Warm Mixes are a Hot Topic

Road Science

Warm Mixes are a Hot Topic

The asphalt industry moves closer to low-emissions asphalt mix
with a triad of new proprietary asphalt mixes from overseas.

by , Contributing Editor

With the introduction into North America of warm asphalt mixes, the asphalt industry is getting closer to a low-emissions asphalt mix that should make siting of asphalt plants easier, expand the construction season, and require less fuel to bring mixes to temperature.

So-called warm asphalt mixes are just that: heated to a temperature well-below the 300-degrees F-plus temperatures of conventional hot-mix asphalt plants. They attract interest because of their potential for reduced plant emissions in different stages of production, benefits in construction in the field, and reduced energy consumption in the plant.

“This technology could have a significant impact on transportation construction projects in and around non-attainment areas such as large metropolitan areas that have air quality restrictions,” reports the Federal Highway Administration’s Office of Pavement Technology.

Since 2000, warm mixes have received public attention in Europe and Australia. Strong interest in North America dates to a presentation at the January 2003 annual meeting of the National Asphalt Pavement Association, where a panel tackled the subject. And in March 2004 a panel on warm-mix asphalt was followed by a demonstration at an asphalt plant at the conclusion of World of Asphalt 2004 in Nashville, and Better Roads was there.
A review of warm-mix performance already is underway by the National Center for Asphalt Technology for one modifier supplier and for the National Asphalt Pavement Association, with an expansion of that effort soon to come with the participation of two more suppliers and the FHWA. The result will be a comprehensive performance report on warm asphalt mixes in the United States.
Warm asphalt mixes produce emissions at a greatly reduced level from conventional hot-mix asphalt plants, thus facilitating the permitting of asphalt plants in air-pollution non-attainment areas, or where there is local opposition.
Warm asphalt mixes can save money in the plant through reduced energy costs, but this is less an issue in the U.S. with its relatively inexpensive energy, than it is in Europe with higher-cost energy sources.
Warm mixes may permit trucking of loads of asphalt over longer distances, without fear of critical loss of temperature, allowing contractors to expand market areas.
Similarly, warm mixes may allow construction of pavements in colder weather, because contractors may no longer fear critical loss of temperature in the cold. The result may be a longer construction season extending into the winter in some regions of the country.
Warm mixes can allow faster construction of pavements made up of deep lifts of asphalt, for example intersections, which need to be opened as soon as possible. Because the mix is not so hot to begin with, less time is required to cool the mix before the next lift is placed.
Warm-mix asphalts are compatible with Superpave mix designs.
One warm-mix provider said warm mixes may enhance pavement durability because the lightest hydrocarbon fractions from liquid asphalt in the mix aren’t driven off by the heat of the burner.
But in life there are always tradeoffs. For the foreseeable future, the benefits of warm-mix technology will be available only through proprietary products, and likely will command a premium in cost per ton, depending on volume, of as much as $3 to $4 dollars by one estimate.

Nothing new

There is nothing new about the concept of cold and warm asphalt mixes; they’ve been with us for years.

So-called cold or ambient-temperature asphalt mixes rely on asphalt emulsions — suspensions of liquid asphalt in water — to distribute liquid asphalt binder among component aggregates. A period of time must elapse for the water to evaporate before the mix has cured and can be opened to traffic.

“Cold mixes use less energy and produce fewer emissions than hot mixes,” reports the Asphalt Emulsion Manufacturers Association. “Cold-mix plants are lower cost, simpler, and more mobile than hot-mix plants, and emulsion mixes lend themselves to on-site and in-place manufacture. The ability to stockpile material for future use leads to less waste and reworking than with hot mix.”

Asphalt emulsions also can be used in hot-mix plants, AEMA maintains. “Asphalt emulsion can be used in a conventional hot-mix plant but requires lower mix temperatures,” AEMA says. “The advantage is greatly reduced emissions and less hardening of the asphalt binder. The higher viscosity of the base binder at the mix temperature allows thicker films to be deposited on open-graded aggregates.”

Likewise, foamed asphalt can be made in cold-mix plants if so-equipped. In this application, foamed asphalt — the product of the injection of a predetermined amount of cold water into hot penetration-grade asphalt in an expansion chamber — replaces asphalt emulsions in the mix.

The expanded asphalt has a resulting high surface area available for bonding with the aggregate, leading to a stable road base or pavement using 100% of the existing in-place materials if done in situ. Foamed asphalt can also be produced by in-place base recycling machines or stabilizers. Unlike asphalt emulsions, foamed asphalt does not require a break period before it can be mixed. The foamed base then is graded and compacted, and can permit traffic — including heavy trucks — almost immediately.

