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|Federal Highway Administration > Publications > Public Roads > Vol. 68 · No. 4 > Recycled Roadways|
Publication Number: FHWA-HRT-05-003
by Jason Harrington
FHWA and the agency's partners are engineering high-quality pavements using reclaimed materials.
Today more than ever before, demands are being placed on the transportation community to find high-quality materials to renew and restore America's highways. Finding available high-quality aggregates has become a significant challenge in many areas of the United States. Because aggregates and paving asphalt are nonrenewable materials, the supply is limited. The United States produces 1.8 billion metric tons (2 billion tons) of aggregate annually. In response to increased demand, aggregate production is expected to increase to 2.3 billion metric tons (2.5 billion tons) by the year 2020—a statistic that raises concerns about the pace at which virgin aggregate is being consumed.
The Federal Highway Administration (FHWA), charged with stewardship of the Federal-aid highway program and environmental quality, has created a national policy on recycling and a team to assist the transportation community. In the March 2002 Administrator's Memo on FHWA Recycled Materials Policy, FHWA's Executive Director Frederick G. Wright, Jr., wrote, "The same materials used to build the original highway system can be reused to repair, reconstruct, and maintain [it]. Where appropriate, recycling of aggregates and other highway construction materials makes sound economic, environmental, and engineering sense. The economic benefits from the reuse of nonrenewable highway materials can provide a great boost to the highway industry. Recycling highway construction materials can be a cost-saving measure, freeing funds for additional highway construction, rehabilitation, preservation, or maintenance."
The memo continued, "Recycling presents environmental opportunities and challenges, which, when appropriately addressed, can maximize the benefits of reuse. The use of most recycled materials poses no threat or danger to the air, soil, or water. Furthermore, careful design, engineering, and application of recycled materials can reduce or eliminate the need to search for and extract new, virgin materials from the land."
Applying sound recycling principles that protect the environment, the FHWA Recycling Team is helping industry reclaim materials like asphalt and concrete pavement, foundry sand, scrap tires, and roofing shingles into new highway materials while conserving the Nation's natural resources.
The FHWA Recycling Team, working with the Recycled Materials Resource Center (RMRC) at the University of New Hampshire and others, finds appropriate highway applications for byproducts that are not used by the industry that produced them, thus keeping those materials out of the waste stream. Additionally, the team recognizes that some recyclable aggregates are already being used successfully in existing highways and bridges. "To derive all of the value from the original materials," says Dr. Constance M. Hill, FHWA environmental protection specialist, "why not use these best performing materials again? The more we recycle today, the fewer environmental impacts we will have in the future. Recycling is not only environmentally friendly but can be economically viable as well."
In some cases, the addition of industrial byproducts may enable bridge decks and structures to last about 75 to 100 years, which is many years more than current lifespans. Penn State researchers credit byproducts with helping to lengthen bridge deck lifespans, for example, because the added material reduces the permeability of concrete, deters salts from entering the concrete, and increases electrical resistance. Pennsylvania is planning to construct 10 bridges along Interstate 99 using concrete mixtures containing industrial byproducts such as fly ash, silica fume, ground granulated blast furnace slag, and an alkaline earth mineral admixture. Because water and salt may not reach the steel reinforcement rods in the bridge deck for 40 or 50 years, corrosion could be slowed and lifespan lengthened. Costs for using the new mix designs are similar to those of conventional mixes, but the estimated lifespan increases could provide Pennsylvania with savings of more than $35 million annually.
Indirect cost savings from using recycled materials also add up. Recycling concrete pavement prevents the recycled concrete aggregate (RCA) from going into landfills, for example, and, if the recycled material is used locally, can lead to decreased energy consumption and other costs associated with hauling and producing aggregate. In cities like Houston, TX, where the demand for new concrete is high, RCA is providing a value engineering solution. According to a 2004 FHWA report, all concrete rubble generated in Houston is reused as RCA within the city. This recycling saves time and money when compared with transporting aggregates from distant quarries. Improved air quality due to reduced transportation emissions is a benefit as well.
