AKRON—Crumb rubber use in asphalt is not a new technology, but it's application is growing in the U.S.
Experts say states are taking an active voice in the ways their transportation departments choose to mix and lay rubberized asphalt.
Historically, crumb rubber has been more expensive to use than other polymer sets in asphalt. However, the tide seems to be turning as more benefits emerge from crumb rubber usage.
The topic of crumb rubber was discussed at length during last fall's International Tire Exhibition & Conference for tire manufacturers in Akron.
Proponents pointed out that rubberized asphalt takes the place of another material, such as a polymer.
“In general, in order to convince a customer to move to a recycled content product, usually that product has to cost less, has to perform better and needs to be recyclable itself,” said Douglas Carlson, vice president asphalt products at Pittsburgh-based Liberty Tire Recycling L.L.C.
The use of rubberized asphalt began in 1966 as a patching material, Mr. Carlson said. Charles McDonald worked for the city of Phoenix as an engineer and was interested in using tire buffings from a local retreading plant as a patching material with asphalt. The mix was about 30 percent rubber at that time.
Mr. McDonald tracked the performance of the road and, according to Mr. Carlson, in some cases the patch performed better—in some areas ending up as the only material left on the road. Mr. Carlson said Mr. McDonald then started looking for other opportunities to use the substance, especially in Phoenix, where the dry environment can cause materials to crack easily.
This led to the material being used as chip seals, and the industry later started making machines that could spray the hot, rubberized asphalt.
Previously, rubber cost more than asphalt. In August 1990, Mr. Carlson said, asphalt cost about $100 a ton. That amount gradually increased as the cost rose for oil, an ingredient in the asphalt. “So now it's very economical to put rubber in asphalt,” he said. “Rubber costs about half of what asphalt costs today.”
When creating rubberized asphalt, the product needs to be recyclable, he said, in order to prevent problems downstream. While rubberized asphalt has not always been able to meet these objectives, Mr. Carlson said it finally does.
Liberty Tire Recycling collects tires from around the country from tire dealers and other sources. Either the company goes out to pick these tires up, or someone is sent to retrieve them and bring them back to Liberty's plants so the tires can be shredded.
“A lot of people just think, 'Well it's no big deal. You just throw rubber into asphalt, right?' No, no. It's not that at all. There's a lot of science that goes into it,” Mr. Carlson said.
He related the process to baking cookies, where it is all the same basic formula. A person can still combine flour, butter, etc., “but you tweak some of the ingredients a little bit, and you get a different result.”
Mr. Carlson said that because rubberized asphalt is an engineered material, “you can't just throw waste product to asphalt and expect a good performing pavement.”
With the process, there are keys to quality control, such as using a paving machine and a roller on the road.
This prevents cracks, he added, and reduces maintenance on the road over time.
A myth about rubberized asphalt is that it only works in hot climates, Mr. Carlson said.
“The important part is just the temperature at the time you place the pavement,” he said. “You can't place it when it's really cold out.”
To achieve optimum results, it must be placed at 45 F or warmer.
Redmond Clark, CEO of CBL Industrial Services, said not much attention is being paid to recycling or moving the Environmental Protection Agency's (EPA) hierarchy, which he claimed is more focused on getting problems out of the way rather then letting markets develop on their own.
He said there are a few “asteroids that are approaching this market.”
A new generation of environmental regulations will enter the tire manufacturing industry that are going to spill over into the tire recycling industry, Mr. Clark predicted. Additionally, some fundamental changes will impact the U.S. economy as well as the global economy.
“So when we talk about climate change, you have to understand that climate change really isn't just climate change,” he said. “It is a recognition that sustainability now has got to be a part of every corporation's ethic in operations.”
The population is growing so much, he said, that a city the size of Chicago could be “built every three weeks for the next 35 years to keep up with what's already in the cards for population development.”
As the global middle class continues to rise, demand will increase on all the environmental systems, he said. Sustainability initiatives recognize that the world cannot tolerate growth as it is occurring and limits carbon fuel.
This will impact the tire manufacturing industry because carbon fuels are going to make for lighter cars, which means tires are going to last longer, Mr. Clark said, adding: “It is going to change where tires go when they are done.”
The supply side of the tire industry will be affected, as well as the waste disposal and recycling side.
Currently, about 57 million to 60 million tires are being used for crumb rubber domestically, Mr. Clark said, and of that, about 10 percent is used in asphalt. Other opportunities exist to push more rubber out of less green applications—such as disposing or burning—into more productive use. “We have to find a way to make these systems work and find a way to get them adopted by a large number of states,” he said.
Learning what customers want is important, according to Mr. Clark. State departments of transportation (DOTs), for instance, are large customers of rubberized asphalt. They use two processes for using crumb rubber in asphalt. A wet process involves mixing rubber in the liquid asphalt before it's combined with the aggregate and then treated, he said. With the dry process—known as the plant method—rubber and asphalt are mixed simultaneously. The mixture does not need to sit for any amount of time, he said.
With a wet process, he continued, the higher the percentage of rubber, the stickier the product is, thus special equipment is needed to lay it down. However, if the amount of crumb rubber in the mix is reduced, standard application equipment can be used.
No special equipment is needed with the dry mix process.
Mr. Clark added that the days of wondering whether rubberized asphalt really works are over. As long as guidelines are followed, the compound will be successful.
“The markets...tell us what we're doing right and what we're doing wrong,” he said.
The state DOTs can dictate what processes can be used on roads. Mr. Clark said California has decided that rubber should be used in asphalt, period. Some other states are on the same page, while others have decided to let the market sort it out.
In 2006, Florida's DOT decided to use rubber in asphalt, Mr. Clark said. It recommended using the wet process, but rubber or a polymer-modified asphalt could be used. The program has worked well.
“As far as Florida is concerned, the roads have been getting steadily better,” Mr. Clark added.
In the wet process, users have found that rubber can be more difficult to work with. Whereas the mixture previously was 2/3 rubber and 1/3 polymer, those ratios were reversed in 2013.
This is why it is important to understand each market, according to Mr. Clark. For example, rubberized asphalt is performing better in Georgia, he noted, but the state DOT is allowing the dry process to be used.
While some states still may be hesitant to use rubberized asphalt, its usage is becoming more popular.
Edgard Hitti, director of asphalt technical services for Paramount Petroleum Corp., said this process has been an evolution. It began more as an experiment but has grown to become more high tech than just blending two components together for pavement. It has grown from a simplified process to using a performance-graded (PG) material that consists of engineered properties and uses computerized equipment for formulation.
Paramount, which Mr. Hitti said produces product in asphalt terminals, incorporates tires into the binder so that it creates 100-percent liquid for the rubberized asphalt, combining the tire rubber—a solid—and the liquid components together. This type of mix has opened a lot of doors, he said.
While making the product, the manufacturer must ensure certain customer qualifications are met. For instance, Mr. Hitti said the qualifications for PG material are different among the states, such as California, Nevada, Arizona, etc. The mix may need to be tweaked based on historical performance, he said.
To ensure it is an approved supplier, Paramount performs theses tests, he said, and will submit weekly samples and other paperwork when necessary. In the case of a grant, the company may have to document exactly where the crumb rubber came from and the percentages in the batches.
He said it is imperative to recycle a tire because of the many expensive components and technology used in its production.
A common misconception is that rubber must be used in certain aggregates, he said. In reality, pavement is a combination of asphalt—which is the glue—and the rubber. Any application can be used, be it a prime coat or layering.
While rubberized asphalt is not yet the main market for recycled tires, experts say it is continuing to grow across the U.S., and they expect this trend to continue.