| Asphalt Producer
Working with
Polymer Modifiers
Like it or not,
polymer-modified asphalt has a place in our future.
Here are some ideas on how to deal with it.
by Daniel C. Brown, Contributing
Editor
 As the years unfold, it’s virtually certain that
we’ll see more and more polymer-modified asphalt. Today, the majority of
states have a specification for such mixes, and recent studies now confirm
that polymers, when properly selected, improve asphalt’s resistance to
distresses like rutting at higher temperatures, low-temperature thermal
cracking, load-associated fatigue cracking, aging, and stripping of the
asphalt cement from aggregates. In addition, polymers generally assure that
you’ll get good stickiness, or adhesive qualities, in your mixture.
The selection of a polymer is important, says Dave
Newcomb, vice president of research and technology,
National Asphalt
Pavement Association. “The improvements come not just from the polymer, but
from the combination of the polymer with the asphalt cement. Various
polymers react differently with different asphalts,” says Newcomb.
“It is likely that modified binders represent as
much as 15% of the total annual tonnage of asphalt binder used in the United
States; this percentage is expected to increase in the coming decade,” says
the staff of the Transportation Research Board in a foreword to NCHRP 9-10,
a report called Superpave Protocols for Modified Asphalt Binders.
To be sure, polymer-modified asphalt costs a bit
more than unmodified, or neat, asphalt cement. Depending on the type of
modification, says NCHRP 9-10, the cost per ton of a modified binder may be
50 to 100% greater than that of neat asphalt cement, translating to an
increase of 10 to 20% in the cost of the in-place asphalt.
The fundamentals
A polymer is a large molecule that is made up of
many small chemical units called “monomers,” says Ron Corun, technical
support manager for Citgo Asphalt Refining, Ocean City, Maryland. He notes
that several types of polymers are available to modify asphalt mixtures:
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Elastomers, or block copolymers. These include
styrene butadiene and styrene butadiene styrene.
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SBR Latex (styrene butadiene rubber).
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Polyolefins, or plastomers. These include
low-density polyethylene; reactive terpolymer (Elvaloy AM, for example); and
ethylene vinyl acetate.
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Engineered binders.
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Crumb rubber.
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Chemical modifiers.
 “A plastomer will generally stiffen the asphalt to
increase its high-temperature performance,” says Dr. Mark Buncher, director
of field engineering at the Asphalt Institute. “For surface mixtures, this
results in greater resistance to rutting and deformation. For intermediate
and base layers that have been modified, it also means an increased
structural strength of the pavement system which the newer
mechanistic-empirical thickness design procedures will consider.”
In contrast, Buncher explains, an elastomer will
generally both stiffen and increase the flexibility or stretchiness of the
asphalt, improving both high- and low-temperature performance. “An
elastomeric binder will rebound after being stretched, so the asphalt
mixture is better able to recover from the stresses that occur under heavy
loadings and from the thermal stresses that build up with extreme cold
temperatures,” he says.
Many states find the improved performance of polymer
modification to be worth the cost, especially for high-traffic-volume
pavements (see chart). Mississippi, for example, requires PMA for the top
two courses of any pavement designed for 3 million or more equivalent
single-axle loadings. In fact, says Jimmy Brumfield, the state materials
engineer, suppliers are required to start with a PG 67-22 binder, then add
polymer to bump it up to PG 76-22.
The PG 67-22 is equivalent to the old AC 30 binder,
which worked well in Mississippi’s hot climate, says Brumfield. “We didn’t
want AC 20; it was too soft,” says Brumfield. “So to eliminate that
possibility, we make the suppliers start with the PG 67-22, then modify that
to get the PG 76-22.” In his state, Brumfield says polymer adds between $3
and $5 per ton of mix, which increases the cost of mix to about $38 to $42
per ton of mix paved.
To specify polymers, most states use what are called
SHRP-plus, or PG-plus, specifications. That’s because the research for the
Strategic Highway Research Program developed tests for neat asphalt binders
only. The new performance grade system did not discern the presence — and
value — of polymer in the binders. It was, and is, possible to manufacture
both a neat PG 64-22 and a modified PG 64-22. And yet, the modified binder
will outperform the neat binder.
“It’s possible to fool the system,” says Citgo’s
Corun. Not only that, but a binder supplier can provide an air-blown
asphalt, one with no modifiers, that will have a higher PG rating than its
original material.
“So, if you want a polymer, you specify that,” says
Don Watson, research engineer for the National Center for Asphalt
Technology. “A state can say that the binder must be straight-run, not
air-blown, to meet the grade. Some states specify the type of polymer, such
as SBS, SBR, or SB, or they may specify it by elastomer or plastomer.”
In Georgia’s department of transportation, a leader
in polymer-modified asphalt usage, engineers go as far as to specify the
so-called “phase angle” of the binder. The phase angle, often called delta,
is an indicator of the relative amounts of recoverable and non-recoverable
deformation, according to The Asphalt Institute’s SP-1 manual. The phase
angle is measured using a dynamic shear rheometer, then plotting the
material’s viscous behavior (y-axis) versus its elastic behavior (x-axis).