Asphalt: One of the Top Two Recycled Materials
Sound engineering principles based on research and testing provide detailed information on mix design, performance, and construction specifications to make recycled materials function as required. One goal is the concept of "closing the materials cycle," or using 100 percent recycled materials in road construction.
"The single most recycled material in the world is asphalt," says Byron N. Lord, a program coordinator in FHWA's Office of Infrastructure. According to Lord, "The U.S. highway community recycles more than 81 percent of all asphalt back into highway use."
In fact, reclaimed asphalt pavement (RAP) is most commonly reused as an aggregate for hot-mix asphalt (HMA). During this process, RAP is added as aggregate feedstock at the drum or batch plant, where it is combined with virgin aggregate and asphalt cement to produce new HMA paving mixtures.
In a 1996 report, Pavement Recycling Executive Summary and Report (FHWA-SA-95-060), FHWA researchers surveyed States that frequently use RAP in HMA. They found that over a 17-year period, the performance of recycled HMA designed and controlled during production is comparable to conventional HMA and actually can improve the materials properties of the existing pavement layer. Data from the Washington State Department of Transportation (WSDOT) supported this finding. In the late 1970s, WSDOT built two projects using more than 70 percent RAP (very high content, experimental) in the HMA. The two projects were monitored and did not show any unusual signs of mixture aging throughout their service life of 16 years. In comparison, the control sections, which included no RAP, lasted for only 10 years of service.
Use of Recycled Materials in U.S. Highways
Note that the use of some recycled materials may not be tracked across the Nation. A question mark indicates that the information is not known.
In the years since RAP first appeared in use, recycling techniques have evolved to create a more consistent and reliable product. In the past, RAP often was crushed but not screened or sized. The resulting recycled product may have been either fine or coarse and contained a varying amount of liquid asphalt.
"That's okay at 10 and 15 percent," says Dr. J. Don Brock, president of Astec Industries, Inc., a Chattanooga, TN, manufacturer of specialized equipment for building and restoring the highway infrastructure, "because it's such a small amount that you can stay within specifications." Adding more recycled material to the mix requires a more consistent recycled product. According to Brock, "When you get up over 20 percent, you need to treat it like any other aggregate. You need to crush it and size it to match whatever size of virgin material that the HMA plant is running."
Brock explains by offering an example of a contractor in Daytona Beach, FL, where rock cost $19 per ton and liquid asphalt was an additional $12 per ton. Staying within specifications enabled use of a maximum of 20 percent of unsized RAP in the mix. Mixes containing sized RAP enabled the contractor to increase the amount of recycled material to 45 percent and still stay within specifications. For this company, which sells 362,800 metric tons (400,000 tons) of product annually, the $7 difference per short ton resulted in $2.8 million in savings.
Today, RAP can be fractionated (separated) and screened. And, like its virgin counterpart, RAP comes in different gradations. "Since RAP usually comes from milling 0.50-inch maximum-sized aggregate surface mix, and by screening the recycle into 0.50 to 0.25 gradation and minus 0.25 gradation, black 78s gradation and black screening can be produced. The 0.50 to 0.25 size screen recycled aggregate contains approximately 3-percent asphalt while the minus 0.25 gradation will contain about 7.5-percent asphalt. If you do it right," says Brock, "recycled product makes an equal, if not better, mix than you would normally get with an all-virgin mix."
Brock points out an added benefit of RAP: In many cases, the liquid asphalt it contains is a better quality than that available today, which is made from a harder crude that contains more liquid oil than its predecessor did in order to soften enough for use. Additionally, if the aggregate is an absorptive material, RAP, which has already gone through the absorption process, produces a better asphalt. This is because old aggregate also might have been a higher quality aggregate than that which is used today.
Creating recycled materials that equal their virgin counterparts has meant making changes to processing, storage, and handling methods. For RAP, the changes include the redesign of plant technology for processing RAP into HMA and the use of drum plants that are capable of increasing the amount of RAP in the mix while staying within specification. Given that the amount of heat energy required during production increases with the proportion of RAP in the mix, some of today's plants use double-barrel mixers that retain more heat than traditional counterflow mixers. According to Brock, "As you superheat the virgin material up to 800 or 900 degrees, the drum shell gets up to 800 or 900 degrees." If the barrel is open, that heat dissipates and more fuel is required. In the double-barrel configuration, a mixer built around the dryer drum minimizes heat loss. "The economics really work," says Brock, who points to the increasing price of aggregate and asphalt. "Recycle is really worth what it replaces," he concludes.
