The first true hot-mix asphalt (HMA) roads laid in the U.S. were put in
place just over a hundred years ago. The technique these pioneering 19th
century contractors employed to get acceptable mixes was, in many respects,
the same used today: aggregate was dried, heated and then mixed with a binder.
However, while the technique was the same, the technology employed is a
far cry from today's portable or stationary asphalt plants: All operations
were fully manual, and the mix was created in shallow, open iron pans situated over coal fires. The work was slow, inefficient, imprecise and-to a certain degree-unsafe.
These early roads proved popular and demonstrated an appreciable increase
in longevity over existing roads. Soon the demand for asphalt roads rose;
this demand was accompanied by an increased need for HMA. From flat iron
pans through truck- or rail-mounted units to large stationary plants, asphalt-production facilities grew to meet the need for HMA. Batch plants first appeared in the 1870s; drum-mix plants were introduced just after the turn of the century.
Just as these early plants evolved to meet the increased demands for HMA,
so have today's plants evolved to meet this decade's particular demands.
While the basic technology underpinning both batch and drum-mix plants is
almost a century old, it is constantly undergoing subtle improvements.
Unlike in the past, however, where HMA-production "breakthroughs"
were generally driven by the need for more or improved HMA, today's asphalt-plant
improvements address a different set of problems: increasing a plant's efficiency
and cost-effectiveness; allowing for the use of new materials, such as reclaimed
asphalt pavement (RAP); and coming up with acceptable solutions created
by the actual or perceived damage these plants have the potential to cause.
First and foremost, the industry seems to be in agreement that the business
of producing HMA is just that: a business. While it's a sound business practice
to make the best possible product-this usually leads to steady sales-the
emphasis is on the bottom line.
"There are a [few] things driving design [of asphalt plants],"
remarks Brian Barlow, marketing manager for Asphalt Drum Mixers Inc. (ADM),
Huntertown, Ind. "One is the issue of RAP and how to get it into the
plants, and how to do it cost effectively. Maintenance costs is another
issue: Everyone is trying to get the lowest cost per ton. It's more of a
crunch today because aggregate is more costly."
Another economic reality facing asphalt producers is the consolidation trend:
As more and more companies combine to form larger units, the competition
can become fierce. Underpricing jobs to prevent smaller companies from getting contracts is not an uncommon ploy. Since larger companies frequently have lower operating costs and greater cash flows, they can afford to take a
job or two at a loss if they know doing so will force a competitor out of
According to Barlow, ADM has attempted to address this bottom-line driven
mentality. "The [unit] we introduced last year was the Milemaker Series, which is a dual-drum counterflow model, available as either a stationary or mobile plant. The whole idea was to have the latest in technology available in an affordable manner."
A Milemaker plant employs a long, completely sealed counterflow drying drum with a preheated outside shell for RAP, and a separate mixing drum. The increased length (over conventional counterflow drums) drum is designed to ensure proper aggregate moisture levels and to preheat RAP with the excess drying-drum heat before entering the mixing drum. The added length also permits a more thorough mixing of the aggregate and asphalt cement for consistent, higher-quality mixes.
Each Milemaker comes standard with a 75-ton, self-erecting silo and a 400
tph slat conveyor that consists of a dual, 6-in. pitch drag chain with chrome
Barlow says these units have proved popular for retrofitting existing units,
as well as stand-alone plants. "Older plants are having trouble meeting
environmental regulations, so they either have to retrofit or get a new
plant. A new plant is about $1 million; the retrofit can be accomplished
for approximately $250,000. And the retrofit is quicker."
The drive for more efficient and cost-effective units is echoed by Jim Rodriguez,
director of corporate communications for CMI Corp., Oklahoma City. "The
parallel-flow plants evolved from 1971 to about the mid '80s; they became
more efficient and cost-effective," he notes. "The counterflow
was introduced in the late '80s, and now they have undergone the same type
of evolution. The most recent design of this type of plant was introduced
three years ago: CMI's Triple-Drum. It, too, has matured as a product-it
now makes mix at a lower cost per ton than other counterflow systems out
The Triple-Drum plant is somewhat of a misnomer: It actually consists of
a lengthy single drum that contains three drying/heating "zones."
