Wherefore art thou aggregate resources for highways?
Crushed stone and crushed gravel are the major sources of most pavement aggregates. Their angular shapes perform well in applications where interparticle friction adds to pavement strength, such as granular bases and asphalt layers. For portland cement concrete, natural sand, gravel, and crushed stone are widely used in pavements and structures as well. Natural sand, as the fine aggregate for concrete, is entrenched in highway agencies' specifications because its rounded shape contributes to concrete workability. Using crushed, angular, and manufactured fine aggregates in concrete, mortar, and grout applications is more difficult, but may be necessary in some areas.
To be useful to highway agencies, first and foremost, aggregates must be of a sufficient quality to meet both initial design needs and long-term, life-cycle performance objectives. Industry decision makers regularly consider alternative blends, recycled sources, and gradings, as well as other aggregates specified for the project designs. Developing specifications that allow more blending to meet performance objectives can help preserve premium aggregates for critical uses.
Ensuring a sustainable supply of aggregates requires advance planning and balancing a complex matrix of engineering, geographical, and geological variables and community interests. Aggregate resources--whether quarries, pits, recycled materials, or industrial byproducts--are more sustainable when located close to projects. In many cases, however, materials must be trucked to project sites from distant locations.
"The highway industry and the public need to become more educated about the importance of aggregates to local economies and regional transportation infrastructure," says Jorge E. Pagan-Ortiz, director of the Federal Highway Administration's (FHWA) Office of Infrastructure Research and Development. "Knowing the locations of current and potential future aggregate sources is important for strategic planning and resource protection."
By knowing more about local resources, officials can plan and design highway projects to optimize the use of various types of locally available natural and recycled aggregates. Using locally available aggregates reduces transportation costs and energy expended in moving these heavy bulk materials. Optimal use of local aggregates also reduces truck traffic and the number of axle loadings on the highway system. Further still, communities can extract high-quality aggregates before committing land to other uses, such as lakes, parks, or new developments. However, advance planning and environmental and landscape architectural considerations are critical in reclaiming and developing aggregate lands.
In January 2011, at the Transportation Research Board's (TRB) 90th annual meeting, experts from the United States and Europe gathered for a workshop on "Aggregate Source Depletion and Future Supply." Representatives from FHWA, the U.S. Geological Survey (USGS), State departments of transportation (DOTs), industry, and academia discussed the future of sustainable sources of mineral aggregates and related issues facing many States and transportation agencies. What follows are highlights from their presentations.
Aggregate Needs for Highways and Structures
Both by volume and tonnage, aggregates surpass all other materials used in the built infrastructure of roads and bridges. As defined by ASTM International in ASTM D 8-02, an aggregate is "a granular material of mineral composition such as sand, gravel, shell, slag, or crushed stone, used with a cementing medium to form mortars or concrete, or alone as in base courses, railroad ballasts, etc."
USGS Circular 1176 Aggregates from Natural and Recycled Sources: Economic Assessments for Construction Applications--A Materials Flow Analysis (1998) further refines the definition as follows: "aggregates are...materials, either natural or manufactured, that are either crushed and combined with a binding agent to form bituminous or cement concrete, or treated alone to form products such as railroad ballast, filter beds, or fluxed material." Treated and untreated aggregates are also used for local gravel roads or other aggregate-surfaced roads, driveways, and parking areas.
In general, natural aggregates are mined from stone quarries and from sand and gravel pits. Increasingly, however, agencies are using recycled, reclaimed, and alternative byproduct aggregate materials, such as blast furnace and steel slag, other mining or industrial byproducts, and reclaimed asphalt pavement and recycled concrete aggregate. However, these alternative materials currently fill only a small fraction of the total aggregate needs for highways. A 2010 survey of State DOT materials engineers by the American Association of State Highway and Transportation Officials' (AASHTO) Subcommittee on Materials reveals use of reclaimed asphalt pavement (in asphalt mixtures) and use of recycled concrete aggregate (mostly in base course applications) in most of the States.