Three versatile methods for in-place recycling of asphalt pavement also have evolved: hot in-place recycling (HIR), cold in-place recycling (CIR), and full-depth recycling (FDR). When compared to the time needed for conventional rehabilitation methods of milling and overlaying with HMA, in-place asphalt recycling processes make it possible to return the roadway to service sooner. All three in-place recycling methods have different uses depending on the depth of repair required.
When shallow repairs are needed, hot in-place recycling can provide an appropriate choice. The HIR process renews a damaged asphalt pavement surface by using heat to soften the existing surface to an approximate depth of 5 centimeters (2 inches). After the pavement is heated, the surface is mixed with a rejuvenator that softens the asphalt, which in turn helps to bind the old paving materials back together to renew the riding surface. Cost figures from Florida, Mississippi, Oregon, and Texas indicate that in comparison to control pavements, HIR offers savings of 17 to 50 percent, depending on whether repaving or remixing is performed.
Deeper pavement recycling uses cold in-place recycling, a process in which the pavement is milled instead of heated and then is mixed with an asphalt binder like foamed asphalt (hot asphalt and cold water combined in a tank to make a foam), emulsion (a suspension of water, oil, and admixtures), or a combination of emulsion, hydrated lime, or portland cement, and laid down as a new pavement. After proper curing to reduce moisture content, a surface treatment or seal coat is applied. CIR can be used to a depth of 13 to 15 centimeters (5 to 6 inches) but is not intended to go into the aggregate subbase. CIR also can incorporate aggregate to change the final materials gradation, resulting in a renewed, stronger base pavement than what was initially in place.
The New Mexico DOT routinely uses CIR as its primary rehabilitation tool. Because CIR avoids the need for milling and hauling the material back as RAP to an asphalt plant, economic savings are realized. According to Pavement Recycling Guidelines for State and Local Governments (FHWA-SA-98-042), compared with conventional mill and overlay projects that need maintenance about every 4 years, CIR projects may only require maintenance every 8 years.
When reworking, rebuilding, or removing all of the existing pavement layers as well as some of the aggregate subbase to widen or change the roadway profile, full-depth recycling can provide the appropriate solution. In the FDR rehabilitation method, the full thickness of the asphalt pavement and some of the underlying base or subgrade material is pulverized and blended to provide an upgraded base material. In Maine, FDR represented a savings of $8.87 per square meter when compared to full conventional reconstruction (excavate, place grade and compact, pave). The Maine DOT is just starting to use FDR with hot foamed asphalt as the primary process to rebuild low-volume road base, in preparation for adding an asphalt riding surface.
Concrete: The Other One Of the Top Two
The Transportation Applications of Recycled Concrete Aggregate: FHWA State of the Practice National Review indicates that building demolition in the United States generates an estimated 112 metric tons (123 million tons) of waste per year and helps to create the second most recycled material by weight worldwide: construction debris and recycled concrete. According to the review, recycled concrete can include old portland cement concrete (PCC) pavement, bridge structures/decks, sidewalks, and curbs that are being removed from service. Any steel in the pavement debris must be removed.
Similarly, commercial construction debris used to create recycled concrete aggregate as an aggregate base for highways and buildings construction must be cleaned of unwanted material such as bricks, wood, steel, ceramics, and glass. After the material is crushed, electromagnets remove any residual metal, and the remaining recycled product is used as fines or can be screened and washed to be used as RCA. States that are high producers and consumers of RCA include California, Illinois, Michigan, Minnesota, and Texas.
