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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - July/August 2000

Scanning European Advances in The Use of Recycled Materials in Highway Construction

by Katherine Holtz and T. Taylor Eighmy

Recycled materials that have suitable engineering, environmental, and economic properties can be used as substitutes for natural aggregates or materials in the construction of highway infrastructure. Historically, both in the United States and abroad, these recycled materials are typically either recovered materials from the transportation sector or secondary or byproduct materials from the industrial, municipal, or mining sectors.

Examples from the transportation sector include reclaimed asphalt pavement (RAP), reclaimed concrete aggregates (RCA) from deconstructed portland cement concrete pavements, and virgin petroleum-contaminated soils. Examples from the industrial sector include blast furnace slag, steel slag, coal combustion byproducts, and foundry sands. Examples from the municipal sector include waste glass, scrap tires, construction and demolition (C&D) debris, petroleum-contaminated soils, roofing shingle scrap, plastics, and municipal solid waste combustion residues. Examples of materials generated in the mining sector include quarry wastes and mill tailings.

Gerry Malasheskie (left) of the Pennsylvania DOT, Gerry Rohrbach (center) of the Minnesota DOT, and Maria Arm of the Swedish National Road and Transport Research Institute examine reclaimed asphalt pavement (RAP) feed stock at the Ammonn double-drum hot-mix plant outside of Amsterdam, the Netherlands. The plant is owned by five large Dutch road construction companies.

In the United States, these process streams are estimated to be between 352 million tons (320 million metric tons) and 859 million tons (780 million metric tons) per year. Compare this to the use of new materials in highway construction — estimated in the mid-1990s to be about 350 million tons (317.5 million metric tons) per year of which 320 million tons (290 million metric tons) are used as aggregate — and it is obvious that there are potentially a number of recycled materials that can be considered as substitutes provided that they are economical, meet engineering and environmental specifications, and perform as well as traditional materials in the field.

There are seven major applications in the highway environment where recycled materials can be used — largely as substitutes for natural aggregate materials. These include use in asphalt concrete pavements, portland cement concrete pavements, granular bases, embankments or fills, stabilized bases, flowable fills or controlled low-strength materials for pipe bedding, and landscaping applications. Other applications also exist — e.g., appurtenances such as curbs and gutters, medians, guardrails, and signs — but these applications use smaller quantities of materials, and their evaluation methods (both testing and evaluation criteria) are dictated by very specialized industrial or new product standards.

Use of Recycled Materials in the United States and Europe

Presently in the United States, accurate data on recycling rates (material used/material produced) are generally not available. Actual data, once compiled, may reflect slightly better recycling rates. For now, data show the following rates:

  • Blast furnace slag — 90 percent, with 12.6 million tons (11.4 million metric tons) used and 14 million tons (12.7 million metric tons) produced.
  • Coal bottom ash — 31 percent, with 4.4 million tons (4 million metric tons) used and 14 million tons (12.7 million metric tons) produced.
  • Coal fly ash — 27 percent, with 14.6 million tons (13.2 million metric tons) used and 53.5 million tons (48.5 million metric tons) produced.
  • RAP — 80 percent, with 33 million tons (30 million metric tons) used and 41 million tons (37.2 million metric tons) produced.

Since the 1970s, a number of countries in Europe have been routinely using recycled materials with a high degree of success. The most remarkable thing about the European experience is that the recycling rate of materials in the highway environment frequently reaches 100 percent. Just a few examples are provided to illustrate this point.

In Sweden, a large country with a relatively low population density and high aggregate availability, some rates are:

  • Blast furnace slag — 45 percent, with 0.5 million tons (0.45 million metric tons) used and 1.1 million tons (1.0 million metric tons) produced.
  • Steel slag — 100 percent, with 0.22 million tons (0.2 million metric tons) used and produced.
  • RAP — 95 percent, with 0.84 million tons (0.76 million metric tons) used and 0.88 million tons (0.80 million metric tons) produced.

