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To design mixes in the laboratory, technicians use a
gyratory compactor. Mixtures are compacted with a specified number of
gyrations, called N-design. The number varies in steps from 50 for low
levels of traffic to 100 or 125 for the highest level of equivalent
single-axle loadings.
Binder content debate
In some areas of the country, pavements designed by
the Superpave system have experienced premature raveling and cracking. “To
solve these problems, some contractors and engineers argue that increased
asphalt binder content in Superpave mixes would improve durability, reduce
permeability, and improve resistance to cracking,” says Gerald A. Huber,
associate director of research at the Heritage Research Group, Indianapolis.
Huber addressed this year’s meeting of the National Asphalt Pavement
Association under a series of sessions entitled, Making Superpave Work.
Increased binder content is not necessarily the
answer, Huber maintains, noting that historically, excessively high binder
content has led to rutting. Inadequate field density is the main problem, he
says. In fact, a survey by the National Center for Asphalt Technology showed
that more than half the pavements sampled had a density of less than 92%.
Mixture compactability is controlled by aggregate
shape, texture, and hardness as well as by gradation and compactive effort
(N-design). All of these factors influence packing of the aggregate
particles and all of them except hardness are addressed in the Superpave
specification.
“N-design controls the structural strength of the
mixture,” says Huber. “Adding asphalt makes the mix easier to compact but
can lead to rutting unless packing of the aggregate skeleton is adjusted to
accept the extra asphalt.” Some people think that reducing N-design will
increase asphalt binder content, Huber says. But designers really cannot do
it by lowering N-design. “The way to accomplish increased asphalt binder
content is to increase the voids in the mineral aggregate — which will allow
the aggregate to accommodate more asphalt binder,” he explains. Decreasing
N-design, Huber says, runs the risk of weakening the stability and
performance of the mixture in the field.
What Colorado did
In Colorado, transportation officials found that
original Superpave mixes were too stiff and didn’t have enough asphalt
binder in them, says Tim Aschenbrener, materials and geotechnical branch
manager for the state DOT. So Colorado increased its VMA specifications by
one percentage point and dropped design air voids from 4 to 3.5%. As a
result, “We probably saw a 0.2 to 0.3 percentage increase in binder
content,” says Aschenbrener.
Why the changes? “We started tracking performances
of Superpave mixtures after four or five years and found that those
pavements still had pretty high air voids,” says Aschenbrener. “They weren’t
failing — and they did achieve the specified density at the time of
construction. Testing indicated they were dry pavements after four to five
years.”
Following are some tips for successful Superpave
design and construction, gleaned from those well-experienced with the
method. This is by no means a comprehensive list; you can also consult
sources listed in the For More Information
sidebar.
1. Control your aggregates
“One of the most important things to know is the
history of your aggregates, and to control your materials,” says Marshall
Klinefelter, quality control director for David A. Bramble Inc., a general
contractor and asphalt producer based in Chestertown, Maryland. “We have a
3D data base with every aggregate gradation we have performed since 1988. We
generally know that our aggregate producers have a problem before they do.”
Superpave steps up the level of control needed over
aggregate gradations and cubicity. For example, the standard Superpave spec
limits flat and elongated particles to no more than 10% having a 5:1 ratio
of dimensions — on all pavements but the lowest design level. For gap-graded
Superpave designs — essentially a stone matrix asphalt — the Maryland limit
is no more than 20% particles with a 3:1 dimensional ratio.
“We check gradations every 500 tons,” says Chris
Spray, project manager for Venture Corp., which runs four portable asphalt
plants in Kansas. “A change at the crusher can affect the volumetrics of the
mix.”
2. Premix your aggregates
Venture uses a loader to mix aggregates within
stockpiles of coarse material. That helps prevent segregation, says Spray.
“If we’ve got 20,000 tons of 0.75-inch and larger stone, we pull the loader
in, pick up a bucket, back up 30 feet, dump it, and keep repeating that
process,” says Spray. “That way you get a more consistent blend.”
3. Put your aggregate stockpiles on paved
surfaces
Coarse-graded, Superpave-designed mixes may be more
difficult to dry, so steps have to be taken to minimize moisture retention
in stockpiles, says Superpave Construction Guidelines, by the National
Asphalt Pavement Association. Building stockpiles on sloped, paved surfaces
with adequate drainage will help to drain moisture away from stockpiles.
Other methods that have been used include roofs over stockpile storage, and
having the loader operator work from the sunny side of the stockpile while
avoiding its bottom.
4. Buy and use more cold feed bins and silos
Superpave isn’t just one mix; it’s a design method
that produces several mixes. “We have high production facilities here in
Maryland,” says Brian Dolan, president of the Maryland Asphalt Association.
“One of our producers has five cold feeds at a plant, and another has 10 or
11, so the latter one can be more flexible in the number of mixes he can
produce.”
5. Carefully calibrate your cold feeds and AC
meter
“The calibration of the plant is of prime
importance,” says Klinefelter. “The aggregate feeders have to be maintained
in good condition and monitored to make sure that changes in their
calibration can be accounted for. And as material goes from the collector
belt to the inclined belt, you want to make sure no material is lost that
you can’t account for.
The AC meter needs to be carefully calibrated, as
well. Klinefelter notes that the thermocouple that reads AC temperature must
be checked. If that thermocouple is wrong, the plant computer will use a
mistaken temperature to calculate binder content, and binder content will be
thrown off. “Superpave is much less forgiving than conventional dense-graded
Marshall mixes,” warns Klinefelter.
6. Maintain a consistent rate of production
“Your mix will be a lot more consistent if you can
run non-stop for 12 hours a day, rather than with 12 shut-downs and 12
fire-ups,” says Spray. “Your plant will produce a more consistent mix, and
your tests will have more consistent results.”
7. Run QC tests frequently
“Our spec in Kansas requires us to pull a
quality-control test on every 1,000 tons of mix, but we perform extra tests
to monitor the mix,” says Spray. “It’s insurance, is what it is. If the mix
changes, you’ll know it in advance.”
8. Use consistent testing techniques
“When we start a job, I do whatever it takes to make
sure the same technician runs the tests for the duration of the job,” says
Spray. “ It makes for more consistent results and fewer problems.”
9. Use representative mix samples for testing
Opinions vary on where to take samples for quality
control testing and acceptance of the mix. Some say samples should be taken
from the trucks; some say take them behind the paver. Klinefelter points out
that one of the smallest samples taken — yet most important — is the sample
for maximum specific gravity. “Make sure you get a representative sample
from the truck,” he says. “There’s a lot riding on it. The maximum specific
gravity test has to be run correctly, because it can raise or lower the bar
for your compaction specification, and you don’t want to raise or lower the
bar inappropriately.”
10. Maintain your gyratory compactor
“Make sure your gyratory molds are round and not
tapered,” says Dolan. “We’re talking about thousandths of an inch here, so
have a machinist do the checking. And check the angle of your gyratory
compactor. If the angle of the gyratory is incorrect, it will throw off your
volumetrics and asphalt content.”
11. Train your operators and technicians
Klinefelter says even quality control technicians
need to understand how the hot-mix plant works. “If the plant technician
suddenly notices that the binder content has dropped, he’ll say something to
the plant supervisor,” says Klinefelter. “Very likely, the supervisor will
be quick to say the problem is the way he sampled the mix. But if the
technician saw somebody working near the asphalt pump with a shovel, he may
notice that the worker broke a wire on the thermocouple at the AC tank. So
the thermocouple reports a low AC temperature, and the plant will put less
AC into the mix.”
Reprinted from Better Roads Magazine
September 2004 |