RAP enters the drum on top of stainless steel combustion drum that surrounds
the unit's burner, sending radiant energy into a RAP/virgin mix blending
zone and then back into the asphalt mixing zone. Virgin aggregates pass
both over and under the burning chamber as RAP contacts its outside surface,
blending with the superheated virgin material.
One of the advantages of this system, according to Rodriguez, is the unit's
drying efficiency: "We have seven plants in Florida right now, with
two more on their way. Florida is a very wet state, and [a contractor] needs
a very efficient system to really dry the aggregate well at a low cost,
and this unit has really caught on." In addition to the Sunshine State
plants, CMI has at least a half-dozen other Triple-Drum plants either in
place or on order in the wetter Southeastern states.
While dry aggregate is a necessity for all quality HMAs, the emphasis Rodriguez
and others place is on "cost" and "efficiency." While
most asphalt plants will do an adequate job of drying aggregate, the key
is to perform this basic task in the most inexpensive manner. Again, economics
is the driving factor.
New mix materials also play a vital role in driving design. The use of RAP
has become commonplace, and use of this and other materials require special
considerations that an asphalt-plant manufacturer has to account for.
"With high rates [of RAP in mixes] you have to have more heat, and
you also have to have a long mixer. This way, the recycle is cooled before
the mix hits the oil, so it won't cook off the oil," says J. Don Brock,
CEO and chairman of the board of Astec Industries Inc., Chattanooga, Tenn.
He notes that accommodating these materials is an economic issue, as well.
These elevated mixing temperatures demand a unit that can provide these
conditions without bankrupting the user.
Astec believes it has found a solution in its new Double Barrel drum-mix
units. "The double barrel captures all the heat all the time,"
Brock notes. "So all the heat goes into the mix-you can mix high rates
of recycle without a problem. With the counterflow [units], you don't capture
the heat out of the shell, so when you have high recycle rates, it is hot
on the dryer shell."
The Double Barrel is available as either a stationary or portable unit (with
Astec's M-Pack and 6-Pack models). The Double Barrel also is offered for
use with batch plants to serve as a counterflow dryer or, for higher production
of a single mix, as both a dryer and mixer-the finished mix goes into a
storage silo or directly to load out.
The Double Barrel mixer has a long mixing chamber; this creates a long mixing
time-up to 75 seconds, vs. an average counterflow mixing duration of 15
seconds. The importance of this long mixing period becomes more important
as more additives are specified. "The counterflows have such a short
mixing time that all the stuff goes in at virtually the same spot; the asphalt
is hitting hot rock, which we don't think is good for the mix," Brock
remarks. The longer path permits the introduction for additives at different
points in the drum, allowing the user to tailor these additions to best
serve the specified mix.
Brock thinks that the use of additives and-especially-RAP will increase
in the future, but warns that some of the newer materials require a learning
curve. For example, the fibers, polymers and angular aggregates used in
stone-matrix asphalt (SMA) mixes can create a very stiff mix. To overcome
this viscosity, a mixing unit must put out more horsepower and heat. Brock
says Astec's Double Barrel units have addressed this problem, and they have
helped produce some of the finest stone-matrix asphalt roads he has seen:
Laid in Georgia several years ago, the roads have displayed less than 1¦10 in. of rutting.
Brock adds one other caveat that SMA-job contractors should heed: Since
these mixes are hotter than conventional ones, cooling between the mixing
plant and the job site can be a real concern. "You pull the mix at
about 360 deg from the drum, and by the time you truck it 5 to 10 miles
to the site the edges of the mix are down to about 300 deg," he says.
"If you don't have some sort of mixer ahead of the paver, you are not
going to be able to get a smooth pavement with this type of material."
