Pourous asphalt pavement can help control parking lot runoff.

At this year’s major European asphalt conference in Copenhagen, there were 14 presentations on warm-mix asphalt. One of them was an overview that I had been asked to give on the state of the practice on warm mix in the United States. Several milestones were cited. The question asked at the conclusion of the talk was, “How has the United States come so far on warm mix in just a few short years?”

A big part of the answer is that the U.S. asphalt pavement industry was truly ready for warm-mix technology. Over the past decades, we have taken many steps to make our facilities an even better neighbor, improve working conditions and protect the environment. Warm mix is the logical next step.

The rest of the answer is partnerships. The U.S. industry has developed partnerships among contractors, government agencies, the labor unions and researchers. These partnerships have worked together on a series of innovations to improve our operations and enhance working conditions for our employees. When warm mix came along, we had the infrastructure – the partnerships – to capitalize on the opportunity.

Big Breakthroughs
Warm mix has the potential to become one of the most important breakthroughs for the asphalt industry in more than 50 years. It could rank with recycling, milling machines, the floating screed, stone-matrix asphalt (SMA) and perpetual pavements.

If you watch some of the historical film footage on asphalt plants, you will note that the industry has made huge strides from the early plants that belched black smoke to our modern-day facilities where emissions are very low and well-controlled. For the period from 1960 to 1999, hot-mix asphalt (HMA) tonnage increased by 250 percent, but total emissions from our operations decreased by 97 percent. Even with the progress that has been made, we recognize that continuous improvement is a key factor for the success of the asphalt pavement industry.

As global environmental awareness brings new challenges, we need to continue to take action and encourage innovations that make our facilities an even better neighbor, improve working conditions and protect the environment. In the mid-1990s, National Asphalt Paving Association (NAPA), the National Institute for Occupational Safety and Health (NIOSH), the equipment manufacturers, the Federal Highway Administration (FHWA) and the labor unions put together a partnership to explore the idea of using engineering controls on paving machines. Each of the six U.S. manufacturers of highway-class asphalt pavers devised ventilation systems to collect asphalt fumes from the screed area and vent them away from workers. These controls have now been in use for more than a decade.

In this time period we also saw that mix temperatures were edging upwards as a result of the implementation of Superpave, SMA and the wider use of polymer-modified asphalt. In 2000, a coalition of NAPA, the Asphalt Institute and the State Asphalt Pavement Associations published Best Management Practices To Minimize Emissions During HMA Construction, which provided guidance on how to reduce plant-mixing temperatures. Also, NAPA and the State Asphalt Pavement Associations funded research at National Center for Appropriate Technology (NCAT) on the effect of temperature on emissions and odors. That research showed that temperature is one of the most significant drivers affecting the quantity and chemistry of fumes. Similar conclusions were reported by Eurobitume in 2007.

By collecting and venting away emissions, the engineering controls could be considered a first-generation “end of pipe technology.” However, it was clear that there were additional benefits if emissions could be controlled at the source. Based on the findings from the NCAT study, one option was to pursue technologies that could lower production and placement temperatures.

New Developments
In the late 1990s, NAPA made contact with Germany’s Dr. Reinhold Ruhl, who had formed a broad industry/labor/government coalition known as the German Bitumen Forum. This is very similar to the coalition NAPA had formed in the United States for the engineering controls project. The German Bitumen Forum had an active Temperature Reduction Working Group that was evaluating technologies that saved energy, reduced CO2 and lowered emissions. In addition to the work in Germany, similar initiatives to reduce temperatures were occurring in France, Holland, Norway, South Africa and elsewhere.

Warm mix asphalt must be heated to roughly 250 Fahrenheit, while hot-mix must be heated in excess of 350 Fahrenheit.

Learning of the developments in Europe, a NAPA study team made up of producer/contractors conducted a warm-mix study tour of Germany, Denmark and Norway in 2002. The technologies viewed included Aspha-min (synthetic zeolite), Sasobit (a wax additive) and WAM Foam. At one production facility we observed a side-by-side comparison of warm mix and hot mix. With the warm mix, there were no fumes, no odors, lower energy consumption, lower CO2 production and no discernable differences in mix workability compared to hot mix. The potential benefits of warm mix for the U.S. industry were self-evident. NAPA’s leadership decided to work with its partners to encourage and evaluate various warm-mix technologies in a U.S. setting.

Considerable progress has been made in the six years since the initial study tour. The various technology vendors have been very responsive. They have invested in research and have worked with contractors and DOTs to conduct numerous field trials. Several new technologies have evolved, and some hot-mix producers have developed their own in-house technologies.
    
Vision for the Future
The current market environment includes high-energy prices, pressures on highway funding, the likelihood of more-restrictive environmental regulations, continued pressure on plant zoning/permitting and a surge in mainstream environmentalism. The planets are aligned for warm-mix asphalt to help with these issues because it can:

• Permit higher content of reclaimed asphalt pavement (RAP) without excessive emissions or poor workability. Also, less aging of the virgin binder in warm-mix production may allow higher RAP percentages without changing the virgin binder grade;
• Provide energy savings of 10 to 20 percent;
• Reduce the carbon footprint of asphalt pavement production and construction by reducing greenhouse gas emissions;
• Minimize or eliminate fumes, emissions and odors; and
• Improve working conditions at the plant and paving site.

In addition, there have been several operational benefits from field projects that are yet to be fully evaluated or understood. These include more time for compaction, the ability to store and haul the mix for a longer time, and the opportunity for extending the paving season.

The environmental benefits are a given. The challenge is clear. We need to continue to evaluate warm mix and be able to demonstrate through field observation and research that warm mix has the same coating, the same workability, the same air voids and the same performance as hot mix. In essence, warm mix must equal hot mix. We must not lose any of the performance benefits we’ve achieved with Superpave, SMA, and open-graded friction courses.

Mike Acott is the president of the National Asphalt Paving Association, the trade association that exclusively represents the interests of the hot-mix asphalt producer and paving contractor on the national level with Congress, government agencies, and other national trade and business organizations. For more information, call 888-468-6499 or visit www.hotmix.org.