Published: Feb 26, 2008 1:00:00 AM
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Source: February 2008 issue of Better Roads Magazine
Highways constructed 100% of recycled materials come near, as stakeholders point the way to green infrastructure systems.
The technology for building or rebuilding highways of 100% recycled or reclaimed materials exists now.
Crushed and screened reclaimed asphalt pavement is stockpiled at Christ Brothers Asphalt, Inc., Lebanon, Illinois.
Pavement bases are incorporating recycled concrete aggregate, or reclaimed asphalt pavement. They also can be reclaimed and stabilized in-place by mobile equipment. Bases even have accommodated exotic materials, like crushed toilet seats. RAP is used for building road shoulders — especially in work zones — plus lane super-elevations, grade separations, and work-zone lane changes.
Intermediate pavement courses are now being made of 100% RAP, and RAP is being accommodated in higher and higher levels in driving or friction courses. New research underway at the National Center for Asphalt Technology at Auburn University, and planned by the National Cooperative Highway Research Foundation, will help determine hot-mix asphalt mix designs that will greatly increase RAP contents in mixes without sacrificing performance.
The private sector is responding by developing and marketing plant equipment that will mix asphalt at considerably lower temperatures than before, thus reducing the amount of volatile organic compounds that are emitted when higher levels of RAP are used in intermediate and friction course mixes. Other equipment will create 100% recycled cold asphalt mixes for bases from either RAP or RCC. And mobile equipment is being refined that will more efficiently reclaim failed pavements and create uniform bases for overlays on busier highways in-place — up to Interstate levels, as was demonstrated recently on I-80 in California, or for surface treatments on less-used roads.
Missouri DOT is even testing zero-VOC pavement markings based on soy products.
But even as industry stakeholders acclimate themselves to the idea of 100% recycled or green highways, national agencies and associations are promoting green highways not just as pavement structures, but as part of a complete environmental package, and this will put new demands on highway agencies and funds.
This package might include not just reclaimed materials in pavement structures, but also portland cement concrete porous pavement shoulders linked to bioretention swales that will microbially reduce pollutants in surface runoff; forest buffers; local stream restoration; and wildlife crossings to maintain contiguous wildlife habitat (see Critter Crossings and Sensors Keep Wildlife, Motorists Apart, November 2006).
The stage for a more demanding green highway design was set in 2006 when the Federal Highway Administration released Eco-Logical: An Ecosystem Approach to Developing Infrastructure Projects, developed in conjunction with other federal agencies and state departments of transportation, including the North Carolina DOT, Vermont Agency of Transportation, and Washington State DOT.
While not actually setting formal policy, Eco-Logical provides a framework for helping federal, state, tribal, and local partners in design, review, and construction work together to make infrastructure more sensitive to wildlife and their ecosystems. As such, it's a bellwether for what's down the road for highway construction that will be more environmentally friendly, and perhaps pass environmental hurdles more easily, but take longer to build and be more expensive. And by increasing the number of stakeholders in a design, ultimate project approval will get more complicated as more opportunities are provided for an opponent to turn thumbs down on design elements.
The federal interagency steering team wants Eco-Logical to cause highway or infrastructure projects to lead to enhanced conservation, as they say, "protection of larger scale, multi-resource ecosystems in which they're located;" provide "connectivity of habitat," that is, reduced habitat fragmentation that can impede animal migration and hunting for food; ensure the predictability of project outcomes, so it's sure that commitments and contracts made by all agencies will be honored as negotiated; and enhance transparency, with better public and stakeholder involvement at all key stages in order to establish credibility, build trust, and streamline infrastructure planning and development.
Demolition concrete awaits processing to recycled concrete aggregate in Maryland.
"These goals all support an ecosystem approach to infrastructure development," the 2006 FHWA document says. "The approach shifts the federal government's traditional focus from individual agency jurisdiction to the actions of multiple agencies within larger ecosystems. It finds ways to increase voluntary collaboration with state, tribal, and local governments, and to involve other landowners, stakeholders, interested organizations, and the public." Thus Eco-Logical's approach distributes decision-making to a variety of stakeholders from one central agency.