In the hot-mix plant

In North America, the vast majority of pavements are made in hot-mix asphalt plants. The liquid asphalt is a relatively small part of the mix (typically 5 to 7%) and performs as a visco-elastic binder between the fine and coarse mineral aggregate.

Conventional HMA production takes place between 250 to 325 degrees F, not to exceed 350 degrees F, and placement and compaction between 260 and 300 degrees F. Before mixing with hot liquid asphalt, fine and coarse aggregates are heated to high temperatures to drive off moisture, to ease coating of the mineral aggregates with the liquid asphalt, and to keep the complete mix fluid enough to be workable during placement.

In addition to consumption of prodigious amounts of natural gas, fuel oil, or powdered coal, heating of liquid asphalt to these temperatures produces volatile organic compound fumes which may either be vented to the exterior, or collected with fume enclosures and re-vented back into the process.

Like any other industrial facility, fumes from asphalt plants are an issue for regional air quality in areas that are not in compliance with federal air quality standards. However, plentiful research indicates there is no evidence that these fumes are harmful to either workers or nearby residents. Documentation to this effect may be downloaded using the links in the For More Information sidebar which accompanies this article.

Fumes and plant siting

While there is no demand for low-emission pavements in bid documents, and no financial incentives to manufacture them, the permitting of emission-producing HMA plants is another matter. Siting of these plants in air-pollution non-attainment areas is problematic at best, and in some areas, impossible.

“The fumes issue [has] been a challenge at the hot-mix plant facilities and with the hot-mix plant manufacturers,” said D. Bill Garrett, vice president, Gencor Industries, in a presentation at the World of Asphalt Pavements international symposium in 2000 in Sydney, Australia.

“The predominance of parallel flow drum mixers in the industry, along with the use of recycled asphalt, continued to aggravate the stack opacity issues and has led to the creation of more stringent gaseous emission control in many countries,” Garrett said. “Many remedies were put in place to try and alleviate and reduce the opacity problems. The drums got longer, targets were tried in the dryers, heavier veiling flights were introduced in front of the recycle entry areas, gases were extracted through a magnum tube and mixing occurred behind the magnum tube, until eventually the whole mixing process was taken outside of the dryer mixer itself.”

The intent was to remove the liquid asphalt from the radiant heat of the burner flame, Garrett said in Australia. “The industry now [has] many options for counter-flow drum mix technologies, and through this technology, we have been improving in our reduction of emissions and also in the quality of mix that is produced through the counter-flow drum mix operation.”

Use of warm mixes has the potential to further eliminate emissions, giving a plant owner a powerful tool to use in the permitting process.

European technologies

These warm-mix technologies were developed in Europe, where fuel costs are much higher than in the United States, and where environmental standards are much stricter.

Three such products being introduced to North America and described at the World of Asphalt Warm Mix Symposium in March 2004 are:

Aspha-Min, a synthetic zeolite which releases water molecules when mixed with liquid asphalt at the plant, achieving a foamed asphalt binder.
WAM Foam, in which two components make up a system which introduces a soft and hard foamed binder at different points during production.
Sasobit, an organic additive, a synthetic paraffin wax which reduces the viscosity of the binder at mixing and compaction temperatures.

All three processes reduce binder viscosity, but in different ways.

Eurovia’s Aspha-Min

A zeolite is a constituent of a group of commercially valuable minerals — metamorphosed crystals of hydrated aluminum silicates — of interest to both industry and for rockhounds. Zeolites have large vacant spaces or cages in their structures that allow space for large, positively charged ions such as sodium and potassium, and even entire molecules such as water.

Aspha-Min, a manufactured zeolite, is available as a very fine white powdered form in 25- or 50-kilogram bags or in bulk for silos. The percentage of water held by the zeolite is 21% by mass and is released in the temperature range of 212 to 392 degrees F, said Max von Devivere, president, Eurovia Infra GmbH, at the World of Asphalt symposium.

“The procedure is to add 0.3% Aspha-Min to the mix, which permits reduction of production and laying temperatures by about 50 degrees F,” von Devivere said. “By adding Aspha-Min to the preheated mixture of sand and stone at the same time liquid asphalt is being introduced, a water-based vapor is created.”

The water released from the crystal causes the binder to expand to a kind of foam, permitting better workability and coating of aggregates at lower temperatures. Eurovia says its tests indicate that 50-degree F reduction in temperature equates to a 30% reduction in fuel energy consumption.

Aspha-Min in the U.S.

Aspha-Min is in the early stages of marketing in the United States by a Eurovia subsidiary, Hubbard Construction Company of Orlando.