One modifier that asphalt-plant manufacturers and operators won't have to
worry about-for the time being-is crumb rubber. Under the National Highway
System bill signed Nov. 28, funds for crumb-rubber demonstration projects
have been suspended-a move applauded by most in the industry. "I think there is a better way for the country to take care of its tire problem than
to dump it on the asphalt industry," comments Nevin Dieffenbach, president of ADM. "That was really nonsensical."
Like Brock, Dieffenbach forecasts an increased rate of RAP in the future:
"I think the RAP percentage specified by states will rise as more data
becomes available that [demonstrates the quality] of this material."
Increased RAP percentages may be a boon from an economic and conservation standpoint, but Dieffenbach notes that the biggest single issue facing asphalt-plant manufacturers and operators is one RAP users will have to overcome: the handling of airborne pollutants.
The high heat required for RAP use, combined with the hot gases released
from the recycled materials, creates a situation that can lead to emissions.
Manufacturers have responded by creating mix plants that recycle-rather
than emit-these gases. For example, Cedarapids Inc., Cedar Rapids, Iowa,
has developed an advanced counterflow system, the E Series Magnum CF Drum Mixer.
Since high-moisture aggregate or high percentages of RAP require a longer
drum than low-moisture virgin materials, the Magnum provides this length.
The result is lower overall stack temperature, and lower fuel and maintenance costs. The Magnum also handles the blue smoke problems: Since VOCs are not exposed to the high-velocity of hot gases normally released from RAP, blue smoke is eliminated. This is particularly important in light of the increased level of local, state and federal scrutiny over emissions.
While the counterflow design does provide increased fuel and process efficiency-items that pay bottom-line dividends-its strongest selling point is its ability to virtually eliminate blue smoke. While Brock prefers Astec's Double Barrel mixer to the counterflow (and Astec has built a significant number of counterflow units), he acknowledges the emissions' improvement counterflow units offer over their predecessors, the parallel-flow mix plants: "The parallel flow is a dead issue because of regulatory issues."
Along with economic concerns and the use of different mixes-such as RAP-amended mixes-regulatory issues have become one of the primary forces driving asphalt-plant design and operation RAP is more a case study of the difficulties this portion of the industry has had to overcome in dealing with the regulatory burden than an example of an industry problem in itself.
The regulatory burden really can't be separated from the new materials in
use today-new materials face more scrutiny than those that years of testing
have proven safe. Nor can this issue be discussed without examining regulations' economic ramifications-increased scrutiny requires more testing, more paperwork, more money. But any way you want to approach the issue of regulations, one disturbing trend is evident: The regulatory burden on the industry is high, and it shows no signs of diminishing.
"We have seen an increase in the regulatory burden in the last 15 to
20 year," confirms Gary Fore, director of regulatory affairs for the
National Asphalt Pavement Association (NAPA). "The disconcerting [aspect of this trend] is that it is paperwork-oriented, as opposed to addressing real pollution problems. Just the paperwork itself is getting to be a significant burden for [asphalt-plant operators]."
One of the ways the industry has found to combat this increased regulatory
load is through education. For example, Title V of the Clean Air Act addresses
the issue of new air permits, which directly impacts asphalt-plant operators.
However, the EPA-which has jurisdiction over these matters-does not really
understand how the industry operates.
"One stipulation was for facilities to calculate emissions on a full-year,
nonstop-operation basis," Fore says. "The net result, of course,
grossly exaggerated the output of emissions from HMA plants-we don't have
any plants that even begin to approach that amount of time running per year."
By educating the agency about the operating methods of asphalt plants, NAPA
was able to secure a more realistic approach to measuring actual emissions.
Data, provided by manufacturers and operators and analyzed by NAPA, appears
to indicate that the adjusted emissions of most HMA plants will not exceed
the threshold that would require a permit.
This type of "partnering" process is occurring with a higher frequency
these days, and this partnering is often used to help combat real-or perceived-harm
created by asphalt-mix plants.
Few question that asphalt plants hold the potential for environmental harm.
The use of solvents, additives, petroleum-based products and high heat can,
if not properly managed, create problems. Plant manufacturers, working with
operators, regulators and association officials, have come up with technological
checks and balances that all but prevent operations from violating regulations.