To identify what infrastructure work is needed and where it would be done, a regional ecosystem framework is required, Eco-Logical states. "Although there is no standard for creating a REF, Eco-Logical recommends that a REF consist of an overlay of maps of agencies' individual plans, accompanied by descriptions of conservation goals in the defined region(s)," Eco-Logical says, perhaps spanning multiple states.
Also calling for a multilateral approach toward making highways greener is the Green Highways Partnership. The East Coast-based Green Highways Partnership is a voluntary, public/private initiative — including Region 3 of the EPA and state natural resources, environmental protection, and DOTs of Maryland, Virginia, Delaware, Pennsylvania, D.C., West Virginia, New York State, and New Jersey — that is attempting to incorporate environmental streamlining and stewardship into all aspects of the highway lifecycle.
The GHP says that green highways should provide superior watershed-driven stormwater management to prevent pollutants from leaching into streams and rivers, use recycled construction materials to prevent landfill usage, and use cutting-edge technologies to protect critical habitats and ecosystems from the encroachment of highway infrastructure (habitat fragmentation).
Green Highways is creating a template for future partnerships between federal/state transportation and regulatory/resource agencies, contractors, the materials supply sector, highway trade associations, academic institutions, and NGOs. Three cross-sector Theme Teams focus on watershed-based stormwater management, recycling and reuse, and conservation and ecosystem management.
The partnership identifies exemplary projects through a recognition program and supports pilot projects like U.S. 301 in Maryland, where the State Highway Administration is building a collaboration that is intended to make U.S. 301 the nation's first truly green highway. Planning, construction, and maintenance of the route will incorporate stormwater management, recycled materials and ecosystem conservation techniques.
That Maryland is in the forefront of creating eco-friendly highways — with an emphasis on watershed protection — likely results from the fact that essentially every drop of water that falls east of the Maryland panhandle and the eastern continental divide eventually makes its way to Chesapeake Bay. That makes the bay a sink for non-source pollutants, with profound future effects on the state's economy, tourism, and quality of life. It's not unusual to see — stenciled on the concrete in front of Maryland stormwater inlets — a warning that says whatever is dumped into the inlet will wind up in the bay.
The partnership defines a green highway as one that enhances the environmental function and value of a watershed, going beyond minimum standards defined by environmental laws and regs.
One watershed green practice for highways is bioretention at locations where runoff collects. Bioretention is a best-management practice developed in the early 1990s by the Prince George's County, Maryland, Department of Environmental Resources.
Bioretention utilizes soils and both woody and herbaceous plants to remove pollutants from stormwater runoff. Runoff is conveyed as sheet flow to the treatment area, which consists of a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil, and plants. Runoff passes first over or through a sand bed, which slows the runoff's velocity and distributes it evenly along the length of the ponding area, which consists of a surface organic layer or ground cover and the underlying planting soil. Water is ponded to a depth of 6 inches and gradually infiltrates the bioretention area or is evaporated. Stored water in the bioretention area planting soil exfiltrates over a period of days into the underlying soils.
Bioretention is facilitated by porous pavement shoulders, either of portland cement concrete, or hot-mix asphalt. A porous pavement is a low-fines, permeable pavement surface with an underlying stone reservoir that temporarily stores surface runoff before infiltrating into the subsoil (see Voids Add Value to Pervious Concrete, August 2003).
The Green Highways Partnership describes many other attributes of green highways on its Web site, www.greenhighways.org.
Despite all this, market forces and project conditions — not government mandates — are driving reuse of materials in today's green highways. For example, about 140 million tons of concrete are recycled in the United States each year, according to the Construction Materials Recycling Association. Impacting the supply are equipment costs, transportation costs, and external landfill tipping fees.
