Overcome Recycled Asphalt Anxiety
Increasing the use of RAP continues to face limitations across the country. However, experts from Ohio, Texas and Michigan offer a message of hope with the advent of BMD.
The Ohio Department of Transportation (ODOT) contracted with RAP Management, Columbus, Ohio, on its first official balanced mix design (BMD) project in August 2021. With its new Ammann high recycling technology (HRT) batch plant, RAP Management would produce a 55% RAP mix for the southbound lanes along 4.6 miles of State Road 664 to compare against a normal medium traffic mix with 20% RAP placed on the highway’s northbound lanes.
According to ODOT’s State Asphalt Materials Engineer Eric Biehl, “The production volumetrics [of the 55% RAP mix] were all spot-on.” Ultimately, the high-RAP mix performed as well as the control mix, if not better. During a recent webinar put on by Asphalt Testing Solutions & Engineering (ATS), Jacksonville, Florida, Biehl said he thinks BMD will open the door to increased RAP use in the future.
Biehl said RAP caps for ODOT jobs range from 15 to 25% for the surface mix and up to 40% for intermediate mix. There are also caps based on traffic load of 55% for light and medium trafficked roads and 40% for heavily trafficked roads. “We don’t technically hit those points, but we do see pretty high RAP,” Biehl said. Instead, the limitations to RAP use in Ohio relate to the requirement that producers perform binder testing and that anything above 25% RAP requires PG -28 or -22.
Binder testing is also a limitation contractors experience in Texas, said Corey Pelletier, director of special projects at Anderson Columbia in Texas. “Texas requires binder testing if a project uses more than 20% RAP in the mix,” Pelletier said, adding that the Texas Department of Transportation (TXDOT) doesn’t have a limit on the amount of RAP other than the binder testing requirement above 20%.
“If there’s a large stockpile [of RAP] at a permanent plant facility, they already know what’s in that pile and have time to do the testing and are able to put in the maximum amount of RAP,” Pelletier said. However, projects in more rural areas rely heavily on the use of portable plants and are primarily using RAP coming off that project. Furthermore, mill-and-fills are common across the state. In those scenarios, the binder testing requirement creates a timing issue, and so those jobs often opt for less than 20% RAP. “That’s what’s hindering our use of RAP in Texas.”
Brett Stanton, executive director of the Asphalt Pavement Association of Michigan (APAM), said the Michigan DOT (MDOT) has tried to simplify binder replacement criteria for RAP into three tiers. “Tier 1 includes mixes up to 17% binder replacement where there are no additional binder grade requirements,” he said. “Tier 2, 18 to 27%, often requires a binder dump on the low end with an option to drop the top grade as well or do a blending chart to show you have not impacted the final grade, and Tier 3, anything above 27%, requires a full binder grading to show what the virgin binder grade must be to meet the planned grade on the lower end with the top end contractor-optioned.”
However, producers in Michigan face other types of limitations, including Department of Environmental limits on RAP usage in order to get a plant approved. “We have a lot of plants bumping right up against [the limits].”
Another challenge producers face in Michigan is a stigma related to past failures perceived to be the result of RAP. “We need to educate people who saw, or thought they saw, a failure and capped RAP at lower percentages,” Stanton said, adding that some of Michigan’s counties cap RAP at 10 or 15%. Other counties use MDOT’s specification, which results in counties bordering one another having very different RAP specifications. Not only does that add cost to the counties as contractors must produce different mix designs, but it’s also arbitrary, he said. A certain percentage of RAP might be fine in one county, but deemed unacceptable one foot into the next county over.
Pelletier believes a better approach would be one based on materials available. That is the common denominator limiting RAP usage he’s seen in the nine states he’s worked (Florida, Texas, Missouri, Kansas, Iowa, North Carolina, South Carolina, Alabama and Georgia). “The mix is only going to be as good as what you put in it, and material deviates so much within a state, let alone state to state,” Pelletier said. He doesn’t think specifications pertaining to RAP percentage should be the same, regardless of the materials. “Being stuck with the standard spec is one of the big problems we run into. I think it needs to be more relaxed so long as you can prove you’re achieving the end results.”
Another limitation in Texas is that many regional engineers prefer RAP from state-owned projects, which means RAP from local roads and parking lots often cannot be used. This requires producers to have separate piles to keep state RAP separate from other RAP sources.
BMD Offers Opportunities
BMD may be the answer to several of those limitations to increasing the use of RAP and it’s one state DOTs are watching closely.
“We’ve come a long way as an industry on how to better process, utilize and design with RAP,” Stanton said, citing the breadth of research showing that medium- to high-RAP mixes can perform. To take advantage of that expertise, he hopes to see Michigan use some sort of BMD approach with performance metrics during design and checked during field production. “If we tell the industry how we want the pavement to perform and let the contractor use the tools in their toolbox, whether that’s warm mix, RAP, etc., that’s how innovation will come.”
Otherwise, he added, there’s a risk that some producers could shoot for a certain RAP percentage that meets all specs while not achieving the performance one can expect of an asphalt pavement. Pelletier cited an example from Texas where contractors overused recycled asphalt shingles in mixes to realize cost savings to the detriment of pavement performance.
As RAP percentages increase, that’s the benefit of a performance-focused BMD approach rather than the process-oriented approaches of the past. “We have the tools to properly engineer these mixes, but we need to be able to test and adjust on the fly like any other QC adjustment. If you see air voids starting to tick down, you know you need to make a void adjustment. We need to be doing the same thing [in regards to RAP],” Stanton said.
On the DOT side of the equation, Biehl said the limiting factor to increased RAP use is DOTs “having the confidence in [their] performance index limits.” ODOT only began searching for its first official BMD project once it was fully confident in its cracking test (IDEAL-CT). “The criteria is what’s holding [DOTs] back right now. If you can get those figured out and relate them to some sort of performance, it becomes a matter of opening up the specs.”
Although there is some progress yet to be made before that can happen, with the recent success on Ohio’s State Road 664, Biehl’s parting message is a positive one: “Don’t let recycling scare the RAP out of you!”
Stanton said there is a possibility, with increased work to come, that it may be possible to run low on RAP. “It depends on how much RAP you’ve got stockpiled in the yard now,” he said, “but we’ve seen in certain areas around our state that have gone to full reconstruct – brand new pavements were removing concrete and putting back asphalt, there’s no RAP coming off the job and putting back 10 or 12 inches of asphalt and you’ve got 25 or 30% RAP in there, you’re starting to burn through RAP faster than you’re bringing it back in. We haven’t hit that yet, but have talked to a few members that are concerned what will be the future for RAP and hedge bets to figure out where to haul it from and where in the state there’s excess to make sure there’s enough supply, because it’s a very big bargaining chip when it comes to bidding work; you’ve got to have it in order to be competitive.”