According to USGS reports, production and use of aggregates in the United States declined during the economic downturn in 2008-2010. However, the demand for all types and uses of aggregates in 2007 and 2008 was on the order of 2.5 to 3 billion tons (2.2 to 2.7 metric tons) per year and may return to that level when construction volumes return. Bill Langer, a USGS aggregates research geologist who delivered two presentations at the TRB workshop, says that to meet the reported current and future infrastructure needs, an increase in annual aggregate production as much as 70 percent may be required over a 5-year period, if infrastructure repair is begun in earnest. Further, he adds, "natural aggregate is widespread through the conterminous United States, but the location of aggregate is determined by geology and is nonnegotiable."
Few, if any, deposits of sand, gravel, and rock suitable for making crushed stone are geologically available in some regions. For example, natural aggregate is in short supply in the Coastal Plain and Mississippi embayment, Colorado Plateau and Wyoming Basin, glaciated Midwest, High Plains, and the nonglaciated Northern Plains. Furthermore, many sources of aggregate in other areas, such as parts of the Pacific Northwest, do not meet physical and durability requirements, or they contain contaminants or deleterious materials that limit use.
In other regions, development or community actions may preempt resource extraction. In populated areas, encroachment of conflicting land uses, community pressures, permitting conditions, environmental issues, and opposition from an increasing number of Web-based antimining groups prevent or limit development of many suitable resources.
The problem of limited supply becomes particularly acute in the case of friction aggregates needed for the wearing surfaces of pavements and bridges, which require aggregates with hard minerals that will not abrade or polish readily under traffic. In many parts of the country, where limestone is the predominant aggregate, polish-resistant materials need to be transported from great distances and at increased cost. As State DOTs continue efforts to improve safety on rural and two-lane roads, higher quality, good-friction aggregates or blends for surfacing will become increasingly important. Blends of durable aggregates with different wear resistance can be used for a multitextural surface.
Use of Recycled Materials
FHWA estimates the U.S. transportation industry's need for aggregates for pavements at about 700 million tons (630 million metric tons) per year. According to a 2009 presentation by Peter Stephanos, director of FHWA's Office of Pavement Technology, there is a tremendous need to reduce the demand for virgin mineral resources in the Nation's highway system, and one way of doing that is recycling.
As reported in the FHWA study Reclaimed Asphalt Pavement in Asphalt Mixtures: State of the Practice (FHWA-HRT-11-021), as of 2007, the highway industry was using as much as 100 million tons (91 million metric tons) of reclaimed asphalt pavement. Similarly, the American Concrete Pavement Association (ACPA), in its 2009 Engineering Bulletin (EB043P), estimates that the construction industry uses another 100 million tons (91 million metric tons) of reclaimed concrete aggregate and other crushed and broken concrete materials per year. The Construction Materials Recycling Association (CMRA) estimates even larger quantities of crushed or broken concrete are recycled into various uses and products (including aggregates) each year. Specifically, the association points to recycled concrete aggregate use in aggregate base course (road base), ready-mix concrete, asphalt pavement, soil stabilization, pipe bedding, and landscape materials.
Estimated Aggregate Use in the United States (Millions of Tons) Aggregate Type 2007 2008 2009 Sand and Gravel 1,380 1,170 921 Crushed Stone 1,820 1,610 1,290 Reclaimed Asphalt Pavement* 11 16 18 Recycled Concrete Aggregate* 11 17 14 Sum of Above 3,222 2,813 2,243 Sand and Gravel Imported into 5 6 3 United StateS Crushed Stone Imported into 21 23 13 United States Sum of Above 3,248 2,842 2,259 Source: USGS. *Converted from metric tons and reported to three or fewer digits without decimals. Estimates by USGS for 2010 are about the same or a little less than 2009: 909 million tons for sand and gravel and 1,320 million tons for crushed stone. Note that these data for reclaimed asphalt pavement and recycled concrete aggregate are as reported to USGS and are likely extremely low, in part due to limited survey information. The highway industry (ACPA, CMRA, FHWA, and the National Asphalt Pavement Association) has estimated the quantities of reclaimed and recycled asphalt and concrete materials used in construction at quantities closer to 100 million tons each. Reuse activities include use by a contractor or maintenance forces on the same project or a nearby project for base course materials or shoulder materials, or use as a select material where subgrade strengthening of modification is required.