The proportion of RCA permitted in the specifications varies by State. Comparing specifications for RCA in Minnesota and California, Charles Luedders, P.E., contract management engineer for FHWA's Central Federal Lands Highway Division, explains, "The specifications in both States allow the contractor to remove a composite pavement, process it, and use it without separate operations. These specifications are providing a base aggregate with superior qualities while providing economic and environmental benefits." Luedders points out that "Minnesota allows 3-percent asphalt cement by dry weight of the aggregate. This allows the inclusion of about 50-percent recycled asphalt pavement." Although the California Department of Transportation (Caltrans) initially limited the amount of RCA to 50-percent by weight of the total aggregate, a special provision in 2003 enabled the use of 100 percent of recycled concrete aggregate, according to the Caltrans report, Summary of California Recycled Concrete Aggregate Review. According to Luedders, "By allowing a mixture of any percentage of recycled concrete aggregate and recycled asphalt pavement, California allows the contractor to use the most economical material in any percentage combination."
Industrial Byproducts: Foundry Sand
When are industry discards not considered waste? Nearly always. "Recycling is not about waste," says FHWA's Lord. "It is about preserving and reusing the value of materials."
Some byproducts produced by industry have been found to be just the right ingredients when applied to highway use. Fly ash, a byproduct of coal-fired electric power production, is routinely used, for example, in the creation of PCC. Silica fume, slag, and sand represent other industrial products that have been recycled for highway applications with success.
Foundry Industry Recycling Starts Today (FIRST), a nonprofit consortium focused on the market development of beneficial reuse of foundry industry byproducts, estimates that approximately 91 million metric tons (100 million tons) of sand are used in production annually. Of that, 5 to 9 million metric tons (6 to 10 million tons) are discarded annually and are available to be recycled into other products. In Cleveland, OH, facilities like Ford Motor Company's motor assembly plant discard 295,000 metric tons (325,000 tons) of foundry sand annually. Used by the auto industry to make the molds used to cast engines, the sand's initial angularity is essential to this process. When worn down by repeated use, it is discarded by the auto industry.
Even after worn smooth, however, the chemical composition of discarded foundry sand makes it an appropriate additive to PCC and for cement production, which, among other components, requires the correct amount of silica. This good source of silica, although no longer useful to the auto industry, provides an essential ingredient for the production of cement and PCC. "Using recycled sand where the material's angularity is unnecessary also conserves virgin material for use by those processes like engine manufacturing that require it," points out FHWA's Hill.
Foundry sand also has been used as fill material in embankments and other similar highway applications. Again, the angularity of the sand is unimportant in this application.
In Ohio, when a major runway extension for the Cleveland Hopkins International Airport required enclosing Abrams Creek by routing a portion of the creek through four 3-meter (10-foot)-diameter concrete pipes, foundry sand was used as flowable fill between the pipes. With only a 305-millimeter (12-inch) gap between the concrete pipes, normal trench backfill material could not have been compacted sufficiently to create the pipe bedding, which needed to provide adequate pipe strength to resist an average of 20 meters (65 feet) of fill that would be placed over the pipes. "In this instance," Hill explains, "byproduct material that presents no environmental problems was reused in another application for which it was just the right solution."
Recent Research: Asphalt Shingles And Scrap Tires
Two abundant materials that have been the subject of recent testing and use include asphalt shingles and scrap tires. Unless recycled, both materials generally are discarded in landfills.
Approximately 10 million metric tons (11 million tons) of asphalt shingles are sent to landfills every year. According to the U.S. Environmental Protection Agency, asphalt shingles are 19 to 36 percent asphalt cement and 20 to 38 percent aggregate—both common highway component materials.
Florida, Georgia, Indiana, Maryland, Michigan, New Jersey, North Carolina, and Pennsylvania specify the use of up to 5 percent of manufacturer's scrap asphalt shingles in HMA. States such as Georgia, Minnesota, North Carolina, Ohio, and Texas have performed laboratory studies on the use of manufactured recycled asphalt shingles (RAS). Field studies conducted in Minnesota, North Carolina, Pennsylvania, and Vermont in which portions of highways or trailways were paved with asphalt containing RAS and then monitored over time have shown the benefits of including RAS in HMA to be increased stiffness of the asphalt, decreased cracking, no effect on moisture sensitivity, decreased susceptibility to rutting, and decreased optimum content of virgin asphalt cement.