Storage of crushed, screened reclaimed asphalt pavement under cover reduces its moisture content and enhances its value, as RAP does not shed water as easily as virgin aggregate.
"When rock was $1 per ton and liquid asphalt cement was $20 per ton, it was cheaper to make mix with virgin aggregate than to make mix with RAP," said J. Don Brock, chairman, Astec Industries, at the 11th Annual Minnesota Pavement Conference symposium in St. Paul in February 2007.
"Today, however, with liquid AC at $330 to $350 per ton and rock at $8 to $25 per ton, the economics of using RAP is attractive to both producers and paving contractors," Brock said. "In addition, city, county, and state DOTs — as well as private developers — can save money by allowing producers to include RAP in their mixes."
Paving materials obtained from RAP are a bargain compared with the virgin materials they replace. In 30,000 tons of recycled pavement, there are typically about 28,200 tons of aggregate and about 70 6,000-gallon transport loads of liquid, worth roughly $1 million, Brock said.
The cost to reuse that RAP is only the cost of the trucking and processing, since the same material is taken out of the road and then put back into the road. He assumed this trucking/processing cost to be $6.40 per ton. Brock then compared that with typical prices of virgin material: rock at $9.40 per ton + asphalt at $18.00 per ton = $27.40 per ton. Thus, the savings from using recycled material would be $21 per ton.
For valuing RAP and RCA, the distance from the project to a virgin aggregate producer versus to the recycled materials processor — or landfill — must be factored into the cost, along with the added labor costs to keep reclaimed materials separate from virgin materials.
Thus, the degree of penetration of RCA or RAP into a local market will depend on availability of demolition materials, its quality after processing, the level of construction (especially road construction, as RCA and RAP are favored for road bases), local labor costs, and local landfill tipping fees.
The use of RCA is being researched by work sponsored by the Federal Highway Administration and other agencies.
Reclaimed concrete material can be used as an aggregate for cement-treated or lean concrete bases, a concrete aggregate, an aggregate for flowable fill, or an asphalt concrete aggregate, the FHWA says. It can also be used as a bulk fill material on land or water, as a shore line protection material (rip rap), a gabion basket fill, or a granular aggregate for base and trench backfill.
In 2003, the FHWA undertook a National Review of Recycled Concrete Aggregate use, and the results were published in September 2004. Its purpose was to capture, for technology transfer, the most advanced uses of recycled concrete aggregate by state highway agencies. The FHWA found that concrete routinely is being recycled into the highways of the United States, and its principal application has been as base material.
In January 2007, a Transportation Research Board paper supported higher substitutions of RCA for virgin aggregate in large airfield applications. According to Saeed, Hammons and Reed, of Applied Research Associates Inc., in their paper, Comprehensive Evaluation, Design and Construction Techniques for Airfield Recycled Concrete Aggregate as Unbound Base, "A small increase in the amount of recycled concrete aggregate to replace the virgin aggregate in pavement construction will have large economic and environmental benefits while extending the supply of traditional construction materials."
RCA "[also] can be used as aggregate in pavement construction if it has suitable engineering, environmental, and economic properties," they said. "There are sufficient published data available to demonstrate that RCA is a viable alternative to virgin aggregate for unbound base course construction."
Thus, RCA from demolition materials is broadly accepted as a base material. But its use as an aggregate in concrete itself has been problematic. "[RCA] consists of high-quality, well-graded aggregates (usually mineral aggregates), bonded by a hardened cementitious paste," reports the FHWA in its User Guidelines for Waste and Byproduct Materials in Pavement Construction. RCA is generated through the demolition of PCC highways, runways, and structures, which necessarily piles up during reconstruction of transportation infrastructure.
"The [RCA] excavation may include 10 to 30% subbase soil material and asphalt pavement," the FHWA says. Therefore, initially the RCA is not pure PCC, but a mixture of concrete, soil, and small quantities of bituminous concrete.