In Germany, a larger country with much industry and high aggregate availability, the following rates are recorded:

  • Blast furnace slag — 100 percent, with 9.2 million tons (8.3 million metric tons) used and produced.
  • Steel slag — 92 percent, with 4.9 million tons (4.4 million metric tons) used and 5.3 million tons (4.8 million metric tons) produced.
  • Coal bottom ash — 97 percent, with 3 million tons (2.7 million metric tons) used and 3.1 million tons (2.8 million metric tons) produced.
  • Coal fly ash — 88 percent, with 3 million tons (2.7 million metric tons) used and 3.4 million tons (3.1 million metric tons) produced.
  • Municipal solid waste combustion bottom ash — 69 percent, with 2 million tons (1.8 million metric tons) used and 2.9 million tons (2.6 million metric tons) produced.
  • RAP — 55 percent, with 7.3 million tons (6.6 million metric tons) used and 13.2 million tons (12 million metric tons) produced.

Denmark, with a high population density and low natural aggregate availability, reports the following rates:

  • Steel slag — 100 percent, with 0.066 million tons (0.06 million metric tons) used and produced.
  • Crushed concrete — 81 percent, with 0.95 million tons (0.86 million metric tons) used and 1.17 million tons (1.06 million metric tons) produced.
  • Coal bottom ash — 100 percent, with 2 million tons (0.18 million metric tons) used and produced.
  • Coal fly ash — 100 percent, with 1.17 million tons (1.06 million metric tons) used and produced.
  • RAP — 100 percent, with 0.53 million tons (0.48 million metric tons) used and produced.

Finally, the Netherlands, a populous country with more limited aggregate resources and a high degree of industrialization, has a 100-percent recycling rate in several categories, including:

  • Blast furnace slag — 100 percent, with 1.32 million tons (1.2 million metric tons) used and produced.
  • Steel slag — 100 percent, with 0.55 million tons (0.5 million metric tons) used and produced.
  • Coal bottom ash — 100 percent, 0.09 million tons (0.08 million metric tons) used and produced.
  • Coal fly ash — 100 percent, with 0.94 million tons (0.85 million metric tons) used and produced.
  • C&D waste — 100 percent, 10.1 million tons (9.2 million metric tons) used and produced.
  • Municipal solid waste combustion bottom ash — 100 percent, with 0.9 million tons (0.8 million metric tons) used and produced.
  • RAP — 100 percent, with 0.12 million tons (0.1 million metric tons) used and produced.

It is interesting to contrast the U.S. data with the European situation and to speculate about the reasons for the successes seen in Europe. What combination of economics, engineering evaluation, and environmental evaluation has allowed the European system to mature to such a degree? Are these circumstances applicable to the U.S. situation?

Why a Scanning Tour? 

Scanning team members and scientists from the Swedish Geotechnical Institute (SGI) examine lysimeters where steel stag is being leached. The study is being conducted to examine the environmental behavior of slags used in road bases and in embankments. In the foreground, Shari Schaftlein of the Washington state DOT talks with Dr. Ann-Marie Fallman of SGI.

Over the last decade, there has been an increasing interest within the U.S. highway community on all levels to learn more about advances in the use of recycled materials in the highway environment. At the local level, road agents are being asked by their communities to use municipal waste streams such as glass cullett (crushed glass) in road construction.

A number of states have passed legislation to look more closely on the state level at recycling within the highway environment. Pennsylvania, for example, recently passed legislation promoting recycling. From recent conferences in Austin, Texas, in 1998; Harrisburg, Pa., in 1998; and Albany, N.Y., in 1999, it is apparent that state departments of transportation (DOTs) and state environmental protection agencies (state EPAs) are trying to balance the desire for increased use of recycled materials with concerns about potential environmental impacts.

At the federal level, the Environmental Council of States (ECOS) is working on agreements between states to allow reciprocity for beneficial use determinations (BUDs) permitting the use of recycled materials in highway construction. The Federal Highway Administration (FHWA) recently participated in the international Organisation for Economic Co-operation and Development project "Recycling Strategies for Road Works," which concluded with a report on the state of the practice (see ISBN 92-69-15461-2).

