How To Make Warm Mix Asphalt Work At Your Plant

Developed in the 1990s, Warm Mix Asphalt is anything but a new technology. However, there's still a fair amount of stigma associated with it in the road building industry, despite the Federal Highway Administration's pivot in 2010.

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If you have ever attended an asphalt conference, or signed up for any webinars as part of your state’s asphalt pavement association, or you’ve ever heard of environmental product declarations (EPD), then warm mix asphalt (WMA) has probably been a concept you’ve heard a lot about.

For the possibly uninitiated, however, WMA as a process was first experimented with and pioneered in the United States, in 1956 at Iowa State University by Professor Csanyi. Although the modern foaming techniques utilized today were developed in the European market in the late 1990s. It wasn’t until 2010, when the Federal Highway Administration (FHWA) incorporated WMA as part of its Every Day Counts project, that the process began to find a bigger foothold in domestic markets. Around the same time, chemical additive technologies were introduced and now make up for 60% of all WMA produced.3

The process is defined by the FHWA as asphalt produced anywhere between 30 to 120 degrees below the standard hot mix asphalt (HMA) production temperatures, and that ranges anywhere from 300 to 350 degrees.4 

However, according to the Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage 2021 created by the National Asphalt Pavement Association (NAPA), “The weighted average temperature reduction achieved among asphalt mix produced at reduced temperature was 23.5 degrees.”3

That appears to indicate that, on the weighted average, the production of WMA is not falling into the FHWA defined range. The NAPA survey went on to show that almost 84 percent of the WMA produced fell between 10 and 30 degrees reduced production temperature. That means the vast majority of WMA made in 2021 potentially, just reached the minimum end of the temperature spectrum.5

While even a small temperature reduction still has energy and environmental significance, who are the asphalt producers that are sitting well above that minimum temperature reduction threshold? Who is taking the big swings? Maybe more importantly, how do they set about to make the transition from HMA in the first place?

Really Big Swings

When Carmine Pace, the Quality Control (QC) Manager at Hubbard Construction’s Tampa and Lakeland Florida asphalt plant locations, decided to bring WMA to their facilities, he chose to do it in what might be considered an unconventional way.

“In January 2023, I went to management, and told them that I wanted to try it,” Pace said, somewhat expecting a little pushback. To his surprise, they were nothing but supportive and encouraging of the experiment. “They told me to go for it, and to see how it might work for us.” 

Hubbard’s parent company VINCI has set Group Environmental Goals to reach by 2030, and WMA is a big step to achieving those reduction goals.

That was one hurdle, and in some ways you might think the more difficult obstruction, but Pace knew the truth. Getting the office to sign-off on the idea was one thing, but bringing that same idea to the paving and plant crews was going to be another task entirely.

“Historically, you get pushback when there’s change,” Pace said. “That’s the way it is in any industry, you try to make a change, you’re going to get some pushback. We wanted to find a way to get around that, and see what would happen without people not even giving it a fair chance.”

To make the switch to WMA wouldn’t be as easy as flipping a switch. Perhaps, when it comes to the asphalt plant itself, sure, but as with all changes in a company team structure, things will go much smoother if you can get broader buy-in from ground level stakeholders. Plus, you’ll have a lot greater chance of success that way, rather than simply forcing a change from the top down.

“My biggest challenge, was that I wanted to initially try it out on a project, but I didn’t want to tell the paving crew ahead of time,” Pace recalled. “We were dropping the mix forty-five degrees from the usual target. Thankfully, our plant superintendent used to run warm mix years ago, so he was on board for making the adjustments. At the time, of course, we were still running hot mix, and we had to be careful not to clog up the plant while we figured out the ideal temperature. We quickly discovered there is more of a tender zone, and you want consistency in that zone in the production of the warm mix.

Once the warm mix design was approved by the owner, Hubbard put their plan into place. For two consecutive days, they produced warm mix for one of their county projects, and their laydown crew was none the wiser. This was despite the fact that the mix temperature was around 265 degrees, a distinct difference from their usual target temperatures ranging between 310 to 320 degrees.

“They had no idea,” Pace said. “The first few days went so good, that we immediately went and had official mix designs made.” The warm mix designs were performing as good as he hoped they would, and, for the most part, they’d conducted their experiment without the knowledge of our field personnel. However, that wasn’t to say that it was completely devoid of suspicion.

Benefits and Opportunities

Veteran asphalt paving crew members have seen just about everything, and there was one visual cue that did at least raise an eyebrow or two. According to Pace, his foreman noticed that the smoke coming off the mix wasn’t as heavy as it normally was, especially for January in Florida. Pace’s foreman revealed that he’d noticed it, only after the warm mix trial was successfully completed.

