In a paper published in the Journal of Cleaner Production titled “Role of the use phase and pavement-vehicle interaction in comparative pavement life cycle assessment as a function of context” by researchers at the Massachusetts Institute of Technology Concrete Sustainability Hub (MIT-CSHub)—Xin Xu, Mehdi Akbarian, Jeremy Gregory, and Randolph Kirchain—examine the use phase of pavements and calculate the influence of context on their environmental footprint. Their work finds that the “use phase”—a pavement’s operational life—is highly context-dependent.
Researchers at the MIT-CSHub:
Although the nearly 21 million miles of paved roads around the globe appear static, their environmental footprints are anything but set. When studying all stages of a road’s life using a technique called pavement life-cycle assessment (LCA), it becomes clear that a pavement’s environmental impact doesn’t end with construction. In fact, there are significant emissions associated with a pavement during its “use phase”. Several context-varying factors contribute to this footprint, making a pavement’s use phase impacts difficult to calculate:
• The pavement quality’s impact on fuel efficiency
• Lighting
• The pavement’s ability to absorb carbon dioxide through carbonation
• Climate
• Volume of traffic
Use phase can have a sizable environmental footprint, but decisions made before a pavement is even constructed can influence the size of that footprint. Jeremy Gregory, Executive Director-CSHub and an author of MIT-CSHub paper said, “It turns out that the design and maintenance of pavements indirectly impact the environment. Some of these impacts include the way that pavements impact climate through their reflectivity, through the absorption of carbon dioxide over time, through the paving materials, and by how they affect the fuel consumption of the vehicles that drive on them—pavement-vehicle interaction (PVI).”
PVI causes excess fuel consumption, and is one of the greatest contributors to use-phase pavement emissions. PVI is the interaction between a vehicle’s tires and the road it drives upon and is a multifaceted phenomenon. The important aspects of PVI:
ROUGHNESS (most apparent)—refers to irregularities in the surface of the pavement. In addition to affecting ride comfort, roughness can have a significant effect on fuel consumption. Gregory explains, “The rougher a pavement is, the more energy dissipation there is in the shock absorber system of a vehicle. A vehicle must then consume more fuel to overcome this additional energy dissipation … referred to as ‘excess fuel consumption’.”
DEFLECTION—“The weight of of very heavy vehicles, primarily trucks, makes a small indentation in the pavement so that the vehicle is always driving up a very shallow hill. Deflection also causes excess fuel consumption,” noted Gregory. Since excess fuel consumption only decreases fuel economy by a few percentage points, it isn’t that noticeable to the average driver. But, when factoring in the often thousands of vehicles that drive across a stretch of pavement every single day, these few percentage points add up! For example, California’s excess fuel consumption on highways totaled 1 billion gallons over 5 years!
CONTEXT—While roughness and deflection contribute significantly to use-phase environmental impacts, a pavement’s context is also in play. Gregory added, “When we look at the overall pavement LCA, we find that the results are very context-dependent—all of these factors combine to determine the environmental impact of a pavement”:
• Climate
• Traffic volume
• Type of pavement design
• Planned future maintenance and rehabilitation schedule
The authors of the paper selected 9 different scenarios to study the impacts of these context-specific conditions and analyzed pavements in 4 U.S. states with different climates: Missouri, Arizona, Colorado, and Florida. Within each climate zone, they then looked at roads with different traffic levels. After studying the data, they found that traffic was the most significant factor affecting pavement environmental impacts. And interstates— which have the most traffic—also had the greatest use-phase impacts—as much as 78% of total life cycle impacts.
“It turns out that for pavements with really high traffic loads, a much bigger fraction of their overall environmental impact is associated with the use phase and the excess fuel consumption of vehicles,” explains Gregory.
“[But], for pavements that have much fewer vehicles that travel on them like state and rural highways, the materials and construction are associated with most of the environmental impact,” reported Gregory. These less-trafficked roads displayed lower use-phase impacts of 38% and 37%, respectively.
The design and maintenance of a pavement also influence its environmental footprint. The roughness of high-traffic interstates pavements is their primary source of excess fuel consumption, and if not regularly maintained, an interstate’s roughness might increase, leading to greater excess fuel consumption. Since truck traffic is higher on rural and state highways than on interstates, the deflection of those pavements may have a greater impact on excess fuel consumption than roughness. So, the pavement must be designed to be stiff enough to withstand a truck’s weight to mitigate the effects of deflection.
Climate also affects the environmental footprint of a pavement’s use phase:
• Colder climates—some pavements can deteriorate more quickly due to freeze-thaw damage and can have higher roughness, increasing the excess fuel consumption.
• Warmer climates—petroleum-based pavement materials deform more easily—especially under truck traffic—increasing their susceptibility to deflection and excess fuel consumption.
Ultimately, this recent paper shows just how many contextual factors must be considered during a pavement’s use phase in order to make it as sustainable as possible. Gregory explains, “It’s important to not assume any environmental impact for any given context … you really have to run the numbers.”
For the MIT News online article, please go to: http://news.mit.edu/2019/mit-pavement-emissions-sustainability-research-0522
For the Journal of Cleaner Production abstract/paper titled “Role of the use phase and pavement-vehicle interaction in comparative pavement life cycle assessment as a function of context”, please go to: https://www.sciencedirect.com/science/article/pii/S0959652619315197
The MIT Concrete Sustainability Hub is a team of researchers
from several departments across MIT
working on concrete and infrastructure science,
engineering, and economics.
Its research is supported by the Portland Cement Association (PCA)
and the Ready Mixed Concrete (RMC) Research and Education Foundation.