FHWA has also supported a number of research projects on recycled materials use — particularly the project "User Guidelines for Waste and Byproduct Materials in Highway Construction." The National Cooperative Highway Research Program (NCHRP) has two projects nearing completion: "Waste and Recycled Materials Information Database" and "Environmental Impacts of Construction and Repair Materials on Surface and Ground Waters." U.S. EPA has established, under the federal government’s Comprehensive Procurement Guidelines, the use of coal fly ash and ground, granulated blast furnace slags as cement substitutes in federal construction projects.

The U.S. Congress, in the 1998 Transportation Equity Act for the 21st Century (TEA-21), established the Recycled Materials Resource Center (RMRC) at the University of New Hampshire (UNH) for research and outreach to reduce barriers to recycling in the highway environment. Congress also stipulated in TEA-21 that recycled materials be researched to improve the durability of the surface transportation infrastructure.

In recognition of these recent developments on all levels and the growing interest within the U.S. highway community, a team of U.S. recycling practitioners visited Denmark, France, Germany, the Netherlands, and Sweden to observe research, policies, and programs promoting the use of recycled materials in the highway environment within those countries. Of particular interest to the scanning team were the economic, engineering, and environmental approaches that enable the high recycling rates and the successes seen in those countries.

The U.S. Delegation

The U.S. delegation was assembled under FHWA’s International Technology Scanning Program. The delegation was sponsored by FHWA and the American Association of State Highway and Transportation Officials (AASHTO) through NCHRP and RMRC. The delegation included members with expertise in materials, pavement engineering, pavement construction and recycling, beneficial use determinations, and environmental evaluation. They represented FHWA, U.S. EPA, state DOTs, the American Public Works Association (APWA), the National Asphalt Pavement Association (NAPA), and academia. The trip took place in September 1999. The scanning trip summary report has been issued and is available at A final report is expected by mid-2000. It will be widely distributed in print, and it will be available at

The U.S. delegation identified a number of determining factors that have played a role in the success of recycling within the highway community in Europe — particularly in the Netherlands. They fall under the general concept of "sustainability" within the highway environment. The major components of sustainability initiatives are the three E’s: economics, engineering, and environment.

Sustainability as a Model for Success

The Netherlands probably best typifies the concept of sustainability and the importance of the three E’s. The recycling or reuse of secondary materials within the building industry is commonplace — more than 10 percent of all granular materials used within the building industry are recycled materials. In the highway construction sector, recycling rates of nearly 100 percent are seen.

The Netherlands is an affluent country with high population densities and limited land resources. The public has elected not to use their limited land resources for landfills or aggregate mining. This led the Dutch to develop the concept of "sustainable development within the building industry," including the highway construction industry. The basic premise of the sustainability concept is that material cycles should be closed (use, reuse, re-reuse, etc.) so that there is less disposal and less consumption of nonrenewable natural materials. A number of legislative initiatives — including the National Environmental Policy Plan, the Waste Materials Policy, the Soil Protection Policy, the Surface Minerals Policy, and the Construction Industry Policy Declaration — contributed to the framework for sustainable construction.

Specifically, the Dutch have adopted a "market" philosophy in which recycled materials are considered "products" and not "wastes." This means that the product will exhibit a typical product life cycle in the marketplace. The market is supported by governmental and private-sector informational campaigns and policies. This concept might be a predictor of how the U.S. market might eventually evolve within states or geographic regions in which population densities are high, natural aggregates are scarce, and sources of suitable recycled materials are plentiful.

In the Netherlands, the government provides clear and unequivocal engineering and environmental standards for all recycled materials. This is usually achieved through governmental research in support of the standards. Furthermore, public or industrial working groups (including contractors) work together to achieve these standards. The recycled materials producers treat their materials like a "product," using certified quality-assurance/quality-control programs so that the materials can compete against natural materials. There is clear policy, planning, and implementation, which allow the producers and contractors to prepare for this new market. There are some disincentives from the government such as hefty landfill disposal taxes on materials that can be recycled and modest taxes on the use of natural aggregates. If these aspects are combined, then a mature recycling market can develop over time.