One way to boost use and consistency of RAP for mixes is for hot-mix asphalt producers to maintain sheltered, blended RAP stockpiles and, if needed, to reprocess, or fractionate, the RAP into individual gradations. Sheltered stockpiles are favored because RAP doesn't shed water as easily as virgin aggregate.
To optimize RAP use, Brock urges processing plants to refine or fractionate RAP, a concept in asphalt recycling which falls back on conventional best practices for virgin aggregate processing.
With fractionation, RAP is screened, with oversize broken into smaller fractions and stockpiled separately. Fractionated RAP may result in more uniform mixes in which RAP fractions can be isolated, compared to general stockpiles in which large and smaller fractions may become segregated.
While fractionation can boost RAP use, its increased use in hot-mix asphalt mixes also must be sanctioned. The hot-mix asphalt industry is looking for ways to incorporate more RAP and is supporting research in that direction.
For example, it's a line item in the new National Asphalt Road Map: Commitment to the Future, produced in 2007 by the National Asphalt Pavement Association, the FHWA, the American Association of State Highway & Transportation Officials, The Asphalt Institute, and the National Stone, Sand & Gravel Association.
High-RAP-content warm mix from Astec's Double Barrel Green System plant is placed in Chattanooga using a Roadtec paver and material transfer vehicle.
As such, it mirrors the Concrete Pavement Road Map released by the FHWA, the Center for Portland Cement Concrete Pavement Technology, the Portland Cement Association, the American Concrete Pavement Association, and Iowa State University (see Concrete Pavement Road Map, March 2006) and the forthcoming Pavement Preservation Road Map of the FHWA, the Foundation for Pavement Preservation, the National Center for Pavement Preservation, and other interests.
The asphalt road map lists Item No. 4.09: Develop High RAP Content Mix Design Procedure as one of its needed high-priority research projects. The road map also urges study on use of recycled materials in asphalt mixes other than RAP.
"The use of RAP in recycled asphalt pavement is well accepted practice by many federal, state, and local agencies," the road map says. "In many areas, almost all HMA contains at least some RAP. However, with a few exceptions, the amount of RAP that can be added in hot plant-mix asphalt mixtures is limited to relatively low percentages and, in some areas, the use of RAP is prohibited in certain types of mixtures, such as surface courses. Typically, the maximum percentage of RAP allowed is anywhere from 15 to 30% by weight of HMA mixture."
The road map would like to see considerably higher percentages implemented. "Laboratory and field studies have been performed on HMA with much higher percentages of RAP," the new road map says. "These investigations have concluded that HMA materials with percentages in excess of 50% can be produced to perform to the same [level] as virgin mixes. It has been well established that agencies that are not currently allowing RAP into their HMA mixtures and those that are only allowing small percentages of RAP can safely increase the amount of RAP used without fear of shortening pavement life, provided that best practices are followed...[t]he state-of-the-practice relative to the mix design procedures using high RAP content mixes needs to be established."
In late 2007, new National Center for Asphalt Technology director Dr. Randy West said NCAT was investigating ways that the RAP content of today's HMA mixes could be greatly increased. Optimized, high-RAP-content mixes will enable an increase from the 15 to 20% RAP content in most asphalt mixes in this country without sacrificing performance or durability, West said.
And increased levels of RAP are the subject of a pending new NCHRP Study No. 09-46, Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content. The $400,000, two-year study will complement the recommendations on incorporation of RAP in the Superpave mix design method in NCHRP Report 452.
"Current crushing and screening technology allows processing of RAP in a more consistent and uniform manner," the NCHRP said. "In addition, hot-mix plants are routinely able to handle higher amounts of RAP. As a result, it is now possible to consistently produce HMA containing 25% to greater than 50% RAP. Such high-RAP-content mixes have the potential to significantly reduce the cost of HMA paving while conserving natural resources."
