Recycled roadways: the FHWA and key partners are engineering high-quality pavements using reclaimed materials.
The United States produces 2 billion tons of aggregate annually. In response to increased demand, aggregate production is expected to increase to 2.5 billion tons by the year 2020--a statistic that raises concerns about the pace at which virgin aggregate is being consumed.
As a result, 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.
POLICY TO REALITY
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."
Applying sound recycling principles that protect the environment, the FHWA Recycling Team is helping the 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, adding industrial byproducts may enable bridge decks and structures to last about 75 to 100 years, which is many years more than current life spans.
Penn State researchers credit byproducts with helping to lengthen bridge deck life spans, 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 life span 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 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, 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.
An FHWA study estimates that building demolition in the United States generates an estimated 123 million tons of debris per year and helps to create the second most recycled material by weight worldwide: construction debris and recycled concrete.
According to "Transportation Applications of Recycled Concrete Aggregates: FHWA State of the Practice National 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 highway and building 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, "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."
THE EXAMPLE OF ASPHALT
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.
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 OK at 10 and 15 percent," says Dr. J. Don Brock, president of Astec Industries Inc., a Chattanooga, Tenn.-based 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 offers an example of a contractor in Daytona Beach, Fla., where rock costs $19 per ton and liquid asphalt was an additional $12 per ton. Staying within specifications enabled use of a maximum of 20 percent of un-sized 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 400,000 tons of product annually, the $7 difference per short ton resulted in $2.8 million in savings.
Today, RAP can be separated and screened. 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 notes 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. Also, if the aggregate is an absorptive material, RAP, which has already gone through the absorption process, produces better asphalt.
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.
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.
STATE BY STATE
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.
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."
In August 2004, the American Association of State Highway and Transportation Officials (AASHTO) submitted a resolution 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, FHWA materials research engineer and 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."
Recycling materials for use on highways supports FHWA'S 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.
Other examples are numerous and reveal recycling's biggest tradeoff: Using recycled material can mean changing familiar methods in order to enhance the environment by conserving natural resources and supporting sustainability.
SAND IN THE MIX
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 portland cement concrete (PCC). Silica fume, slag and sand represent other industrial products that have been recycled for highway applications.
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.
The author is the team leader of the FHWA Recycling Team. He can be reached at email@example.com. This story is an excerpted reprint from Public Roads magazine, Vol. 68, No. 4.
RECYCLED MATERIALS IN U.S. HIGHWAYS Amount Recycled for Overall Highway Production Use Byproduct (metric (metric Material tons) tons) Applications Recycled Asphalt Pavement (RAP) 41 33 Asphalt, Base Reclaimed Concrete i.n.a. i.n.a. Base, Concrete Blast Furnace Slag 14 12.6 Concrete Coal Bottom Ash 14.50 4.4 Asphalt, Base Coal Fly Ash 53.50 14.6 Cement Production, Structural Fill Foundry Sands 9 to 13.6 i.n.a. Flowable Fill, Asphalt Cement Kiln Dust 12.9 8.3 Stabilizer Bottom Ash i.n.a 8 Small Amounts Asphalt, Base Nonferrous Slags 8.10 i.n.a. Base, Asphalt Steel Slags i.n.a. 7.5 Base, Asphalt, Concrete Note: The use of some recycled materials may not be tracked across the nation. "I.N.A." indicates that the information is not known. Source: T. Taylor Eighmy
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|Title Annotation:||CONCRETE/ASPHALT RECYCLING FOCUS; United States Federal Highway Administration|
|Publication:||Construction & Demolition Recycling|
|Date:||Mar 1, 2006|
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