| Asphalt Producer
Succeeding at SMA Production
Unless you’re experienced at
producing stone matrix asphalt, it can be a challenging mix to make,
handle, and deliver.
by Dan Brown, Contributing Editor
 Stone matrix asphalt basically consists of a coarse aggregate skeleton
and a binder-rich mortar. The higher concentration of coarse stones create
an interlocking skeleton that makes the mix rut-resistant, and the high
binder content gives the pavement durability.
The coarse aggregate is usually 72 to 80% of the blend, says the
technical document, Designing and Constructing SMA Mixtures — State of
the Practice, published by the National Asphalt Pavement
Association. The
gradation of the coarse aggregate has a tremendous effect on the mixture
produced. So it is critical that SMA aggregates be carefully handled,
stockpiled, and blended. “The necessity of closely controlling the
gradation cannot be over-emphasized,” says the NAPA document.
Experts recommend that an SMA producer frequently check and control the
material passing the 4.75 mm (No. 4) and the 0.075 mm (No. 200) sieves.
“SMA is very sensitive to aggregate gradation,” says Richard Schreck,
executive vice president of the Virginia Asphalt Association. “A 5%
change on the No. 4 sieve means it’s no longer an SMA. That mixture
could fail.”
Achieving heat transfer
 Because SMA is gap-graded, with a higher concentration of larger
stones, it can prove more difficult to heat and dry the aggregate blend,
says Brad Garbelman, a technical instructor at Terex
Roadbuilding, which
markets both CMI- and
Cedarapids-brand asphalt plants. “Heat from the
gas stream tends to go between the larger stones and shortcuts straight
out of the drum,” says Garbelman. As a result, the inlet temperature at
the baghouse may rise too high.
One answer, especially for large-stone SMA mixtures, is to flight the
drum to achieve the maximum length of aggregate veil (cascading stones).
“SMA may require flighting changes in the drum, and you may have to slow
down your production rate,” says Garbelman. “The answer is NOT to put
more material in the drum and apply more heat to it. That will just
increase the gas velocity and drive the hot gases through faster.”
One solution from CMI is a cold air damper that can be used for SMA or
for running high percentages of RAP. The damper, located in the ductwork
between the drum and the baghouse, admits cold air to the gas stream and
lowers its temperature. “That way you don’t have to re-flight your
drum every time you run SMA,” says Garbelman.
Cedarapids asphalt plants have an indexing slinger conveyor that can
insert aggregates at different points in the drum. For a large-stone SMA,
you would position the slinger all the way to the back of the drum, to
achieve the maximum length of veil. Or for a virgin aggregate surface mix,
you would index the slinger further forward, because finer aggregates
absorb heat more quickly.
Running hotter
Especially if SMA mixtures contain polymers, they may need to be
produced at higher temperatures than dense-graded asphalt. That’s
because polymers raise the temperature at which the material is workable.
However, the NAPA document warns against using excessively high
temperatures, because rapid oxidation begins to occur at higher
temperatures.
“Usually the state would like to see 335 to 345 degrees on a polymer
mix,” says Joe McGuire, plant superintendent for Callaghan Asphalt,
which runs two asphalt plants in the Chicago area. Often SMA is paved at
night, to minimize traffic disruption. And because night-time temperatures
are cooler, “You’ve got to make the mix hotter,” says McGuire. “We’ve
never paved SMA during daylight hours — it’s always been at night.”
Garbelman says that he has seen producers run polymer-SMA at 350
degrees F, or even higher — to make the mix workable for the paving
crew. “If you start SMA out at 300 degrees, the paving crew will call
you on the first load and ask what in the hell you’re sending us,” he
says.
Polymerized asphalt cement should be agitated to keep the polymers in
solution, experts recommend. Such material is best stored in a vertical
tank with an agitator, says J. Don Brock, chairman and CEO of Astec
Industries. “You need to keep your polymers agitated,” he says. “We
sell a lot of vertical tanks with mixers in them for polymerized binder.”
Mineral filler
SMA gradations usually require approximately 10% passing the 0.075
sieve, says the NAPA document. Even with the use of baghouse fines
returned to the mixture, the high fines content means that at least 5%
commercial mineral filler must be added to the mixture. NAPA says that in
most SMA construction projects in the United States, the ability to add
mineral filler governs the plant production rate. To produce 300 tons of
mix per hour, you must deliver mineral filler at about 15 tons per hour.
