Study finds simple changes in road resurfacing practices could improve gas mileage for heavy vehicles and reduce greenhouse gas emissions.
A big truck roars by. A deep rumble. Feel the house shake. The reason? In part, the weight of heavy vehicles causes a slight deflection in the road surface under them—enough of a dip to make a difference to the trucks’ overall fuel efficiency.
A theoretical study by Massachusetts Institute of Technology (MIT) researchers suggests that small changes in roadway paving practices could reduce that efficiency loss, potentially eliminating a half-percent of the total greenhouse gas emissions from the transportation sector, at little to no cost. The findings are detailed in a paper in the journal “Transportation Research Record“, by Hessam Azarijafari, Postdoc-MIT; Jeremy Gregory, Research Scientist-MIT; and Randolph Kirchain, Principal Research Scientist, Materials Research Laboratory-MIT and Co-Director-Concrete Sustainability Hub (CSH)-MIT. The study modeled different scenarios for pavement resurfacing practices based on examinations of state-by-state data on:
• Climate conditions
• Road lengths
• Materials properties
• Road usage
Kirchain explained that they found that
one key to improving mileage efficiency:
“make pavements stiffer”.
Results are amount of deflection reduced, reducing wear on the road, as wells as the slight uphill motion the vehicle constantly makes to rise out of its own road depression. He stated that when individuals walk on pavements, they feel perfectly rigid because pavements “do not respond to us”. But, that is not the case for trucks because there is enough of a deflection in that surface that some amount of energy is expended to overcome the little divot that is created as the truck drives along. He compared it to walking on a hard surface versus walking on sand, which takes more effort because you sink in with each step.
Kirchain said that while projections show a slight decline in passenger car travel over future decades, they show an increase in truck travel for freight delivery—where pavement deflection could be a factor in overall efficiency.
The researchers said that there are several ways to improve roadways’ stiffness:
• 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.
ISCP articles on fibers in concrete:
Jan 2020: “Structural Synthetic Fibers in Thin Concrete Overlays: Keep Cracks Tight, Increase Load Transfer, & Reduce Slab Displacement”
Jun 2019: “Joint Activation Case Study: Fiber-Reinforced Concrete & Joint Sealant Testing”
Nov 2019: “Concrete Reinforced with Life-Extending Basalt Fibers—Meshes, Geogrid Meshes, Scrims, Rebar, Chopped”
May 2017: “TCP Fiber-Reinforced Heavy-Truck Traffic Project is First OptiPave Project in the U.S.”
May 2017: “ABSTRACT: “Mechanical Properties of Roller-Compacted Concrete with Macro-Fibers”
• 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.
Azarijafari said, “If there are high quality local materials available” to use in the concrete (or asphalt) 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. 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.”
ISCP articles on aggregate mixes:
Nov 2019: “NRRA “Research Pays Off Webinar Day” November 19—“Determining Pavement Design Criteria for Recycled Aggregate Base & Large Stone Subbase””
Jul 2018: “New Website, Videos, Paper: Tarantula Curve—Mix Designs & Enhancing Aggregates”
Aug 2015: “New FHWA Tech Brief: “Blended Aggregates for Concrete Mixture Optimization . . . Best Practices for JCP”“
• 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. But in the south, asphalt is especially subject to deflection in hot weather, whereas concrete surfaces are relatively unaffected by heat. The study showed that by just upgrading the road surfaces [to concrete] in Texas alone—due to its high temperatures—could make a significant impact on the state’s large network of asphalt roads.
Kirchain said that in carrying out this study, the team is “trying to understand what some of the systemic environmental and economic impacts are that are associated with a change to the use of concrete in particular in the pavement system.” Some challenges may be faced in changing the mix of materials in asphalt because it might affect its workability in the field, perhaps requiring adjustments to the equipment used. Kirchain added, “That change in the field processing would have some cost to it as well.”
He said “if you take into account that the pavement will be in place for [40-50] years with thousands of cars driving over it daily, a small effect on each one of those vehicles adds up to a significant amount of emissions over the years.” 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.
The team calculated: With a 10% improvement rate, a total of 440 megatons of carbon dioxide-equivalent emissions would be avoided over the 50 years—about 0.5% of total transportation-related emissions for this period. In many cases, changes could be as simple as changing the specifications required by state or local highway authorities. Kirchain stated that various effects could be considered part of the managed performance. It would largely be a choice from each state to include either fuel use or climate impact in the management, as opposed to just the surface performance of the system.
The research was supported through the Concrete Sustainability Hub by the Portland Cement Association (PCA) and the Ready Mixed Concrete (RMC) Research and Education Foundation.
Please see two articles:
MIT News article titled “Stiffer roadways could improve truck fuel efficiency”: http://news.mit.edu/2020/stiffer-roadways-improve-truck-efficiency-emissions-0611
Science Daily online news article titled “Stiffer roadways could improve truck fuel efficiency”: https://www.sciencedaily.com/releases/2020/06/200611183929.htm