Within the construction and demolition industry, recycled asphalt, known as reclaimed Asphalt pavement (RAP), has been successfully used for decades and grown slowly over the past six or so years that the National Asphalt Pavement Association (NAPA) has been tracking trends.
In fact, according to NAPA and their latest survey data, during the 2016 construction season more than 76.9 million tons of RAP and nearly 1.4 million tons of recycled asphalt shingles (RAS) were put to use in new pavements in the U.S., saving taxpayers more than $2.1 billion. Also, more than 30 percent of all asphalt pavement mixture produced in the country that year was made using warm-mix asphalt (WMA) technologies.
According to Randy West, Ph.D., director of Auburn University’s National Center for Asphalt Technology, the amount of concrete that is recycled is miniscule compared to asphalt.
Other recycled materials that are sometimes used in asphalt paving mixtures include recycled asphalt shingles, recycled tire rubber, slag, foundry sand, re-refined used engine oil, paper (cellulose fiber), and to a much lesser extent, glass and plastics (low-density polyethylene).
“RAP is primarily generated by milling existing pavements as part of the rehabilitation of roads and highways,” West said. Milling removes distressed (cracked, rutted, potholed) pavement layers and provides a better surface for an overlay. This operation is done within the asphalt paving industry, so it is a self-contained part of the C&D industry.
“There are not a lot of regulations around milling other than state DOT specifications regarding the smoothness and texture of the milled surface,” West said.
“There are some new regulations regarding the generation of silica dust that will impact milling operations.”
Over the last 10 years asphalt recycling has increased across the construction industry. More contractors, engineers, municipalities and private owners have become aware of the value of recycled asphalt.
As Kris Bernens with Mac Asphalt in Clark County, Indiana, explained, asphalt recycling comes in different methods, in-place (hot or cold), incorporating in new hot mix asphalt, and manufacturing of cold mix asphalt.
“All of these methods are reducing cost of constructing new or revitalized roadway surface,” Bernens said. “Recycling asphalt is also reducing our carbon footprint. We use less virgin liquid asphalt in our new hot mix asphalt (HMA) mixes because we are reclaiming the existing asphalt from the RAP. When we recycle the asphalt in-place, the RAP is processed on-site with rejuvenator liquids added to the process.”
Also, by processing on-site the RAP is not trucked to the asphalt plants processed and then hauled back to the site. All asphalt removed from Mac Asphalt’s construction projects is recycled at the company’s asphalt plant or processed to become a new sub-base for parking lots or roadways.
And when it comes to asphalt recycling, regulations vary from agency to agency. State agencies set limits of RAP usage in new HMA mix designs. For instance, Kentucky and Indiana have different limits but when removing asphalt from projects all typical regulations apply.
“Most of the regulations for these recycling operations pertain to dust control and keeping the asphalt away from any water sources,” said Gianni DiFranco, president of DiFranco Contractors Inc. in Chardon, Ohio. “It’s a relatively clean process as long as it’s done properly. Locally we have a recycling plant near a residential area that doesn’t cause much uproar with the residents.”
Recycled asphalt has been used for some time in DiFranco’s area for aggregate in hot mix asphalt. When the material is heated up it’s reintroduced into new hot mix and does well as the oil is still in the old material. Recently, some ready mix concrete operation are adding recycled concrete aggregate to their mix designs to concrete that doesn’t need to be freeze/thaw cycle-resistant like concrete footers.
“This a great way to use these materials as it replaces the original virgin material used with a recycled one,” DiFranco said.
Of course, construction and demolition contractors are always more motivated to recycle when there is a value or cost reduction to their services. “MAC Construction buys asphalt removed from demolition and construction projects at a low cost per ton,” Bernens said. “Often this sale of the demolished asphalt covers the cost of trucking the material to our plant site.”
Trends To Notice
State DOTs and the asphalt industry are very interested in further increasing recycling RAP, shingles, rubber, etc. because of the economic and sustainability benefits. According to West, the constraint has been that we lack test methods that can be used to ensure that asphalt mixtures with higher recycled contents will perform equal to or better than mixtures with all virgin materials.
“That has been a key focus area of researchers at the National Center for Asphalt Technology (NCAT) at Auburn University,” West said. “We are working with state DOTs and the asphalt industry to validate and refine lab tests that will be used to design and for day-to-day quality control of asphalt paving mixtures.”
As the NAPA website shows, across the U.S., the average RAP content of asphalt mixtures is about 21 percent.
“Once we nail down the laboratory ‘performance tests,’ it will open the door for much higher RAP contents and better utilization of other recycled materials,” West said. “Japan and the Netherlands currently have average RAP contents of about 50 percent. NCAT has built and trafficked test sections on its world famous Test Track with 50 percent RAP mixtures that outlasted all virgin asphalt mixtures, to prove it can be done in the USA with existing plants and construction methods.”
Another proven recycling technology that is gaining momentum is cold recycling of RAP as a base layer in pavements. This approach uses about 97 percent RAP with approximately one percent foamed asphalt or asphalt emulsion and one percent cement.
As West explained, the approach was developed in the late 1950s and has been used around the world, but somewhat forgotten in the U.S. until recent work by the Virginia DOT.
“VDOT worked with NCAT to prove the cold recycled layer would hold up to heavy traffic and develop input properties of the material for pavement design,” West said. This work was recognized in 2017 by the American Association of State and Highway Transportation Officials (AASHTO) as a Sweet Sixteen High Value Research Project.
Also, many engineers and designers are starting to let these materials be used where they were originally not. For instance, in Ohio, most public works projects would not let a contractor use recycled concrete aggregate for sub base in roads.
“This is starting to change, which is great for the building green trend,” DiFranco said. “Most aggregate used in residential construction is recycled in Northeast Ohio.”
In the asphalt industry a huge effort is being made to better utilize RAP. Manufacturers of HMA plants are working on better ways to process the materials. According to Bernens, manufacturers of equipment are working on ways to better process the materials on-site.
“Asphalt rejuvenators are becoming more widespread across the country,” Bernens said. Asphalt rejuvenation is a process of adding oil back into the demolished asphalt to increase the asphalt binder content. The asphalt binder is lost over time due to oxidation caused by UV light from the sun and scrubbing from traffic.
“Some of our asphalt rejuvenators are made from recycled products as well,” Bernens said.
According DiFranco, it does seem that there has been some improvement of the machines that are used to grind up the recycled asphalt and concrete materials.
“The new machines require less maintenance and do a much more efficient job making the aggregate,” DiFranco said. “Previously, concrete pavement that was recycled usually had a lot of steel reinforcement material where today it is virtually free of any metals. The amount of asphalt and concrete has dramatically increased in the last 15 years. Virtually all this material is recycled and turned into aggregate.”
It’s important to note that, in order for these materials to be recycled properly, they need to be kept clean. Many contractors tend to mix too much dirt and other organic materials when removing the pavement. “This creates a problem when the material is recycled as this organic material is not to be used in aggregate as it doesn’t compact properly,” DiFranco said. “Care must be taken when the concrete or asphalt is removed or it can’t be recycled properly.”