that could be achieved by policies emphasizing the use of stiffer road surfaces. (MIT)

Paving the way to improved truck fuel efficiency

Reducing greenhouse gas (GHG) emissions and improving fuel efficiency are much-sought-after goals in the commercial-vehicle industry. Various methods have been proposed to achieve these objectives – often centered around advanced technology on the truck itself – but researchers at Massachusetts Institute of Technology (MIT) focused their attention to what’s beneath the vehicle: the road surface.

The weight of heavy-duty trucks and buses causes a slight deflection in the road surface that leads to efficiency loss. A recent study by MIT postdoc Hessam Azarijafari, research scientist Jeremy Gregory, and principal research scientist in the Materials Research Laboratory Randolph Kirchain, theorizes that changes in road resurfacing practices could alleviate that inefficiency and potentially eliminate a half-percent of the total GHG emissions from the transportation sector in the process.

The findings, detailed in a paper in the journal Transportation Research Record, examined state-by-state data on climate conditions, road lengths, materials properties, and road usage, and modeled different scenarios for pavement resurfacing practices. One key to improving fuel efficiency is to make pavements that are stiffer, Kirchain explained. That reduces the amount of deflection, which decreases road wear but also reduces the slightly uphill motion the vehicle constantly makes to rise out of its own depression in the road.

Challenges of changing the road system are unlike other strategies that require the involvement of millions of users. According to the MIT researchers, improving the road characteristics needs the attention of the industry and government. “The construction industry has been known as one of the most conservative sectors in terms of changing from traditional practices. To implement this practice widely, it is important to expand the scope of industry and agencies’ decision-making from considering only the construction cost to the ‘lifecycle’ cost,” the researchers told Truck & Off-Highway Engineering.

With the lifecycle approach for pavements, the researchers explained that vehicle fuel consumption is incorporated into the decision-making process. Also, the emissions and cost of pavement use will be considered; it can induce larger benefits for a given payback period. Looking to the future, Kirchain shared in a release from the MIT News Office that while projections show a slight decline in passenger-car travel over coming decades, they show an increase in truck travel for freight delivery – enough so where pavement deflection could be a factor in overall efficiency.

Making roads stiffer
There are several ways to make roadways stiffer, the researchers said. One method is to add a very small amount of synthetic fibers or carbon nanotubes to the mix when laying asphalt. Just a tenth of a percent of the “inexpensive material” could dramatically improve its stiffness, they said. Another way of increasing rigidity is to adjust the grading of the different sizes of aggregate used in the mix, to allow for a denser overall mix with more rock and less binder.

“If there are high-quality local materials available” to use in the asphalt or concrete mix, “we can use them to improve the stiffness, or we can just adjust the grading of the aggregates that we are using for these pavements,” said Azarijafari. And adding different fibers is “very inexpensive compared to the total cost of the mixture, but it can change the stiffness properties of the mixture significantly.”

Yet another way is to switch from asphalt pavement surfaces to concrete, which has a higher initial cost but is more durable, leading to equal or lower total lifecycle costs. Many road surfaces in northern U.S. states already use concrete, but asphalt is more prevalent in the south. There, it makes even more of a difference, because asphalt is especially subject to deflection in hot weather, whereas concrete surfaces are relatively unaffected by heat. Just upgrading the road surfaces in Texas alone, the study showed, could make a significant impact because of the state’s large network of asphalt roads and its high temperatures.

Kirchain, who is co-director of MIT’s Concrete Sustainability Hub, explained that in carrying out this study, the team is “trying to understand what are some of the systemic environmental and economic impacts that are associated with a change to the use of concrete in particular in the pavement system.” Even though the effects of pavement deflection may seem miniscule, he said, “when you take into account the fact that the pavement is going to be there, with thousands of cars driving over it every day, for dozens of years, a small effect on each one of those vehicles adds up to a significant amount of emissions over the years.”

For purposes of this study, the team looked at total emissions over the next 50 years and considered the reductions that would be achieved by improving anywhere from 2 to 10% of road surfaces each year. With a 10% improvement rate, they calculated that a total of 440 megatons of carbon dioxide-equivalent emissions would be avoided over the timeframe, which is about 0.5% of total transportation-related emissions for this period.

Strategy implementation
The proposal may face some challenges, because changing the mix of materials in asphalt might affect its workability in the field, perhaps requiring adjustments to the equipment used. “That change in the field processing would have some cost to it as well,” Kirchain said. But overall, implementing such changes could in many cases be as simple as changing the specifications required by state or local highway authorities. “These kinds of effects could be considered as part of the performance that’s trying to be managed,” Kirchain said. “It largely would be a choice from the state’s perspective, that either fuel use or climate impact would be something that would be included in the management, as opposed to just the surface performance of the system.”

Unlike emerging technologies for GHG reduction of vehicles, which are under development or in the process of scale-up, stiffening pavements can be widely implemented quickly, after reaching an agreement and enactment by the industry and governments, the researchers shared with TOHE. There is larger potential for those states with a warmer climate to reduce their GHG emissions via this strategy. Also, as rural interstates annually service a larger number of heavy trucks, it would be beneficial to prioritize the change for these road types, the research team explained.

The research was supported through the Concrete Sustainability Hub by the Portland Cement Association and the RMC Research & Education Foundation. The paper, titled “Potential Contribution of Deflection-Induced Fuel Consumption to U.S. Greenhouse Gas Emissions,” is available for purchase.

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