“Usually you auger in the mineral filler,” says Garbelman. “It
has to be precisely controlled; it has a very narrow tolerance band. Too
many fines will ruin your mix, and too few fines will ruin it also. You
also have to meter the baghouse fines that you put back in the mix.”
Commonly, producers monitor the amount of baghouse fines being added, then
subtract that from the total needed to determine the amount of mineral
filler to use.
Mineral filler should be coated with asphalt cement before it is
exposed to the high-velocity gas stream through the drum, NAPA notes. That
means placing the mineral filler line next to the asphalt cement line into
the drum. As well, cellulose or mineral fibers are used in SMA in the
United States. “Ideally you want the fibers and mineral filler and AC
and baghouse fines to all arrive at the same place at the same time —
before the gas stream gets to it,” says Garbelman.
Another cautionary note: be sure to preheat your drag chain conveyor
sufficiently. If the drag conveyor and SMA mixture are not hot enough, the
mix will cool, stick, and plug up a drag chain quickly — boom! — says
Garbelman.
SMA cannot be stored for extended periods of time at high temperatures.
The result of that could be unnecessary draindown of the asphalt cement.
NAPA says that SMA can be stored for two to three hours without a problem.
“You almost have to ship it out as soon as you make it,” says
Garbelman. “And if you ever fill up a silo with the intention of keeping
it overnight, you’re going to keep it a lot longer than that.”
Successful delivery
“You need to make sure you have a good release agent in your trucks,”
says McGuire. “We use what’s called E-Soy oil — it’s a
biodegradable oil that we’ve had a lot of luck with.” The product is
sold by Basic Chemical Solutions, he says.
And it’s best if truck beds are perfectly flat — not sunken between
the rails, because the swales will collect release agent and allow SMA to
stick on the ridges, McGuire says.
Solving the Aggregate Problem
Aggregate quarries are presenting a challenge to the growth of stone
matrix asphalt in this country, says J. Don Brock, who is chairman and CEO
of Astec Industries,
a major paving equipment manufacturer. SMA, he says, requires quarries to
make a high volume of a single-size aggregate to form the stone skeleton
in the mix. In the process of crushing rock for that single size, the
quarries must make several other sizes of aggregates that are difficult to
sell.
Now, Brock says he has the answer to that problem. “We can make a
stone skeleton out of any single-size rock,” he says. “When I was a
25-year-old in this business, they told me you can make a good mix out of
any size rock, and that’s still true.”
Current research by Pavement Technologies Inc., which until recently
was owned by Astec, backs up Brock’s assertion. “I asked Ron Collins,
who now owns PTI, to take seven single-sized aggregates, compact them in a
vibratory compactor, and find the air voids in each one,” says Brock.
The sizes compacted ranged from 1.25-inch topsize to 1-inch, to 0.75-inch,
and so forth down to number -8 aggregates. Each aggregate was placed in an
enclosed chamber and vibrated until it locked.
“Ron Collins called and asked me which one has the most air voids,”
Brock recalls. “I answered that the larger stones do. And he told me
they all have 28% air voids. So, since SMA requires asphalt cement and
some kind of filler in the voids, the next question was what size of
filler, or smaller aggregates, can maintain the aggregate interlock.
Again, PTI went to work on the question. Single-sized aggregates were
used to make a variety of trial asphalt mixtures. With each single-sized
aggregate, three different sizes of smaller aggregates were mixed. PTI
made 12-inch-long beams of each trial asphalt mix and tested each one for
rutting with a loaded wheel tester. Mixes with 5% in-place air voids were
used, and the remaining space (23%) was split about evenly between asphalt
cement and filler aggregates.
What extensive rutting tests showed is, if the filler aggregate is any
larger than one-sixth the size of the single-sized aggregate, aggregate
interlock will be spoiled. The mixture will not create a stone skeleton
and it will rut.
The bottom line, says Brock, is that quarries can fractionate their
aggregates into seven or more single sizes and make SMA mixtures with each
of them. Base mixes can be made with the larger stones, binder courses can
be made with intermediate-sized stones, and surface mixes can be made with
0.5-inch topsize or 0.375-inch topsize aggregates. And the fines can be
used in each one.
That way, aggregate producers get to sell all of the sizes of
aggregates they crush — and the price of SMA will come down in the
process.
“Our challenge as an industry is to make SMA less expensive. “SMA
will compete under heavy loads anywhere you want to put it.
Reprinted from Better Roads Magazine
March 2004 |
