Applications & Innovations: Off Roof, Into Road
Applications & Innovations
by Tina Grady Barbaccia
Using RAS
Recycled asphalt shingles are an acquired taste
W
hen going from the roof to the road in recycled
asphalt pavement (RAP) mixes, “it’s all in the
details,” says John Christensen, technical marketing manager for MeadWestvaco (MWV).
It’s not impossible to get density in RAP mixes incorporating recycled asphalt shingles (RAS), “but it helps to pay a little
more attention to details. It’s definitely a more difficult mix to
work with once you put shingles in there.”
But agencies and contractors are able to save money on
virgin asphalt by using RAS. “This is what pushes a lot of contractors to use [RAS] now that it’s about $600 per liquid ton
for asphalt,” Christensen says.
“When I first started working with contractors, RAP was a
‘four letter word,’” Christensen says. “It was a waste material
that was usually given away or used by state or local DOTs
who retained ownership. If it was used, it wasn’t more than
10 percent. It was easier to just use virgin asphalt and not deal
with the headaches of the inconsistencies caused by putting
RAP in a mix, because virgin asphalt was cheap and readily available. Those days are gone. Liquid asphalt is not cheap
anymore, and sometimes it’s not as readily available and many
specifications would not allowmore than 10 percent – if any
at all .”
RAS has provided an environmentally responsible option,
and its economy is added-value for agencies, says Steve Jackson, P.E., quality control manager of Missouri-based NB West.
But like Christensen says, Jackson notes that RAS must be dealt
with carefully to ensure it will meet specs and perform well.
Jackson says he is seeing RAP mixes in early spring and
in late fall, when temperatures aren’t as hot. In the summer,
more shingle mixes are being used. “When temperatures get
below 40 degrees F, people look at not using shingle mixes or
[contractors] using ones with 6 to 7 percent going down to 2
to 3 percent because [the mixes] are getting stiffer,” Jackson
says. “In spring and late fall, it’s wetter so it’s harder to dry
them.” Jackson suggests keeping the shingles under a tarp or
in a storage building to help deter moisture.
But there’s more to using recycled asphalt shingles than just
32 July 2012 Better Roads
keeping them dry. “You have to determine the specific gravity
for your mix design,” Jackson says. This can get tricky because
different states use different methods. Missouri uses the effective specific gravity as the value for the bulk-specific gravity to
determine the voids in the mineral aggregate (VMA). This is
just one method that may be used for VMA calculation.
“This is kind of a nationwide issue people are dealing with
in determining the bulk-specific gravity for RAP and shingles,”
Jackson points out. “The problem is that the fine particles in
the RAP and shingles float when you submerge them in water;
this makes it very difficult to determine the bulk-specific
gravity directly using the traditional AASHTO T-84 and T-85
test methods.” The National Center for Asphalt Technology
(NCAT) has a method where 1 percent virgin binder is added
to the RAP to keep the fine particles from floating. However,
the methods all have different shortcomings, Jackson says. It
makes it difficult when you are trying to come up with a hard
and fast number and there are different approaches that people
are using.
The RAS, “eliminates the potential for rutting in the roads
because of the stiffer asphalt binder and the fibers they add to
the road, but there is still concern about cold weather cracking
and fatigue cracking,” says Jackson.
In 2012 the Missouri Department of Transportation (MoDOT) revised its specs and raised the minimum VMA requirement in lower-volume mix designs so cracking issues do not
occur, says Jackson. “I think the ultimate solution is that we
go to straight mixture performance testing instead of looking
at each component,” Jackson says. “There are several performance tests out there. People have their own opinions on
what is the best method; I think that a combination of fatigue
cracking and rutting potential tests will provide the best performance predictors in the future.”
Go to www.facebook.com/BetterRoadsMagazine for a
RAS photo album. For coverage of Illinois’ use of RAS and
Illinois Tollway Authority, EPA and Illinois DOT testing, go to
www.betterroads.com, then click on “Web Exclusives.”