Heartland Asphalt Pushes the RAP Envelope with Invigorate
BY Rick Zettler
The asphalt industry, and asphalt producers in particular, are continually searching for ways to increase recycled asphalt pavement (RAP) content in mix designs. “It makes sense to recycle as much asphalt as we can,” said George Jessen, president and general manager for Heartland Asphalt Inc., Mason City, Iowa. “It will extend the life of our natural resources, and it will help our customers by stretching their dollars farther.”
The catch, according to Jessen, is to increase RAP use in a responsible way. When increasing the amount of recycled asphalt in mix designs, producers can end up introducing oxidized binder that is harder and stiffer and that affects the performance characteristics of the asphalt. “It can make the asphalt more brittle in cold temperature times of the year,” Jessen added. This can lead to premature road failure in the field.
To prevent adverse road performance, state departments of transportation (DOTs) place limits on the percentage of RAP used in asphalt mixes for state and Federal projects. For asphalt producers in Iowa, this comes in the form of capping binder replacement.
“The Iowa Department of Transportation specifications allow us to include as much RAP as possible, but we must use a minimum of 80 percent virgin binder,” explained Rich Millard, quality control manager for Heartland Asphalt.
It is possible for Heartland Asphalt and other producers in the state to drop to a minimum of 70 percent virgin binder, but it comes at a cost. “We must use a softer grade binder,” Millard continued. This means dropping the standard PG58-28S to a more expensive PG52-34S binder. “Often, it is cost-prohibitive for us to bump to a softer grade to increase the percentage of RAP used in the mix,” Millard said. Therefore, the asphalt producer will typically run roughly 25 percent RAP in the mixes to adhere to the 80 percent virgin binder threshold.
Rejuvenate the Binder
Late in the 2020 Iowa paving season, Heartland Asphalt had the opportunity to work with a new type of binder rejuvenator—one that worked differently than most petroleum-based products. The trial offered the opportunity to boost RAP content and lower the virgin binder percentage and aggregate required without the need to bump to the more costly softer binder.
“I’m on an Asphalt Pavement Association of Iowa (APAI) committee to find ways to increase the use of RAP in the state of Iowa, so this presented a great opportunity to get involved,” Jessen said.
Another positive for Jessen and producers in Iowa and throughout the Midwest is the rejuvenator used in the trial is a soy-based product, designed to be sustainable. Invigorate was developed by researchers at Iowa State University (ISU) in Ames, Iowa, along with Colorbiotics, also based in Ames, which is a brand of the MBCC Group. It is soybean oil-derived and creates a chemical reaction to reverse the effects of oxidation on recycled asphalt.
Chris Williams, professor of Civil Engineering at ISU, has been working in the asphalt research paving industry for nearly 25 years. Throughout his time with ISU, he has studied rejuvenator performance. He noticed that most rejuvenators work to average out a balance between asphaltenes and maltenes in old, oxidized recycled asphalt binder.
To account for the brittle, oxidized asphalt binder in RAP, most rejuvenators work by injecting maltene-rich light material to make it softer. “They take middle molecular weight molecules,” Williams said. “It’s like taking something that is heavy and something that is light, and the average weight is something in the middle to create something that is ‘soft.’” These types of rejuvenators don’t necessarily address the asphaltene agglomeration from oxidation.
Invigorate’s soybean oil molecules create a chemical reaction with the asphalt binder to reverse the effects of aging. “What Invigorate does is take something heavy, break it down and reduce the overall weight into something lower,” said Dan Staebell, Colorbiotics asphalt business development manager. “The chemistry creates value by extending the life of the pavement.”
Field Trials with Invigorate
Heartland Asphalt picked two county road projects in Cerro Gordo County in north central Iowa, less than 40 miles from the Minnesota border, to test the new rejuvenator. Millard explained that the producer has more flexibility with commercial and county mixes when it comes to trying out new mix designs with higher amounts of RAP.
“We often work with county engineers to test new ideas,” he said.
The Cerro Gordo County Road S62 overlay project required 5,000 tons each of a surface and intermediate mix. The mix design included a maximum 0.5-inch virgin aggregate size and RAP size of 1.0-inch minus.
“We have a small crusher and screen on the plant to properly size the RAP before it goes into the drum,” Millard said.
Heartland Asphalt used its standard PG58-28S binder for the trials. In the Invigorate trial mixes, the rejuvenator was pumped from a separate on-site tank directly into the asphalt cement (AC) line before being pumped into the drum mixer.
“All asphalt was produced by our five-year-old DuoDrum plant, and injecting the rejuvenator into the AC line gave it extra time for the rejuvenator to mix with the binder,” Jessen said.
Staebell explained that Invigorate can be injected into the mix in multiple ways to meet the producer’s needs. “It can be blended with the binder at the terminal, blended in the tanker or injected separately in the AC line at the plant, like Heartland Asphalt did for the Cerro Gordo Country project.”
The trials included mixes with several different levels of RAP content. The control sample was mixed with 20 percent RAP, while three different levels of RAP—30 percent, 40 percent and 45 percent—were mixed with and without the Invigorate rejuvenator. The mixes were laid in 600- to 700-ton strips.
“We targeted an Invigorate rejuvenator dosage of 5 percent weight by volume of binder,” Staebell explained.
Jessen added: “We discovered a metering issue with the rejuvenator tank pump early on but were able to get that quickly fixed to get the target rate.”
Other than adding the binder, everything else remained the same as if Heartland Asphalt were running its typical mix with RAP. The asphalt was mixed at the same temperature, transported to the jobsite in the same belly-dump trucks and laid out in the typical windrow as any other county road project.
The paving crew used a pick-up machine to transfer the windrowed asphalt into the hopper of the 10-foot paver. Heartland Asphalt used the same rollers and rolling pattern for a county project to compact the mix to spec densities.
“The only difference the crew noticed,” Jessen mentioned, “was the Invigorate mix smelled differently, and not in a bad way.” With the soy base versus petroleum base, “it kind of smelled like a hamburger stand,” Millard joked.
Heartland Asphalt gathered asphalt samples from the plant and at the jobsite for both lab and field tests. In total, eight cores were drilled in each section of the different mix designs with different percentages of RAP.
Millard mentioned that the Invigorate samples with varying percentages of RAP tested similarly to the control sample with virgin binder and aggregate. “The film thickness run was good, and the samples passed density tests,” he said.
Using the rejuvenator significantly lowered required virgin oil content. “The 45 percent high RAP content mix with 5 percent Invigorate brought virgin oil content to 61.7 percent with 38.3 percent of binder coming from the RAP. This can result in economic savings,” Millard said.
Jessen added, “If using the rejuvenator is cost-efficient and good for the environment, that makes us more competitive, so it makes a lot of sense to consider using more RAP.”
In the field, spec compaction densities were 92.5 percent. Running two days of density tests for the Invigorate mix samples showed density readings of 94.4 percent and 93.8 percent, indicating that even with higher RAP content, density was readily obtained by the rollers.
ISU engineers also ran performance grade and Delta Tc tests on the lab samples. All Invigorate samples with the proper dosage, even those with high percentages of RAP, improved the performance grade of the binder over the standard PG58-28S spec.
“The Invigorate samples also passed all Delta Tc tests, showing its efficacy with high-RAP-content mix designs,” Staebell said.
Beyond performance, Staebell pointed to the sustainability factor for the industry using a soybean oil-derived rejuvenator. “The ability of being economical and environmentally friendly really is the homerun on sustainability for the industry,” he said. “The asphalt industry is certainly looking for those avenues to become more and more sustainable, which is obviously great for the future.”