In a Vermont field study from July 1999 to April 2000, the State collected 357 metric tons (394 tons) of tab shingles (remnants from shingle manufacturing plants, not postconsumer goods) for processing into recycled road materials. In a demonstration project, about 3,628 metric tons (4,000 tons) of mixed RAS/RAP/gravel were manufactured and installed. Town officials reported that the driving surface was hard and durable, potholes and washboard effects were less evident than on a nonrecycled surface, grading was less frequent, and the material was not as dusty as natural aggregate.
Similarly, in Minnesota where 453,500 metric tons (500,000 tons) of postconsumer shingles are created annually, the Minnesota Department of Transportation (MNDOT) allowed 5-percent recycled shingle into HMA in 2003. MNDOT specifications further required that the shingles used must be scrap from manufacturing plants. Although it is possible to use postconsumer tear-offs, these materials often must be tested for asbestos, are of varying quality, and contain the added complication of nails that need to be removed before processing. RMRC concurs that although there are many opportunities for postconsumer asphalt shingles to come in contact with contaminants, scrap from manufacturing plants is unlikely to be contaminated. In Minnesota, although production of the material was slow at only about
As for scrap tires, the New York Department of Environmental Conservation, for example, estimates that the State has 29 million tires in approximately 95 locations with an additional 18 to 20 million waste tires (approximately one tire per State resident) generated annually. States like New York are highly motivated to eliminate tire piles and the hazards they pose.
"When tire piles catch fire, they create noxious fumes and difficult-to-extinguish blazes that can smolder for extended periods of time," says Hill. "They also provide a fertile breeding ground for mosquitoes and increase the health risks of mosquito-borne diseases such as West Nile Virus." Not surprisingly, 38 States ban whole tires from landfills.
According to U.S. Scrap Tire Markets 2003, a report by the Rubber Manufacturers Association, expanding markets now consume four out of five scrap tires with 80 percent or about 233 million of the 290 million scrap tires generated in 2003 going to an end-use market, compared to just 11 percent in 1990. Scrap tires can be recycled as shredded tires or crumb rubber, depending on their origin. Due to concerns over contamination—dirt, rocks, and other road debris trapped in the tread or inside the tires from a discarded tire pile—this material is shredded. Recently removed tires that have never been in a tire pile—called new takeoffs—can be ground to make crumb rubber.
Since 2001, there has been a 41-percent increase growth in the use of tire shreds in civil engineering applications. Highway applications include using shredded tires as lightweight fill over weak soils in bridge embankments and subgrade, in retaining wall reinforcements, or in very cold climates as insulation of the road base to resist frost heaves and as a high-permeability medium for edge drains. Crumb rubber is used in rubberized asphalt concrete (RAC), which can be applied to the road surface and subsurface.
Arizona, California, Florida, and Texas have had success using RAC for highway resurfacing. According to California's Rubberized Asphalt Concrete Technology Center, rubberized asphalt was developed by Charles McDonald, a materials engineer for the City of Phoenix, AZ. In the mid-1960s, McDonald blended approximately 18-percent crumb rubber from scrap tires with asphalt cement. This material was applied to 140 test sections on a 0.8-kilometer (0.5-mile) segment of a busy Phoenix street. The results were so impressive that between 1967 and 1988 the city constructed more than 4,830 lane kilometers (3,000 lane miles) of rubberized asphalt chip seal. Over the years, modifications have resulted in increased durability and flexibility of the pavement, increased resistance to reflective cracking, and a reduction in the pavement's tendency to ravel.
By the end of 2004, the Arizona Department of Transportation (ADOT) Quiet Pavement Program will cover 105 kilometers (65 miles) of freeway with a 25-millimeter (1-inch) surface of rubberized asphalt and an additional 40 kilometers (25 miles) by the end of 2006. According to ADOT, paving with rubberized asphalt will recycle approximately 10,000 tires per mile. An additional and initially unintended benefit: Studies have shown that rubberized asphalt can reduce traffic noise levels by 3 to 5 decibels. A 3-decibel reduction in noise equates to reducing freeway traffic volume by half.