Use of Aspha-Min was demonstrated during the field trip following the warm mix symposium, in which bags of Aspha-Min were emptied into a bin, from which it was conveyed pneumatically into an Astec Double-Barrel drum mix plant. Injection of Aspha-Min in this manner is no different than adding any other modifier to any other modern plant of any make. At the demo, conventional hot-mix asphalt was produced and placed on the plant’s driveway first as a control section, followed by placement of the Aspha-Min warm mix.

“Aspha-Min is very environmentally friendly,” said R. Wayne Evans, senior vice president, asphalt operations, Hubbard Construction. “It will allow plants to be permitted in non-attainment areas because of its much lower emissions compared to conventional hot-mix asphalt.”

Aspha-Min helps reduce energy costs, from both lower fossil fuel consumption, and even electricity costs, Evans told Better Roads. “We found it reduced the amperage consumed on the drag slat carrying the mix from the drum to the silos,” Evans said. “The material simply flows better.”

And Evans said early tests show that Aspha-Min may improve compaction efforts. “It appears to improve compaction,” Evans said. “The material becomes more workable than conventional mixes. We’re experimenting with that now because we don’t have enough history to prove that attribute, but that’s what we saw with our test sections here in Florida in February.” In February the mix was placed in ambient temperatures in the 60s F and placed with a Blaw-Knox 3172 paver and compacted by two Hypac Model 340 static rollers.

Evans said in the United States, the product may be sold to asphalt producers on a licensee basis in specific regions.

NCAT research

The National Center for Asphalt Technology at Auburn University was present at those February tests, took cores, and is developing a technical report, Evans told Better Roads, adding that additional testing of Aspha-Min, with the participation of the Florida DOT, will take place this summer.

But the work by NCAT for Hubbard Construction is only the start of a major review of warm-mix technology being undertaken by that asphalt research facility. In addition to Eurovia/Hubbard, a funding arm of NAPA is underwriting more general research in warm mixes at NCAT. And while not finalized, at press time it’s anticipated that both Hubbard and NAPA will be joined by Shell Bitumen (WAM Foam, below) and Sasol Wax GmbH (Sasobit, below), and the FHWA, in broadening the NCAT research. The result will be a comprehensive report.

“We will be working with three suppliers, Federal Highways, and NAPA and the State Asphalt Pavement Asssociations to do research on warm asphalt,” said Brian Prowell, P.E., NCAT assistant director. “Hubbard is supporting research on Asphalt-Min zeolite, and we anticipate Shell and Sasol will be underwriting research on their products.”

Hubbard’s work began in September and is scheduled for completion in summer. “NAPA’s/SAPA’s work started in September and it is supporting an overall literature review and concept,” Prowell said. “We understand that the FHWA will supplement work in both the laboratory and field phases. And the three suppliers will be underwriting laboratory evaluation of their products.” Individual reports will be written and an overall report will be developed.

Shell’s WAM technology

Shell Bitumen and Norwegian contractor Kolo-Veidekke are marketing WAM Foam as their entry to the warm asphalt sweepstakes. As with Aspha-Min, warm-temperature workability of the asphalt mix is facilitated by a chemically induced foam, but the process of getting there is entirely different.

WAM Foam (WAM = warm asphalt mix) is a patented process jointly developed by Shell and Kolo-Veidekke, and produces high-quality asphalt mixes at 212 to 250 degrees F, the joint venture reports. “The significant temperature reduction translates to an improved environmental product with lower energy requirements and emission levels and at a reduced cost,” they say.

“WAM Foam can be compacted at only 175 to 194 degrees F, saving energy and money, without affecting the asphalt’s properties,” said Shell International Oil Products’ Carl Robertus at the World of Asphalt symposium. “Carbon dioxide emissions are reduced to half, and fumes emitted from the bitumen are halved for every 50-degree F reduction in temperature.” An important feature is that conventional asphalt mix plants and normal paving equipment and techniques can be used.

Robertus said WAM Foam can be an integral part of environmentally sustainable road construction. “Compared to traditional hot mix, the new process is cost-effective and better for the environment,” he told the symposium. “It is a huge step towards a viable future for road production. WAM Foam is ideal for the road ahead.”

With WAM Foam, the binder is derived from two separate components — a soft binder and a hard binder in foam form — during the mixing stage. At a temperature between 212 and 250 degrees F, the soft binder is mixed with aggregate in the first stage. Then, a hard bitumen emulsion is added and a foaming commences, which lubricates the mix and provides its workability at relatively low temperatures.

Sasobit wax modifier

Unlike Aspha-Min and WAM Foam, which rely on a foam to enhance mix workability at lower temperatures, Sasobit is a paraffin-wax compound derived from coal gasification.

A product of Germany’s Sasol Wax GmbH, and sold in Germany since 1997, Sasobit and a variety of related asphalt modifiers are marketed in the United States by Sasol’s subsidiary Moore & Munger, Inc. of Pass Christian, Mississippi.

