By Bruce Barkevich, Contributing Author
Warm Mix Asphalt (WMA) entered the asphalt industry’s national conversation more than two decades ago.
Since then, millions of tons of WMA have gone down across the country, and industry experts still debate the definition of warm mix.
If you put 10 asphalt experts in a room, it would be hard to pin them down on one clear concept of what meets the criteria to be a warm mix.
Once in that room, those same asphalt experts also would likely talk about products that reduced the production temperature of asphalt dating back to the 1950s and 1960s. Names like “steamed dispersion” or “emulsified asphalts” might be discussed. Many of the products were proprietary and didn’t stick around. Others found local acceptance, and you can see hints of them in work being done today.
But the more recent warm mix story begins in Europe. The concept of WMA was developed in response to environmental concerns following the 1997 Kyoto Protocol, which gave European countries reduction targets for greenhouse gas emissions.
Companies in Germany experimented with additives like Aspha-min zeolite, while Shell Bitumen and Kolo Veidekke in Norway worked on WAM Foam. Sasobit, a Fischer-Tropsch wax, was used in Germany as early as 1997.
In 2002, leaders from the National Asphalt Pavement Association (NAPA) went on a European study tour to Denmark, Germany, and Norway to examine warm-mix technologies. This marked the introduction of WMA technology to the U.S.
The first field trials of WMA were conducted in the United States in 2004 and 2005. Research sponsored by various organizations, including NAPA and the Federal Highway Administration (FHWA), began to investigate the performance of WMA and the potential implementation for use in everyday applications.
I remember vividly being on one of the first pilot projects in the state of New York in Cortland in 2005. New York Construction Materials Association (NYMaterials), asphalt producers, and the New York State Department of Transportation (NYSDOT) were working together on these experimental projects.
The producer/contractor was Suit-Kote Corporation. In less-than-ideal circumstances, the project pushed ahead as it was raining on and off. With many white hardhats watching the paving progress (from DOTs and the industry), the material went down as if it was being produced at 320 degrees, but it actually was being produced and placed at 100 degrees less than that.
The pavement behaved no differently than convention hot mix asphalt. The screed laid a smooth mat, and the rollers compacted it to achieve proper density. And that pavement performed for many years.
The traveling public would never understand it was the beginnings of a major shift in the paving world.
For the next several years, NYSDOT, the asphalt industry, and many local municipal partners worked to placed WMA pavements around the state. And roughly one million tons of warm mix was placed in the following decade.
But we still hadn’t defined what WMA was. In the end, we landed on this definition: an asphalt mixture utilizing a warm mix technology. But this begged another question: What qualified as warm mix technology?
In a typical process, NYSDOT and NYMaterials worked together to come up with a testing protocol to approve technologies for use. The tests included the effects on the liquid asphalt binders, the rutting potential for the mix, and crack testing.
Technology would be approved for use once it was provided a mix with a warm mix technology in it, and it was compared favorably to a control mix. This list grew as acceptance for warm mix grew. We have had up to 15 approved technologies over the years.
WMA continued to gain acceptance for use around New York. But were we really getting warm mix?
Most of the asphalt laid had a warm mix technology in them, but there wasn’t always a temperature reduction associated with the mix production or placement. Still, it became apparent the WMA technologies offered other benefits. Workability of the mix and compaction were enhanced.
We could pave in cooler working conditions and still achieve a good, finished product. Raking and shoveling was a little easier and blended better. All these factors continued to build on the confidence in the product. Again, was it really warm mix, though, if we didn’t reduce temperatures.
In 2013, NYSDOT made another progressive move toward the adoption of WMA. They issued an Engineering Bulletin (EB) that made warm mix permissive. Basically, a producer or contractor could switch from hot mix to warm mix as needed. They just had to communicate any changes to the DOT.
NYSDOT also decided an approved warm mix technology could be introduced to the mixture as a drop in product. This means that a new asphalt mixture design for warm mix didn’t have to be produced or submitted for approval.
If the hot mix design was approved, the producer could introduce the approved technology in production to the mix and they could use it as they liked. This was a progressive move, but it also showed confidence in the product and process that had gone on for the last handful of years.
Over the next eight years, warm mix asphalt was interchangeable with hot mix asphalt. Contractors continued to realize the product could perform as good if not better than conventional mix.
Many agencies followed the DOT with this specification allowance. Local towns and counties adopted the DOT’s permissive approach. In the end, there were millions of tons of asphalt mixture with a warm mix technology dropped in, placed, and performing under the rigors of Northeast traffic and weather conditions.
What was happening in New York was going on across the U.S. States were piloting projects, adopting specifications, and realizing that Warm Mix Asphalt was a viable option to conventional hot mix.
Research continued, but the best research is always placing mix on a road and watching it perform. With all this work going on and acceptance taking place, we still never got to a point where there was a consensus definition for warm mix.
Should it be defined by a production temperature range?
Or was it just mix with a warm mix technology being utilized?
The latter makes it more of a mix enhancer than a true warm mix. Was it a difference or delta between conventional productions temperatures to reduced temperatures?
Even today we struggle with defining it, but each state was and is working toward what warm mix means to them.
Back to New York: On July 18, 2019, the Climate Leadership and Community Protection Act (CLCPA or Climate Act) was signed into law. The state’s Climate Act is among the most ambitious in the nation and requires New York to reduce economy-wide greenhouse gas emissions 40% by 2030 and no less than 85% by 2050 from 1990 levels.
The enactment of the CLCPA forced industry and agencies to look closely at all their operations, functions and specifications and determine compliance with the law and its goals.
NYSDOT and the asphalt industry decided it was time to make another aggressive move. We knew WMA had many advantages whether at reduced temperatures or not. We had done millions of tons to prove its performance and value. It was time to move to full adoption of warm mix, but we needed to mandate a temperature reduction. So, on Sept. 21, 2021, Engineering Bulletin EB21-047 was issued.
EB21-047 effectively changed the design manual’s requirements for paving project. This revision moved NYSDOT to 100% WMA with a true temperature reduction (295 degrees at the paver).
For all practical purposes, this was a 20-to-30-degree reduction in temperatures. New York was the first state to move in the direction of placing a temperature requirement to WMA. And the specs now housed new paving items that are referred to as “404” items.
The “404” designation required the use of an approved warm mix technology along with the temperature reduction. For a short period, NYSDOT kept “402” items (conventional hot mix asphalt) in the specification book. This was to allow for existing projects to be completed.
But, as of Jan. 1, 2023, “402” items no longer exist in the specification book. Designers no longer even refer to pavements as warm mix or hot mix. They are now asphalt mixtures which are placed at 295 degrees or lower.
It may look like NYSDOT was moving aggressively to WMA. But if you follow the timeline, it was a methodical thought-out process that allowed for industry and the agencies to have complete confidence when this final move was made.
The change has been in place for the past two years. As with any big adjustment, there has been learning curves to the process. New York has climatic challenges. We call early and late season (spring and fall), the shoulders of the paving season. Paving in 80-degree weather is ideal.
When temperatures drop, a paving contractor must be diligent about their paving and compaction practices. This doesn’t change with WMA In the end, NYSDOT took steps to comply with the state’s climate goals along with ensuring that pavements continue to perform as they should.
NYMaterials and industry leaders continue to work closely with NYSDOT to move the latest and greatest technologies forward to be sure that taxpayers money is spent appropriately.
The warm mix story is just one of many positive and progressive moves the asphalt industry is taking. I can’t wait to be part of what is next! RB
Bruce Barkevich is Vice President – New York Construction Materials Association