But current AASHTO recommendations make it difficult to design HMA mixes with high-RAP contents, the NCHRP said. Modifications to the current specifications may be needed to assure agencies that satisfactory performance will result from the use of high-RAP-content HMA mixes. In response, this research would develop a mix design and analysis procedure for HMA containing high-RAP contents that provide satisfactory long-term performance, and propose changes to existing specifications to account for HMA containing high-RAP contents (high-RAP content is defined as greater than 25% and may exceed 50%).
Manufacturers are getting on board the high-RAP movement as well. In summer 2007, one of the first truly green plants for manufacturing hot-mix asphalt was unveiled by Astec Industries. Astec's Double Barrel Green System incorporates the use of hot foam asphalt to lower the mix temperature without increasing the cost of the product, Astec says.
"The Double Barrel Green System mixes a small amount of water with the liquid AC, thereby producing microscopic foam as the liquid AC enters the mixing chamber of the dryer/mixer," Brock said. "This concept lowers the viscosity of the resulting mix so it can be compacted at temperatures as low as 220 degrees F."
By running the mixing process at less than 280 degrees F, blue smoke and pungent smells can be effectively eliminated, worker comfort and safety can be enhanced, and fume-capturing systems on silos, load-out areas, and mobile equipment can be eliminated, Brock said. Most importantly, he added, the industry would be able to raise percentages of RAP in mixes to higher levels than in the past.
The system was demonstrated in June 2007 in Astec's home town of Chattanooga. "The city of Chattanooga allowed us to mill 2 inches of asphalt from a high-traffic street, separate it, and crush it back to its original sizes," Brock said. "The material was then mixed with virgin rock and liquid AC that had been foamed. It was then replaced on the street. The mixture was produced at approximately 270 degrees F and placed on the road producing a Tennessee Type D surface mix identical in performance and appearance to that of a normal Tennessee 100% virgin mix."
Wirtgen Group is bringing German cold mix, foamed asphalt technology to the United States via its KMA 200 portable plant, and other recycling equipment from Wirtgen America.
Global's portable KMA 200 recycling plant making a foamed asphalt mix with 100% RAP is computer-controlled and usually requires no permits for operation.
In late 2007, green mix from the KMA 200 was approved by the U.S. Green Building Council as eligible for points toward LEED certification of a structure or project. That means that specification of Wirtgen Green Mix technology in significant volumes in project roadways and parking areas may help a project attain LEED certification, or may bump a certified structure into a LEED Silver, Gold, or Platinum premium certification.
LEED certification is a new driving force behind value-added private- and public-sector infrastructure design and construction. LEED stands for Leadership in Energy and Environmental Design, and the building industry has turned to the LEED system to evaluate the degree of green design a structure or development incorporates.
"It's easy to see why green mix was certified," said Harold C. Green, chief executive officer, Chamberlain Contractors, Inc., a Laurel, Maryland-based pavement maintenance contractor who spearheaded the approval of Wirtgen foamed-asphalt green mix as LEED certifiable. "The technology uses up to 100% recycled asphalt pavement removed from existing roads and parking lots, diverting the RAP from landfills while it reduces the demand for virgin materials extracted and processed from quarries or pits. We've had great success in marketing green mix from our KMA 200 throughout the Baltimore-Washington D.C. region."
Wirtgen green mix technology incorporates liquid foamed asphalt, in which hot penetration-grade asphalt is foamed with water and air, and is injected into reclaimed materials and aggregate in a mixing chamber. This green mix is placed, graded, and compacted, and can accommodate traffic — including heavy trucks — almost immediately. Most often it serves as a durable base to support a thin asphalt wearing course or surface treatment.
Wirtgen green mix technology also is implemented in-situ using mobile reclaimers like Wirtgen's WR 2500 S or the WR 2400. These mobile machines grind up weathered, deteriorated asphalt pavement in-place, and incorporate foamed liquid asphalt into the mix inside the machine on the fly. The ecological benefits of this in-place recycled foam mix are such that it's become a preferred means of rebuilding roads in ecologically sensitive areas such as Zion National Park, Utah; Canyon de Chelly National Monument, Arizona; I-80 through California's High Sierras; and in 2007, Colorado's Mesa Verde National Monument.