Sasobit’s melting point is just under the boiling point and is completely soluble in asphalt binder at temperatures in excess of 240 degrees F, permitting asphalt production temperatures to be reduced by 18 to 54 degrees F, said Sasol’s business manager for bitumen additives, Matthias Nolting, at the World of Asphalt symposium. “Below its melting point, Sasobit forms a lattice structure in the asphalt binder that lends stability and resistance to rutting,” he said.

“Sasobit is excellent for high temperature stiffness in both pavement applications,” Nolting said in March. “Sasobit makes it possible to upgrade softer grades of asphalt to harder grades while overcoming deformation and bleeding at high ambient temperatures.”

For optimum performance, the manufacturer recommends adding Sasobit at 3% of mix by weight, not to exceed 4%.

Asphaltan B wax derivative

In addition to Sasobit paraffin wax, the FHWA describes another organic compound, Asphaltan B, a product of Romonta GmbH, of Germany. Not described at the World of Asphalt symposium, it’s available in granular form in bags. Asphaltan B, the FHWA reported, is a low molecular weight esterified wax, a mixture of substances based on Montan wax constituents — a product associated with lignite deposits — and higher molecular weight hydrocarbons.

Romonta recommends adding Asphaltan B at 2 to 4% by weight, the FHWA said. “It can be added to the asphalt mixing plant or directly at the binder producer and can also be added to polymer-modified binders,” he said. The melting point of Asphaltan B is approximately 210 degrees F. Similar to Sasobit, it acts as an “asphalt flow improver” with associated reduced production temperatures, the FHWA said, with the manufacturer reporting increased compactability and resistance to rutting.

No guarantee of adoption

Warm mixes offer an exciting new dimension to construction of asphalt pavements, but their adoption will depend on contractors’ abilities to sell new, proprietary processes to government road agencies, and those agencies’ openness to accepting them. Historically, the track record for such products is mixed at best.

Thus while there are many benefits to new warm-mix asphalt technologies, and their performance on these shores already is being documented, their success will depend on whether road agencies turn a cold shoulder to warm mixes. But the wheels are turning.

For More Information

Much more information on the topics discussed is available off the Internet and by contacting individual entities. Here are some places to start:

The Asphalt Emulsion Manufacturers Association’s Basic Asphalt Emulsion Manual is available on CD-ROM. The disk includes everything in the bulky book, in addition to new video clips and other enhancements. Ordering information is available at www.aema.org/cdorder.htm.

A growing body of resources on foamed asphalt mixes is available on the Internet. For an overview of the FHWA’s application of foamed asphalt in a national park setting, visit www.tfhrc.gov/focus/aug03/04.htm. And an FHWA review of foamed asphalt using reclaimed asphalt pavement in Louisiana may be found at www.fhwa.dot.gov/pavement/foamrap.htm.

Additional information is available at www.soter.com/foamstab.html, which describes the use of a foamed asphalt system in conjunction with a Caterpillar RR-250 stabilizer, and on the Web site of Wirtgen America Inc., www.wirtgengroup.com (follow link to foamed asphalt).

Astec Industries Inc. provides an abundance of technical papers on the operations of asphalt plants available in .pdf format on its Web site. Of great value is its glossary of asphalt terms. View them at www.astecinc.com/literature/default.htm.

Information on fumes and asphalt workers will be found in the landmark National Institute of Occupational Safety and Health report (2001) which found there was no evidence that such fumes harmed the health of workers. To view it, visit www.expresswaysonline.com/expwys/niosh.html, or go directly to www.cdc.gov/niosh/pdfs/01-110.pdf.

A newer report on fumes, Evaluation of Worker Exposure to Asphalt Paving Fumes Using Traditional and Nontraditional Techniques in the AIHA Journal (American Industrial Hygiene Association), may be downloaded off the asphalt industry’s public outreach Web site, BeyondRoads.com. Access it in .pdf format at www.beyondroads.com/visual_assets/ Asphalt_Fumes_Study.pdf.

More information about Sasobit asphalt modifiers in the United States is available from the distributor at www.mooremunger.com. Or contact Prem Naidoo or Jeremy A. Day, Moore & Munger, Inc., voice 228-452-0009, cell 228-493-2851, fax 228-452-2201. Information also may be obtained at www.sasolwax.com.

More information about Eurovia’s Aspha-Min is available from R. Wayne Evans, Hubbard Group, P.O. Box 547217, Orlando, Florida, 32854, voice 800-476-1228, e-mail info@hubbard.com.

More information about Shell Bitumen’s WAM Foam is available from www.shell.com/home/ Framework?siteId=bitumen-en, or simply Google Shell Bitumen and click the first link.

Reprinted from Better Roads Magazine
June 2004