Stiffer roadways could improve truck fuel efficiency

Date:
June 11, 2020
Source:
Massachusetts Institute of Technology
Summary:
A theoretical study 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.



FULL STORY ========================================================================== Every time you hear a deep rumble and feel your house shake when a
big truck roars by, that's partly because the weight of heavy vehicles
causes a slight deflection in the road surface under them. It's enough
of a dip to make a difference to the trucks' overall fuel efficiency.


==========================================================================
Now, a theoretical study by 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 MIT postdoc Hessam Azarijafari, research scientist
Jeremy Gregory, and principal research scientist in the Materials Research Laboratory Randolph Kirchain. The study examined state-by-state data on
climate conditions, road lengths, materials properties, and road usage,
and modeled different scenarios for pavement resurfacing practices.

They found that that one key to improving mileage efficiency is to make pavements that are stiffer, Kirchain explains. That reduces the amount of deflection, which reduces wear on the road but also reduces the slightly
uphill motion the vehicle constantly has to make to rise out of its own depression in the road.

"When we as individuals walk on pavements, they seem like perfectly
rigid things. They're not responding to us," he says. "But for trucks,
that is not the case. There is enough of a deflection in that surface
that some amount of energy is expended to overcome the little divot that
you create as you drive along." He likens it to the difference between
walking on a hard surface versus walking on sand, which takes more effort because you sink in with each step.

Looking to the future, Kirchain says that while projections show a slight decline in passenger car travel over coming decades, they show an increase
in truck travel for freight delivery -- the kind where pavement deflection could be a factor in overall efficiency.



========================================================================== There are several ways to make roadways stiffer, the researchers
say. One way 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 say. Another way of increasing rigidity is simply 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," says 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,
says 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 tiny, he says, "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, so 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, they looked at total emissions over the next 50
years and considered the reductions that would be achieved by improving anywhere from 2 percent of road surfaces to 10 percent each year.

With a 10 percent improvement rate, they calculated, a total of 440
megatons of carbon dioxide-equivalent emissions would be avoided over
the 50 years, which is about 0.5 percent of total transportation-related emissions for this period.

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 says.

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 says. "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."

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by David
L. Chandler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hessam AzariJafari, Jeremy Gregory, Randolph Kirchain. Potential
Contribution of Deflection-Induced Fuel Consumption to
U.S. Greenhouse Gas Emissions. Transportation Research Record:
Journal of the Transportation Research Board, 2020; 036119812092616
DOI: 10.1177/ 0361198120926169 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200611183929.htm

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