The switch from parallel-flow drum-mixers to counterflow units is an example
of manufacturers working with regulators (and their guidelines) to produce
units that will provide quality mix with less harm to the environment.
Perceived risk is a more difficult challenge for asphalt-plant manufacturers
and operators, since the "problems" the public perceives are not
necessarily supported by data-they are just subjective views.
"I think that smell and noise and emissions are the issues locals are
worried about," says CMI's Rodriguez. "Permitting new plants is
a very difficult process today. So the manufacturer and operator are working
together to convince people that we are dedicated to building a quiet, safe
plant." He adds that the "not in my back yard" mentality
does apply to asphalt plant location: "Yes, people want the smooth
roads but not the plant next door."
Asphalt plants, just like most large industrial operations, have a very
negative connotation in people's minds: Many imagine them to be metal-and-concrete
behemoths, with multiple stacks belching thick, acrid smoke. To combat this
stereotypical view of asphalt plants, there has been a trend toward improving
the environment surrounding these plants, which is contrary to the public's
expectation of the plant-site's degradation.
For example, an Ultraplant mixing facility in Mexico recently won NAPA's
International Ecological Award (see "Gator Asphalt, Mexico's Plantasfalto
earn top ecological awards," January 1995, p 42). The 275 tph stationary
plant, manufactured by the Orlando, Fla.-based Gencor Co., is designed to
both prepare batch or continuous operation. It's a large, high-volume unit,
one of the types that might be preconceived by the public as noisy or dirty.
However, the plant's counterflow technology is designed to meet the most
stringent of emission requirements, including those of New Jersey and California.
Despite the clean design, all many individuals see is a large metal plant
surrounded by unsightly piles of aggregate.
The award-winning Ultraplant installation fought this prejudice by surrounding
the plant with extensive plantings, skilled grading and a decorative fence.
Looking at the site, the public's perception of an asphalt plant is immediately
challenged and, as the plant continues operation in a clean manner, ultimately
changed. In this instance, the partnering process could be said to have
"included" the public.
While the end product-and the profit it will bring-is ultimately the highest
priority of asphalt-plant manufacturers and operators, dealing with the
real and perceived risks such operations entail are vital to the industry's
success. Partnering can help industry address issues and overcome these
Despite partnering's success, considerable challenges still face asphalt-plant
manufacturers and operators. New regulations, particularly a title of the
Clean Air Act that addresses toxic emissions, could force design and operation
modifications very shortly. Paving warranties have become more commonplace
today; contractors entering into these contracts are going to want the highest
quality HMA possible to help them fulfill these warranties.
In addition to federal regulations (such as the Clean Air Act), asphalt-plant
operators have to be aware of the patchwork quilt of local or state regulations
that often exist. Crossing the border into another state (or even another
county) for a paving job can expose a contractor to a different set of regulations;
those who fail to make a point of finding out what the regulations are expose
themselves to fines or litigation.
However, the future for asphalt plants is anything but bleak: Contractors
seem to feel that 1996 will be a banner year for paving; this bodes well
for HMA producers and plant manufacturers, as well. In addition, the overseas
market appears to be opening up. Astec's Brock says the company built seven
batch plants in China in 1995; 10 drum plants went to Germany.
While the devaluation of the peso has created a recession that has hobbled
Mexico's transportation industry, American firms are still looking favorably
on foreign sales, and plant sales to foreign countries are up.
The bottom line for asphalt-plant producers and operators, however, remains
the bottom line. Existing technologies, such as plant computerization, have
been continually modified to produce equal or better products in the least
expensive manner. The many new materials introduced over the last few years
will require plant engineers to continue to tweak today's plant designs
to provide the most flexible, cost-effective production of HMA in the future.
Industry success stories have demonstrated that partnering is both successful
and necessary, but the potential of economic damage from future regulation
will require even more extensive and pro-active use of this process.
For restaurants, the three secrets of success are location, location and
location. For HMA-plant manufacturers and operators, the secrets are no
secrets at all: cost, cost and cost.