California, like New York, also must manage large quantities of scrap tires generated annually within the State. According to the Rubberized Asphalt Concrete Technology Center, California "is faced with the challenge of diverting or safely managing more than 33 million reusable and waste tires generated in the State each year." RAC is playing a considerable role in keeping new tires out of scrap piles in California. In 1985, the County of Los Angeles, CA, resurfaced a street with 38 millimeters (1.5 inches) of RAC over lightly alligatored, but mostly sound, pavement. Nineteen years later, the roadway shows no visible reflective cracking. Since 1993, the county's Department of Public Works installed 805 lane kilometers (500 lane miles) of RAC resurfacing that used more than 1.1 million scrap tires. The Department of Public Works specifies RAC for approximately 75 percent of all arterial highway resurfacing projects. An additional 50,000 tires have been used to create a rubberized asphalt slurry for more than 30 projects involving county streets, as well as major and secondary roads.
Similarly, California's Sacramento County Department of Public Works uses RAC on maintenance overlays and capitol improvement projects. Since 1989, the county has placed more than 150,000 tons of crumb rubber asphalt concrete, or the equivalent of 338 lane kilometers (210 lane miles) of RAC resurfacing projects that have used nearly 500,000 scrap tires. A key factor in the successful use of RAC was determining the correct overlay thickness. Recently, the county began a standard maintenance treatment in which 25-millimeter (1-inch)-thick asphalt overlays are placed on residential streets.
To improve upon traditional crumb rubber, the Rhode Island DOT is using chemically modified crumb rubber. The rubber is being modified to create a mixture that is homogeneous. Although 60 percent more expensive than conventional crumb rubber asphalt, longer pavement life and a lower separation range after heated storage are thought to have been worth the additional cost.
Getting the Word Out to States
There is no way to avoid the fact that, in the past, recycling has battled a bad image. The concept of adding material to the Nation's roads that otherwise would sit in a pile at a landfill as garbage is not appealing. Even when research shows that what the Nation throws away contains valuable material that can be processed to be as good or, according to some demonstration projects, better than new, some barriers still prevail between research and practice. In fact, Lord says, "One of recycling's greatest barriers has been the stigma of using waste products. If recycling is to become a standard practice across the industry, it must be approached with all the quality controls and sound management practices that virgin materials receive."
RMRC research and outreach aim to do the required convincing. "One of our experiences with barrier reduction," says Taylor Eighmy, "is simply getting the right specifications in place. If the specification for the use of a recycled material exists, that material has a better chance to be used."
On August 13, 2004, the American Association of State Highway and Transportation Officials (AASHTO) submitted a new resolution on recycling that states: "Resolved, that the AASHTO Subcommittee on Materials recommends that the AASHTO Strategic Plan include specific language promoting the use of recycled materials where technologically, environmentally, and economically appropriate."
Terry Mitchell, an FHWA materials research engineer who is secretary of the AASHTO Subcommittee on Materials, says, "The fact that AASHTO's Standing Committee on Highways and its Board of Directors endorsed the statement shows the high level support the use of recycled materials has in the State highway agencies. We're hoping it will further encourage the States to have their engineers consider the appropriate use of recycled materials on every project."
Recycling materials for use on the Nation's highways supports FHWA's strategic goal of environmental stewardship. "The bottom line," says Lord, "is that recycling is everybody's business. The DOTs are strong proponents. FHWA promotes, the DOTs promote, but the industry recycles."
Using recycled materials made available by industry is a balancing act, a series of tradeoffs. For example, with modified crumb rubber, costs are higher than for conventional crumb rubber asphalt, but service life is longer. With RCA, the recycled product is less expensive to produce than virgin material and may even be a better aggregate, but it requires a high level of quality control during production and construction. Foundry sand can be a superior fill material, but it is not always available when and where needed. Other examples are numerous and reveal recycling's biggest tradeoff: Using recycled means changing familiar methods in order to enhance the environment by conserving natural resources and supporting sustainability.
Jason Harrington is the team leader of the FHWA Recycling Team. He has devoted the last 9 of his 19 years with FHWA to recycling technology and recycled materials. A civil engineering graduate of the University of Alabama at Birmingham, he also is a member of the Transportation Research Board's committee on Waste Management in Transportation, TRB ADC60.
For more information, visit the Recycling Team's Web site at www.fhwa.dot.gov/Pavement/recycle.htm, or contact Jason Harrington at 202–366–1576 or email@example.com.
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