The evidence for a mature market in the Netherlands is shown by the fact that a number of materials are recycled at rates greater than 90 percent: steel slag, blast furnace slag, phosphorus slag, coal fly ash, construction and demolition aggregates, municipal solid waste incineration bottom ash, and RAP. Others, such as contaminated dredge spoils, are not used as widely because of environmental concerns.


From an economics perspective, engineering and environmental life-cycle costs and benefits are the basis for many of the recycling initiatives in Europe. The free market generally plays a central role in all aspects of the highway recycling industry. Tax structures (both incentives and disincentives) have played an important role in promoting recycling in the highway environment in Denmark, the Netherlands, and Sweden. Denmark and the Netherlands tax the use of natural materials. Restrictive landfill taxes and policies in Denmark, France, and the Netherlands (and soon within the European Union) are also promoting recycling. Companies that supply natural materials also tend to supply recycled materials. They typically have the equipment to be able to process the recycled materials to meet certain engineering specifications. A number of materials, such as RAP, blast furnace slag, crushed concrete, and high-quality C&D aggregates, are of high engineering and environmental quality and compete favorably with natural materials in a typical supply-and-demand situation. For instance, demand for some of these recycled materials in the Netherlands is so high that there are anticipated shortages for the next few years.


Katherine Holtz (left) of the Texas DOT and Gerry Malasheskie of the Pennsylvania DOT examine processed C&D aggregates at the Jean LeFebvre processing facility near Paris.

A number of interesting observations can be made about the engineering aspects of recycling. A few countries require that recycled materials meet the same specifications as natural materials and provide equal performance in the field. An approved product list is generally not used; ultimate performance is the determining factor in promoting the use of recycled materials. As in the United States, there is still concern that many engineering test methods do not predict true field performance, but ongoing research in Germany and Sweden with load simulators is, in part, addressing this issue.

Of particular interest was work conducted by Dr. Heinrich Werner at the German Federal Roads Institute (Budesanstalt für Straßenwesen, or BASt). Their work involved testing full-scale model pavements made with RAP, crushed concrete from pavements and building slabs, and two classes of C&D aggregates as an unbound base. Testing was conducted under controlled freezing and thawing conditions. An impulse loader was used to produce design-life simulated loads (truck and vehicle) over very short time periods. Surface deformation, pressure distributions, bearing capacity, and moisture profiles were examined. RAP and crushed concrete were found to be comparable to control-group materials in performance. One class of C&D aggregates was more susceptible to rutting, deformation, and freezing/expansion.

In virtually all of the countries, crushed concrete and C&D aggregates are high in quality and are widely used in road construction. Processing facilities can produce wide ranges of aggregate sizes for blending with natural materials. In many of the countries, foamed bitumen is used to treat certain recycled materials (e.g., tar pavement, municipal solid waste bottom ash) for use in a stabilized base course. However, unlike the United States, portable plants are used to treat storage piles, and the material is placed for up to 30 days provided that it is stored in an un-compacted state. In additional, some new equipment facilitates the use of foamed bitumen during in-place recycling.

In the Netherlands, an innovative, Swiss-designed, double-drum hot-mix plant capable of recycling up to 70-percent RAP was observed. The double-drum plant heats up RAP in the lower drum. The off-gases are used as fuel to dry the natural aggregate in the upper drum prior to blending with the RAP in a mix box. Such an arrangement produces very clean air emissions from the facility, but such plants may be expensive to purchase.


An environmental approval process that is now used in the Netherlands and under consideration by the European Union involves mechanistic leaching tests and application-specific evaluation of incremental impacts on background soil and groundwater. This process might be appropriate in the United States.

A large mechanistic leaching database for more than 25 recycled materials (both as unbound material and in various concrete and asphaltic applications) is maintained by Dr. Hans van der Sloot at the Netherlands Energy Research Foundation. Data about materials from many countries have been collected for about 10 years. This may become a Web-based database that may be of use to state regulators.