This is consistent with what asphalt science says about the production of WMA. According to the European Asphalt Pavement Association’s website, there is a direct causation between the reduction of temperatures and the fumes and odors released by the final product.1 For approximately every 50 degrees you reduce the temperature, you also reduce the fumes released by ~50 percent. That’s not just an environmental benefit, it’s a direct benefit to the people on the crew itself.2

“The workability of the asphalt was exactly the same,” Pace said, which probably contributed to the crew’s suspicions not being raised any further. “They paved on a couple cul de sacs, as well as making some straight pulls, and it was a total success.”

Their initial warm mix project saw about a half of a percent jump in density. As they’ve continued to refine the process their compaction averages jumped from between 90-92 percent, to upwards of 93-94 percent, on a regular basis.

In addition, Pace was able to compare the previous days hot mix runs side-by-side with the warm mix, and the results were stunning.

“We actually saw that the warm mix side was a darker, deeper color,” said Pace. “It took a longer time to oxidize than the hot mix.”

Now, after seeing it perform in the field, the majority of Pace’s team was satisfied with how its worked out. That doesn’t mean they are content to sit back and coast from here on out. In fact, the success of this warm mix experiment seems to only have amplified their motivations towards further experimentation.

“We were already using Ingevity’s Evotherm WMA additive as a liquid anti-strip, we just were not taking advantage of its full potential,” Pace said. “Now, we are seeing just how low we can get the warm mix target temperature. We’ve pushed it down as low as the mid 260s, and we had no issues while doing some straight pulls.” That’s nearly a drop of 45 degrees from their original HMA productions temperatures, which would result in an astounding ~75 percent drop in fumes and odors released.1

According to the National Asphalt Pavement Association’s 2021 Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage, only 2.4 percent of reported WMA was produced at a reduced temperature greater than 50.1 degrees.3 This would place the WMA produced by Pace’s team in Tampa in the top three percentile of producers in the country. They’re on the cutting edge of WMA production.

If you are considering adopting WMA or increasing the production of WMA at your plant, but might need something to sweeten the deal for those making the key decisions, here’s something to consider. While you would need to do a more detailed lifecycle analysis to get an exact figure, NAPA’s research suggests that for every 10 degrees of production temperature reduction, there is between a 2 and 3 percent fuel consumption savings.

Using Pace’s team’s warm mix production temperature reduction, they are likely seeing between 14 and 21 percent fuel consumption reductions. Those type of savings for costly fuels and their unpredictable changes in price and availability can translate into millions of dollars in savings.

With temperatures as low as Pace is currently trialing, it might make you concerned about travel and storage times, but Pace says the opposite has been true, reporting no issues with that at all. In fact, they report improved overall workability with their product due to the use of the WMA additives. They’ve stored the mix overnight sometimes up to twenty-four hours, and also sent warm mix out to some job sites with haul distances reaching up to an hour from plant to pavement. This is consistent with long establish science on WMA, but there are many other documented benefits as well. 

Due to the decrease in temperature, WMA compliments mixes utilizing higher reclaimed asphalt pavement (RAP). That’s good news, because RAP stockpiles have been on the rise, trending upward since 2016. The NAPA industry survey from 2021 reported that, ever since then, more RAP has been stockpiled by asphalt plants than was utilized in production. The reported 2021 stockpile level reached a national high of 137.5 million tons, which was approximately a 2 percent increase from the previous year.3

It’s important to note that overall asphalt production has also increased during that same period by 57.5 million tons, or a 13.3% rise from the period between 2016 and 2021. For those looking to up their percentages of RAP and make a dent in their current stock, while additionally reducing their need for expensive virgin aggregates, WMA is should absolultely as be considered as an option.3

It Hasn’t been all smooth sailing.

“Producing warm mix has had its issues,” said Pace. “There was a learning curve, adjusting temperatures up and down and between multiple mixes throughout a shift can lead to mistakes.” One of the biggest challenges they faced was how to handle the volume of outside sales, and how their customers would react.

“That took a lot of education, communication, and continued follow-up with our FOB customers across our market,” He continued. “They’ve only paved with hot mix their entire careers. We have mostly had success, but we still have to be proactive, checking on the customers while championing the technology.”

Pace concluded by saying, “Hubbard is pleased with this year’s successful venture into warm mix and establishing themselves as a warm mix producer in the state of Florida.”