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America's public road network spans more than 2.75 million miles of paved roads.
This system connects every major population center and is vital to our
economy, defense, and mobility.
With fiscal constraints and increasing infrastructure needs,
agencies are incorporating life-cycle thinking into their planning.
Marshall Klinefelter: "It's just like owning a house. Everybody knows that every
eighteen to twenty years, you're gonna replace the shingles on the house.
Because once you let water into the house,
the cost of repairing the house goes up exponentially."
[asphalt roller passing]
Long-life pavements, traditionally called perpetual pavements,
are pavement designs that provide agencies with an approach that
reduces the life-cycle costs of a pavement.
Ray Brown: "So there's a lot of emphasis on keeping the initial cost down,
but if we look at the life-cycle cost, long-life pavements
are going to be cheaper in the long run. And then we can clearly see
that if we build longer-life pavements we're gonna get a lot more for our money."
Long-life, flexible, pavements are multi-layered pavement designs.
They are designed and built from the bottom up
with the lower layer designed to resist bottom-up fatigue cracking,
which limits damage to the easily repaired surface layer.
Richard Willis: "So at the bottom of the pavement is
where you're gonna see the most tensile strain.
It's really like the pavements being pulled apart every time the vehicle runs over it,
kind of like a rubber band. And so you want something that's flexible to resist
that pulling motion every time it's loaded. A little bit of polymer or a
little bit of extra asphalt can be very beneficial in increasing that pavement
flexibility which will in turn increase its fatigue life."
Trenton Clark: "I can fix the top easily
but if my base and my sub-base are
the source of my failure. Now I have created a huge maintenance issue for the
owner and the users in the future. So start with the proper structures,
the proper foundation, when you build a house you have solid footers."
The middle layer uses an asphalt mix designed to support
anticipated traffic loads.
And the top layer can be customized to
local conditions, environment, and requirements.
Buzz Powell: "Good mix designs, good production, good construction practices.
Make sure you have a multi-layered pavement system
that is well built and well bonded between all those individual layers
so that it can respond to passing loads monolithically
and then good maintenance, good preservation
and maintenance practices on the surface."
Long-life pavement design takes into account external factors
so agencies can forecast pavement performance
and then specify an appropriate pavement structure for current and expected future needs.
[car passing]
Debbie Schwerman: "Some of the critical keys are traffic loading, knowing what you're designing that for.
You know, the uniqueness about perpetual pavements is that we design it per layer
to accommodate the the overall loading of the pavement."
David Timm: "So the fatigue endurance limit is central to perpetual or long-life pavement design.
You have to have an estimate or somehow quantify the level at which cracking is not going to occur
or if you design it in such a way that the stresses are low enough
and the strains are low enough you're not going to have cracks initiate
and propagate to the pavement surface."
Long-life pavements optimize pavement structure through the right
combination of thickness and materials. With bottom-up failure mitigated,
agencies can focus on the relatively quick and simple maintenance of the
surface riding layer. This design approach minimizes maintenance costs
and reduces user delays for many highway agencies and owners across the U.S.
Bill Vavrik: "The Illinois Tollway in designing its full depth asphalt pavements uses a
long-life pavement design approach. We do look at all of the layers of the pavement system.
So we've got a number of different full depth asphalt pavements that I would
consider long-life pavements. So we designed with the materials and the
thicknesses so that bottom-up distress doesn't happen in at least the 50-year
design period that we're looking for."
Industry best practices and proven resources are available
to help agencies better design long-life pavements
and get longer service life from their roads.
Some pavement design software applications
developed through test track research, include features like
layered elastic analysis that enable users to design individual pavement layers
taking into account all appropriate variability.
"Variability with how thick the layers are, variability in the material properties,
variability of traffic,
it puts all that together in a Monte Carlo simulation
and estimates for you what the strain distribution is. That strain distribution
can be compared back against a design distribution that we developed
here the test track to know whether your pavement passes or fails
the long-life pavement design criteria.
Richard Willis: "So as we start to look at mechanistic procedures you can really
see how your materials choices impact performance. So as states start to move
towards that type of procedure, I think you're really going to see people to
be more creative and open to innovation
because they'll see the results of performance."
With long-life pavement designs, asphalt roads can be
built with a structure that lasts many decades with only periodic surface
renewal and maintenance required, making it an ideal choice for
drivers, engineers, and the traveling public.
"All these have to come together in your long-life.
It's not just designing a thicker pavement. It's designing a smarter pavement with
the right materials and not thinking thicker is always better because there
is a point that you have diminishing returns
that you're just spending money that you don't need to."
Dan Staebell: "Building a long-life pavement for the user is really critical
because they want to have something that they're driving on that's
economical, smooth, quiet, and long-lasting. And building it correctly, spending that
next incremental dollar, if you will, to doing it that way really does provide
the user a long-term performing pavement."
For more information about asphalt best practices and emerging technologies,
visit the Federal Highway Administration's website.
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