An asphalt binder with relatively more elastic behavior than viscous
behavior will have a lower phase angle.
Some states use a forced ductility test, Watson
says. That test measures the force required to stretch a binder. With a
polymer-modified binder, the stretched strand will be thicker than with a
neat asphalt. “With a neat asphalt, you get a hair-like strand,” says
Watson.
And some states have a separation test, which
measures the degree of separation of the binder from the polymer when the
blended material is stored. “Some states require that contractors have
agitation systems that prevent separation,” Watson says.
Higher temperatures
 Most contractors and state officials with PMA
experience will tell you that running polymer mixes requires higher mix
temperatures in order to assure that the paving crew can achieve the
required density. But how much higher is quite another question.
“If the producer doesn’t make hotter mix, then the
mix could require him to put forth more compactive effort,” says
Mississippi’s Brumfield. But making hotter mix can raise environmental
issues, such as blue smoke, or issues with fumes and worker health. What is
more, overheating the PMA mixture can prematurely age the material.
So, Mississippi limits the high-end mixture
temperature to 340 degrees F. At higher temperatures than that, Brumfield
says, “You’ll age-harden the mix. I want to preserve that durability for use
on the roadway!”
Dan Gallagher, vice president of Gallagher Asphalt,
a hot-mix producer based in Thornton, Illinois, advises communication. “Work
closely with your binder supplier,” he says. “They know the
temperature-viscosity curve of various binders. You want to make sure you
achieve your compaction temperature. Polymers tighten the range of
temperatures for compaction.”
Typically, says Gallagher, a polymer asphalt might
require a mix temperature leaving the plant of about 320 degrees F and 305
degrees behind the paver.
Tips for producers
In a presentation at World of Asphalt in March,
Citgo’s Corun made the following recommendations for producers making
polymer asphalt.
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Use a dedicated storage tank for your polymerized
binder. Not doing so risks contaminating your polymerized material and
changing its temperature grade.
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Either vertical or horizontal binder tanks are
satisfactory, but a vertical tank provides more efficient agitation.
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When reheating modified binders after a cold
winter, heat it up slowly, in 20-degree increments. Allow about a week to
get it working.
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Polymer-modified binder is more difficult to pump
than neat binder, because it’s stickier. So if your pump runs near the upper
limit of amperage for neat binder, the circuit breaker may kick off the pump
for polymer.
-
The same pump calibration should work for polymer
as for neat binder. Put the strainer in if you’re running polymer.
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PMA will increase the electrical draw on your slat
conveyor. If you run near the upper limit with unmodified asphalt, you may
need to warm up the slat conveyor first with regular asphalt, then switch to
the PMA.
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Don’t store PMA overnight in the silo.
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Be sure your release agent works on truck beds,
because the polymerized material is stickier.
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Tighten down the tarps on trucks so that air can’t
dry out the mixture.
 At Callaghan Asphalt, Chicago Heights, Illinois,
plant superintendent Joe McGuire heartily recommends E-Soy, a biodegradable
release agent. The material is approved by the EPA and the Illinois DOT, and
McGuire says “It’s worked really well for us.”
In Mississippi, Brumfield has confidence in the
value of polymers. “We have done some research on different types of
polymers,” he says. “I believe that polymers will increase the life of any
asphalt pavement. A polymer will not make a bad mix good, but it can make a
bad mix better.
“We feel that a polymer can extend the life of a
pavement by 10 to 15%,” Brumfield adds. “If we can get one more year out of
an HMA pavement, it saves the taxpayers millions of dollars.” BR
AMAP (below) is dedicated to informing the industry
about modified binders.
For More Information
Association of Modified Asphalt Producers: P.O. Box
270006, St. Louis, Missouri 63127; www.modifiedasphalt.org; 314-843-2627.
Bob Berkley is executive director. A stated purpose of AMAP is “to inform
and educate owners, contractors, and specifying agencies about
the...benefits of modified asphalt binders.” Members include modified
asphalt producers, suppliers, companies that provide services and equipment
to regular and supplier members, and Friends of the Association.
Asphalt Institute: Research Park Drive, P.O. Box
14052, Lexington, Kentucky 40512-4052;
www.asphaltinstitute.org. Members are
international petroleum asphalt producers, manufacturers, and affiliated
businesses. AI’s mission is to promote the use, benefits, and quality
performance of petroleum asphalt.
National Asphalt Pavement Association: 5100 Forbes
Blvd., Lanham, Maryland, 20706-4413; 301-731-4748;
www.hotmix.org. NAPA
members include the nation’s leading hot-mix asphalt producers and
contractor firms, as well as those who supply equipment and services for HMA
pavements.
NAPA publication: Best Management Practices to
Minimize Emissions During HMA Construction is available for $2.50 from NAPA.
It contains a table of storage temperatures and plant mixing temperatures
for various PG ratings of binders, up to PG 82-22. The table is helpful in
setting your mix temperatures, but experts recommend working closely with
your binder supplier.
Reprinted from Better Roads Magazine
June 2004 |