There is consistent agreement to move from laboratory work to performance modeling based on field validation. A European Union Fourth Framework project, Alternative Materials (ALT-MAT), illustrates this approach and is a model for U.S. consideration. ALT-MAT involves researchers from nine federal research laboratories in seven countries. The materials that are being explored include C&D aggregates, municipal solid waste incineration bottom ash, steel slag, ferrochrome slag, air-cooled blast furnace slag, and crushed glass. Researchers are looking at mechanical properties, environmental properties (including unbound and bound leaching behavior), aging in climate chambers, leaching in lysimeters, and mechanical and environmental evaluations of field pavements made with some of these materials. (See The ALT-MAT project will be completed this year.


The U.S. delegation consisted of Vincent Schimmoller, program manager, Office of Infrastructure, Federal Highway Administration (co-chair); Katherine Holtz, director of the Materials Section, Texas DOT (co-chair and representing the American Association of State Highway and Transportation Officials); Dr. Taylor Eighmy, director of the University of New Hampshire's Recycled Materials Resource Center (report facilitator and representing academia); Carlton Wiles, Recycled Materials Resource Center (representing academia); Gerald Malasheskie, Pennsylvania DOT (representing AASHTO); Gerald Rohrbach, Minnesota DOT (representing AASHTO); Shari Schaftlein, Washington state DOT (representing AASHTO); Bob Campbell, Snohomish County Public Works, Washington state (representing the American Public Works Association); Greg Helms, U.S. Environmental Protection Agency; Charles Van Deusen, National Asphalt Pavement Association; Michael Smith, FHWA Southern Resource Center; Robert Ford, FHWA Office of International Programs; and Jake Almborg of American Trade Initiatives (coordinator).

Much of the text for this article came from the Scanning Trip Summary Report, a document that the entire delegation authored.

Next Steps and Follow-On Demonstration Project

The U.S. delegation is working with their respective constituencies to promote recommendations from the summary report and final report. Specific implementation strategies include Web, electronic, and printed distribution of the final report, numerous presentations, and published articles.

To further promote technology transfer and to start reducing the barriers, a demonstration project is planned for this fall in Houston, Texas. At this time, it is being sponsored by FHWA and RMRC with participation from AASHTO and the Association of State and Territorial Solid Waste Management Officials (ASTSWMO). The project will target 10 to 12 states — both state DOTs (materials engineers and environmental staff) and state EPAs — to bring them together to learn about European and U.S. advances in recycling within the highway environment and to identify strategic activities to further promote recycling. In addition, field demonstrations of new technologies, such as the use of foamed bitumen as a stabilizer for recycled materials in base course and the use of C&D aggregates in pavements, will be conducted.

Katherine Holtz is the director of the Materials Section, Construction Division, of the Texas Department of Transportation. She is a registered professional engineer.

T. Taylor Eighmy is a research professor of civil engineering at the University of New Hampshire (UNH) in Durham, N.H. Dr. Eighmy currently directs the Recycled Materials Resource Center (RMRC) at UNH; RMRC is a partnership with the Federal Highway Administration (FHWA) to promote recycled materials use in the highway environment. He also directs the Environmental Research Group (ERG) at UNH; ERG is one of the university’s formal research centers and the parent organization to RMRC. His research interests include materials characterization, geochemical modeling of leaching, and leaching of highway products containing recycled materials. He is the principal investigator (PI) on two FHWA-funded projects: Development of a Predictive Approach for Long-Term Environmental Performance of Waste Utilization in Pavements Using Accelerated Aging and Development of a Consensus Framework for Waste Utilization Evaluation Procedures. Formerly, he was a PI on the Laconia, N.H., Bottom Ash Paving Project, a member of the International Energy Agency’s International Ash Working Group (IAWG), and a member of FHWA’s Expert Advisory Panel for the User Guideline for Waste and Byproduct Materials in Pavement Construction project. Dr. Eighmy received both his master’s degree and doctorate in civil engineering from UNH. He is a member of The International Society for the Environmental and Technical Implications of Construction with Alternative Materials (ISCOWA).