Imagine producing and placing hot-mix asphalt (HMA) pavement material at temperatures up to 100°F lower than currently considered feasible. Perhaps the most obvious benefit would be the reduction in fuel consumption, which would conserve precious natural resources.
In addition, production of asphalt fume would decline. The relationship of temperature to asphalt fume production has been confirmed by research at the National Center for Asphalt Technology (NCAT) and elsewhere. Experts say that as the temperature of asphalt cement is raised, the amount of fume increases and the chemical makeup of the fume changes. With lower temperatures, both emissions and odors sometimes associated with asphalt are reduced or practically eliminated. Total emissions from asphalt plants, including greenhouse gases, also would be reduced.
The leaders of the National Asphalt Pavement Association (NAPA) learned of cutting-edge technologies in Europe for bringing down temperatures and, during the summer of 2002, conducted a European study tour to research and evaluate three of these processes.
"The impact of warm-mix technologies on the industry may be as significant as the introduction of stone-matrix asphalt and engineering controls," Peter Wilson, NAPA's 2003 chairman, commented.
While the warm-mix asphalt technologies seem very promising, it will be necessary to study them thoroughly to determine their applicability to the U.S., according to NAPA President Mike Acott. "We need laboratory and field research to evaluate the material characteristics and performance," he commented.
"There are questions about whether the processes and products are compatible with the mix designs, equipment, climate conditions and work practices in the United States, which are quite different from those in Europe. We need to find out whether the products perform up to U.S. expectations.
"And we have to look at plant operations, control of the mixing process, workability at the paving site and the ability to turn pavements over to traffic quickly. In other words, it's going to take us some time and some work to see how these European developments may be applied in our country."
NAPA plans to initiate a research program at NCAT, in cooperation with the Federal Highway Administration and European technology sponsors, to begin to answer some of these questions. Demonstration projects to evaluate performance in the field are a logical next step. Validating and implementing these technologies will be a years-long proc-ess.
The three technologies are quite different from each other and are marketed in Europe as proprietary technologies. They include:
* A synthetic form of zeolite called aspha-min is added during mixing at the HMA plant. The zeolite slowly releases a small quantity of moisture to create a foaming effect in the binder. This reduces the viscosity and permits a reduction in temperature;
* WAM-Foam is a two-component system which uses both a soft asphalt and a hard asphalt, introduced sequentially during the mixing process. First, the aggregate is coated with the softer binder; then, the introduction of a foamed hard binder enables lower mixing temperatures; and
* A third method is the use of organic additives with the asphalt cement to reduce the viscosity of the binder at mixing and compaction temperatures.
NAPA invited experts from Europe to San Diego to share the European experience with low-temperature asphalt mixes in a special workshop at NAPA's Annual Convention.
More of less
Dr. Reinhold Ruhl of the Bitumen Forum, Frankfurt, Germany, set the stage for the presentations.
The Bitumen Forum's highest priority is the development of low-temperature asphalt, with "low temperature" defined as close to 100°C, or 212°F. The benefits of reducing the temperature of asphalt production and placement include:
* Less or no fumes;
* Less emissions at the plant;
* Less energy consumption;
* Less wear and tear on the plant; and
* Less aging of the binder.
The Bitumen Forum's goals include further development and promotion of low-temperature asphalt to minimize emissions, energy consumption and carbon dioxide (greenhouse gas) production.
Dr. Heinrich Els of DAV (German Asphalt Pavement Association) in Bonn, Germany, gave an overview of the German experience with low-temperature mixes, with an emphasis on the use of organic additives.
Several methods for lowering mix temperatures are being developed in Germany. These include a method for adding aggregates in sequence, a two-phase bitumen mixing method (see WAM-Foam below for an example), and both organic and mineral additives.
The two types of organic additives are synthetic paraffin waxes and low-molecular-weight ester compounds. The paraffins are long-chained aliphatic hydrocarbons derived from coal gasification using the Fischer-Tropsch process. The ester additives consist mainly of esters from fat acids and wax alcohols and are produced by toluene extraction of brown coal. These additives increase viscosity and penetration of the bitumen at low temperatures.
These additives have been researched in the lab and in the field for about five years. The performance goals for low-temperature mixes include having the same (or better) resistance to fatigue and deformation and comparable workability at the paving site, said Els. Field experience with stone-matrix asphalt has shown that compaction can begin at 215°F to 250°F.
Els cautioned that over-compaction should be avoided. For example, pneumatic rollers can sometimes "pump up" the binder. Prompt delivery of paving material to the paving site is important, and he added pointedly, "Do not lose time in transporting, laying and compacting the material."
Producing foam
Burgard Koenders of Shell Global Solutions in Petit Couronne, France, and Olle Larsen of Kolo-Veidekke ASA in Oslo, Norway, teamed up for a presentation on WAM-Foam. WAM-Foam is the product of a joint venture between Shell and Kolo-Veidekke which began in 1995.
Koenders and Larsen explained that European companies are required to keep accounts of their carbon dioxide emissions. In addition, "green purchases" are required--that is, at an equal price, the most environmentally friendly solution must be chosen.
The developers of WAM-Foam set goals that include being able to produce low-temperature mixes in existing HMA plants to current HMA specifications. They focused their efforts on dense-graded mixes for wearing courses. WAM-Foam is a process that uses a blend of hard and soft asphalts. It is produced in a conventional HMA plant (batch or drum) that has been modified with the addition of foaming equipment.
A typical WAM-Foam mix described by Koenders and Larsen is produced with 90% aggregate, 4% filler and 6% asphalt binder. The aggregate is heated to about 260°F in the dryer, then mixed at about 225°F. In their example, 1.8% soft binder and 4.2% hard foamed binder were used. After mixing, the material is stored in a silo and ultimately is placed at 175°F to 200°F.
Several demonstration projects using WAM-Foam pavements were put into service in Norway from 1999 to 2002. The pavements are performing well in Norway, where the climate is extreme and studded tires are common, said Koenders and Larsen.
Crystal's clear results
Max von Devivere of Eurovia Services GmbH in Bottrop, Germany, discussed using a synthetic zeolite that Eurovia has trade-named aspha-min. He said that in Germany, asphalt mixes are typically produced at 302°F to 482°F. With aspha-min, production and placement temperatures can be reduced dramatically, to 266°F to 293°F.
Zeolites--crystalline hydrated aluminum silicates--release water at different rates. Eurovia's synthetic crystal offers more uniformity and predictability.
In the aspha-min process, 0.3% of the additive is added to the mix at the same time the asphalt binder is introduced. All known bitumens can be used in this process, said von Devivere, and all aggregates and fillers can be used. There is no need to change mix designs. He added that aspha-min has been observed to improve the workability of the paving material at low temperatures.
In tests conducted by Eurovia, lowering the mix production temperature by 55°F reduced energy consumption by about 30%. It also reduced the production of asphalt fume by 75%. Measurements at the paving site, where the material had cooled even further, showed a 90% reduction in fume. Significant reductions in odor also were documented, said von Devivere.
At least eight test sections have been constructed, and no difference in performance has been seen. Eurovia expects to build a 50,000-ton test section on the German motorway this spring.
Reprinted by permission from HMAT magazine, published by NAPA.
