Ask anyone in the asphalt industry what the drawback of using crumb rubber additive for a recycling project is and they’ll tell you the temperature creates a “scent” issue. While the aroma of highly heated crumb rubber may smell like saved money and environmental sustainability to those of us in-the-know, it smells stinky to the general population. Enter the teams from Liberty Tire Recycling, Pittsburgh, and Odor Solutions Group, Phoenix.
When Doug Carlson, now the vice president of asphalt products for Liberty Tire, was the executive director of the Rubber Pavements Association (RPA) from 2005 to 2010, he encountered a situation with one of the association’s members whose rubber blending plant in a California bay area had housing development build up around it. Those familiar with California’s specs know asphalt rubber (AR) there is usually blended at temperatures around 350 to 375oF. When the binder has about 20 percent AR in it, it is held in agitating tanks for about 45 minutes, and then pumped to a drum for mixing with the aggregate.
“Neighbors around the mixing plant noticed the different odors coming from the AR production and made complaints to the local air quality management district,” Carlson explained. The project at the time was shut down while the district investigated the complaints. At that time, “The contractor manufacturing the AR binder elected to use the AS Cherry product [from Odor Solutions Group] as an additive to the binder to help reduce the rubberized asphalt odors.”
That story had a happy ending. With odors reduced and complaints resolved, the project was allowed to continue, according to Carlson. When he had a crumb rubber project coming up in 2015 for Michigan’s Ingham County that would use Liberty Tire’s new Mix Maxer product, Carlson remembered that success and decided to get proactive.
“Based upon my experience with rubberized mixes, I know that plant neighbors and paving crews may sometimes object to the smell of tire rubber, and I didn’t want that to happen on this first project with the new material.”
Liberty Tire and its technology partner, Innovative Polymer Solutions, Valrico, Florida, had developed the Mix Maxer pre-swelled rubber technology that mimics the AR process by reacting dry rubber powder with a small amount of liquid at elevated temperatures. “The benefit of the pre-swelled rubber is that it is instantly ready to use in a mix to provide a low cost method of modification,” Carlson said.
The product is added to a mix at a rate of about 10 pounds per ton. “The 10 pounds per ton of mix formulation was equivalent to a PG70-28 type of mix,” he shared. “The biggest benefit is that this next generation, improved dry process, eliminates the logistical problems associated with modified liquid storage. It does not require an extra asphalt tank at the plant to deliver a modified asphalt to the customer. The pre-swelled treatment of the rubber ensures that asphalt absorption problems and extra binder requirements that plagued the old dry process of the 1990s do not occur.”
The small batch mixing and pre-swelling process took place at a toll processing site under Liberty Tire’s direction in an enclosed warehouse. “I was concerned about the potential fume and odor that could result from the high temperature mixing and swelling process, and elected to use the AS Cherry product, as it had worked so well before,” Carlson shared.
He didn’t need to add a lot. He said they used “about half a medicine dropper full” of AS Cherry added to each batch of the liquid that was used to pre-swell the rubber.
“No liquid or rubber fume or odor was noticeable during production,” Carlson said. “The end product—a dry, free-flowing rubber powder—actually has less of a tire rubber smell than the original tire rubber. The Mix Maxer material was developed to make rubber much easier to use at the mixing plant. The pre-reacted rubber does not require any processing at the mixing plant, and can be added directly to the asphalt mix through the RAP collar.”
This is how Michigan Paving, Lansing, used the product for the reconstruction project in East Lansing.
“The project was part of a Michigan DEQ-funded Michigan State University research program that has been evaluating various types of rubberized asphalt mixes over the last few years,” Carlson explained. “The Mix Maxer material was selected for use on a small project in East Lansing on Lake Lansing Road. It was a reconstruction project, which involved new sewer and drainage along with widening and a new curb and gutter.
“Mix Maxer was used in all three lifts in various test sections—one with Mix Maxer in the base layer only, one with Mix Maxer in the structural layer, one with it in the surface mix, and various combinations in between with regular unmodified mix. The project owner, Ingham County, followed MiDOT specs, with a special provision to include the rubber.
“The mix type was a MiDOT 5E Superpave mix with a PG58-28 binder with up to 33 percent plant RAP. Michigan Paving was the prime contractor supplying the mix out of the old Spartan Asphalt Plant, which was a short 2-mile haul away from the paving site. The paving took place during the daytime from Aug. 22 to Sept. 7, 2015.”
To make the Superpave mix with crumb rubber additive, the plant operator used a slightly modified fiber feed system to weigh the rubber, meter it and blow it into the RAP collar Carlson mentioned above. The plant ran at about 300 tons per hour and the Mix Maxer was added at about 10 pounds per ton. That comes to about 50 pounds per minute, which meant the crew put one 1,000-pound Gaylord box on the lift every 20 minutes or so to keep rubber in the system while running crumb-rubber modified mix.
“Larger hoppers are available and can scale up,” Carlson explained. “The system is simple to operate and provides rubber modification with a flick of a switch. On for rubber, off for conventional mix.”
The plant operator only has the Mix Maxer to add. Liberty Tire has made the AS Cherry odor-remover part of its product. “The AS Cherry additive is part of the Mix Maxer formula,” Carlson said. “I will continue to use it as it solves odor and fume problems and eliminates that risk at the plant and paving site.”
While on the Lansing project, Carlson experienced none of the discomfort too-often associated with crumb rubber mix laydown. “I could hardly smell the asphalt at all when the rubberized asphalt was being placed.”
Daniel Chapman, state trunkline superintendent for Calhoun County Road Department, worked on the E Lake Lansing Road project, and recalled the lack of typical crumb rubber scent with the new product. He compared the few times the treated mix had a noticeable scent to “a hung up tire on a low boy.” What Chapman described is a far cry from the days when contractors handed out masks for employees.
“Since the first experience I had with crumb rubber asphalt, there is a very noticeable difference in the smell of the material that is delivered to the site,” Chapman shared. “I am guessing that factors such as temperature and injection method may have something to do with it also, but the smell of the material is far less putrid. Back in 2011, it was obvious by the smell that there was rubber in the mix. Reith Riley even supplied masks for their employees while we were paving.”
Back when the AS Cherry and AS Pine odor removing products were introduced to the marketplace in the 1990s, some asphalt professionals noticed enhancements in the workability of mixes that included the additives. That anecdotal good news appears to be present to some degree in crumb rubber mixes now.
“The mix is a lot more workable than previous; however, it still will leave blemishes in the mat when you have to fix an area,” Chapman shared. “Workability used to be very poor. The mix seemed a lot stickier; however, now it is tender a little longer than before, but more forgiving with the blemishes, but not as forgiving as, say a 13A mix.”
Another positive for working with the treated crumb rubber mix is the temperature. “Temperatures at time of placement are lower than before,” Chapman shared. “They average, I would say, 290 degrees.”
To work with a mix that stays tender for longer than usual, Chapman offered this advice: “I would recommend the breakdown roller stay close to the paver and have the density technician check behind the breakdown. Once temps reached around 170 degrees, any additional compaction effort by the finish roller was unsuccessful.”