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The backbone of America's economy is its vast roadway system.
Our roads connect people and goods from one end of the country to the other
and asphalt is key to facilitating that connectivity.
More than 18 billion tons of asphalt pavement
are in place on American roads. Asphalt offers a high-performing, strong,
smooth, durable roadway surface for the traveling public.
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Long-lasting, durable asphalt pavements are the result of collaborative partnerships between
researchers, manufacturers, paving contractors, and agencies.
These groups work together to engineer and develop optimal pavement and mix designs,
materials, and equipment, to improve asphalt durability and performance.
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In addition, they engage in research and collaboration about best
construction practices and techniques to enhance asphalt's performance.
When striving to make asphalt pavements more durable, research and lessons learned show
density is an important factor.
William Pine: "There's a very direct relationship the higher we go in
density, the lower we go in permeability, which keeps water and air out of the
pavement structure to prevent moisture damage from a stripping standpoint to
reduce oxidation of the binder that we've coated the aggregate with.
From a stability standpoint, any mix is stronger to a point the higher density we achieve in place."
The Federal Highway Administration directed an initiative to
examine the impact of improved density on the durability of asphalt pavements.
The study leveraged the collaboration between state DOTs, industry partners,
and the National Center for Asphalt Technology at Auburn University.
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Randy West: "We're actually working on that now.
We're working on some projects around the country where we try to work
with contractors to reach higher density levels, and so that we can
follow those projects and see what the impact in performance is."
Danny Gierhart: "And we have found through studies that if we get about a percent more density,
we can add 10 percent more longevity in the field.
So, we can add a lot of years just by getting proper density in the field."
Achieving proper density starts long before the paving equipment reaches the construction site.
Bill Vavrik: "So density is a construction issue and it really starts at the plant.
It starts with making the mix in the right way, with loading the trucks the right way,
so that you don't have segregation -
the big rocks and the little rocks are all mixed together real well
and when they get to the paver,
they come down in a nice uniform way."
Debbie Schwerman: "There are some things we can do in a lab to predict that.
That'll tell us this mix either consolidates too quickly,
it's going to take a little bit more effort and time to achieve
adequate compaction in the field. So a lot of that again comes
with really knowing your materials."
Once the asphalt reaches the paving site, it's important the
contractor follow best construction practices.
From properly applying a tack coat,
to correct asphalt placement, to appropriate roller operation,
each step works toward the goal of providing optimal pavement density.
"And then it comes down to good paving practices and good compaction practices
by the contractor to make sure that they are using a good rolling pattern
that they understand how many passes it takes with the rolling train
that they have, to reach the highest level of density they can."
Buzz Powell: "There's a strong relationship between the cooling temperature of the mat
and your ability to densify the mat. So there's a range of temperatures
behind the paper where you can go in and aggressively do break down
rolling and achieve a big bump in density, behind the screed. And then
as the mat cools, you'll come in potentially with some intermediate
rollers, like pneumatic rubber tire rollers, and knead the mat as it cools
and then you come back with a finished roller and take the marks out of the
pavement. And you get an increase in density with all three
stages of compaction, but the biggest opportunity to increase the density is
with break down rolling right behind the paver."
Across the asphalt industry, new technologies help contractors deliver a
high-performing product and ensure agencies
make good use of taxpayer dollars.
One tool to help monitor asphalt mat temperature is infrared scanning technology.
"That really gives us an opportunity - we know temperature is that critical factor
in getting density. So if we monitor the temperature at the back of the paver,
we know what the product is that's hitting the road.
We know how closely we need to keep those rollers,
where they need to stay to ensure that we get a long-life product."
There are also innovations that help improve roller operation and provide
real-time data to the operator.
Dan Staebell: "Intelligent Compaction has been a really a good add to our construction industry.
We understand where we've traveled, where we've been
and where we need to go. So it is another important tool for
our contractors to have in their arsenal to obviously achieve what we're all after."
A survey of asphalt mix producers found that more than 99% of the asphalt
pavement material removed from roads and parking lots in 2017 was put back to
productive use in new pavements.
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But how does the use of reclaimed and recycled materials
affect the density of roadway pavements?
Richard Willis: "So the use of recycled materials really doesn't impact density
when we put recycled materials into an asphalt mixture
the mix can become a little bit stiffer, but there're
technologies such as recycling agents and using softer binders allow
contractors and agencies to still get great compaction along a mix with
recycled materials, which will lead to better performance and better durability."
As the asphalt industry moves forward, partnership and clear communication
among agencies across the country as well as between those agencies
and contractors are imperative.
Ben Bowers: "I think it's really important that DOTs specify
their target so exactly what they're looking for, whether that's a percentage
of the maximum theoretical density that's achievable, or a percentage
density of what can be achieved based on the roller pattern that's established
through the test section, you want to make sure there's no lack of clarity for your
contractors as well as the inspectors that are out there on-site.
So that's one really important thing to write into and make clear in your specifications."
Increasing asphalt durability and longevity benefits everyone -
contractors, agencies, and the driving public.
"So agencies and contractors can work together to implement ways to increase density
which will increase our pavement performance and allow us to get more life out of the mix."
For more information about asphalt best practices and emerging technologies,
visit the Federal Highway Administration's website.
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