Base and intermediate road layers incorporating 100% in-place recycling was used recently in the reconstruction of Christian County Highway 1 in central Illinois.
In Champaign County, Illinois, a cold mill cuts a widening strip on either side of the pavement, which is immediately filled by millings off the pavement; then full-depth recyclers, working in tandem, reclaimed and remixed the entire width and stabilized using proprietary emulsion.
Cold planers played the role of a road widener by taking millings from the center of the thick hot-mix asphalt pavement and placing them in a 10-inch-deep widening trench — also cut by a planer — on either side of the existing pavement. Then, twin WR 2500 S reclaimer/stabilizers — operating in tandem — pulverized and full-depth recycled the road and new shoulders using a proprietary asphalt emulsion, prior to its resurfacing with hot-mix asphalt.
The innovative operation drew government road agency observers from all over central Illinois and gained kudos for its contractor, Dunn Company of Decatur, Illinois. Among the visitors was the esteemed Marshall Thompson, P.E., professor emeritus, civil engineering, of the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign, member of the Hot Mix Hall of Fame of the National Asphalt Pavement Association, and now associated with the Illinois Center for Transportation, founded in 2005. In July, he and Professor Sam Carpenter began researching cold in-place recycling with asphalt products at ITC. Their project is scheduled to conclude by June 30, 2008. His visit to Highway 1 was part of that research.
"We're studying cold in-place recycling with asphalt emulsions, and foamed asphalt, and Dunn Company has extensive experience with both processes," Thompson said. "Our ultimate goal is to develop local road and street specifications that could be used throughout the state, without the agency having to go through a so-called experimental project review. It would make CIP a routine construction process, rather than something exotic and new."
This project used value-added Fortress proprietary asphalt emulsion from SemMaterials, and manufactured under license by Emulsicoat Inc. of Urbana, Illinois. The Fortress product has an engineered design, adheres to a set of performance-related specifications, and is a new-chemistry emulsion formulated specifically for the stabilization process, according to SemMaterials.
Tremendous savings can benefit local governments if they can reliably use these CIP recycling processes, Thompson said. "They will be able to reclaim and reuse their old asphalt pavements," he said. "You can use them as a road base with a 3-inch hot-mix asphalt surface, as here, or you can go through an existing thin asphaltic concrete surface and blend it with an aggregate base, and the total blend of AC surface plus agg base can be stabilized with either emulsion or foam. It's very cost effective and allows you to maximize your in-place recycling potential. And while we're not evaluating them for our study, there are environmental problems that cold in-place recycling can solve."
Zero VOC lane striping is a goal of the Missouri DOT as it tested four types of yellow and white soy-based paint that contain varying amounts of soybean oil to see how they work for highway striping in late 2007.
The soy-based paint is made by using the same raw materials and processing techniques as traditional, water-based paint, except the soybean oil replaces some of the petroleum ingredients. Soy paint is considered to be more environmentally friendly than other paints because it's made from soybeans, a renewable agricultural crop that is plentiful in Missouri.
The 12-foot test stripes, which are located on U.S. 63 just north of Jefferson City, run across the roadway from the shoulder to the center stripe. That way, the paint can be tested for how well it sticks to the ground and holds up against tire wear. MoDOT will also be looking to see how the paint withstands the ultimate test: snowplow blades.
"Snowplows are the Achilles heel of any highway paint," said Todd Bennett, MoDOT's chemical laboratory director. "As we monitor the wear and tear of the paint over the next year, we'll especially be looking to see how durable it is in winter weather."
MoDOT has been working with the Missouri Soybean Merchandising Council to test the paint, which was developed by Cargill.
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