Providing precise quantities is difficult because recycled and reclaimed materials often are reused on the same project. The USGS estimates of recycled and reclaimed materials are based on quantities stockpiled and marketed for use elsewhere, only by producers or contractors who replied to its annual survey, so the true volume of reclaimed materials in use is likely much higher. In fact, only about one-third of construction companies and aggregate recyclers surveyed responded to the questionnaire. USGS now is annually surveying these companies that produce recycled materials and is working to improve the data collection on the use of reclaimed asphalt pavement and reclaimed concrete aggregate.
States' Sustainability Efforts
Evidence suggests that reclaimed aggregate use at the State level is on the rise. Public Works magazine reports in its March 2011 issue that the Texas Department of Transportation (TxDOT) increased its use of reclaimed asphalt pavement from 467,000 tons (424,000 metric tons) in 2008 to 827,000 tons (750,000 metric tons) in 2010. Despite this large increase, reclaimed aggregates still only meet a small percentage of TxDOTs needs.
The Oregon Department of Transportation's (ODOT) materials sustainability program aims to reduce, reuse, recycle, and "proactively manage all earthen materials needed for and/or generated by ODOT construction and maintenance activities." The objective of the program is to identify and meet the department's material source and disposal needs through site identification and management, strategic planning, and salvage and utilization of excess or waste materials from one project to another.
According to Russell Frost, statewide aggregate resource coordinator at ODOT, in 2009 the department's bridge construction program reused or recycled more than 21,000 tons (19,000 metric tons) of clean fill, 40,000 tons (36,000 metric tons) of concrete, and 44,000 tons (40,000 metric tons) of asphalt materials.
Oregon, like a number of other States and agencies, examined its aggregate resources and set aside a portion of that supply for future uses to protect it from competing land uses. In 2002 the State produced a report in cooperation with FHWA, Aggregate Resource Inventory and Needs Forecast Study (FHWA-OR-RD-03-03), based on Oregon's planning goal to protect natural resources and conserve scenic and historic areas and open spaces. The report explains how ODOT can evaluate aggregate-producing sites and initiate land use actions to conserve and protect significant sites. Oregon also maintains an Aggregate Source Information System database housed on its intranet site. The database is the primary tool ODOT uses to manage its nearly 700 material sources statewide.
Other States such as Alaska, California, Maryland, and the six New England States have also conducted studies or passed legislation regarding aggregate resources. California, for example, passed the Surface Mining and Reclamation Act in 1975, requiring counties to have sufficient permitted aggregate resources to meet the demand for the next 50 years. Furthermore, most States require reclamation and reuse plans for sites after permitted aggregate resources have been extracted. In some cases, a State or local agency will take over the land for public purposes such as roads, parkland, water storage, or groundwater recharge facilities.
Alaska's Materials Inventory Management Program
According to David Stanley, chief engineering geologist with the Alaska Department of Transportation & Public Facilities, and Peter Hardcastle, senior engineering geologist at R&M Consultants, Inc., Alaska is developing a program to manage material sites within the framework of geo-technical asset management. Geo-technical assets include materials sites and others that require monitoring, such as rock and soil slopes, rockfall mesh, rock bolts and anchors, embankments and pavement subgrades, retaining walls, foundations, tunnels, and geotechnical instruments. The project includes assessments of inventory and site conditions guided by the principles of transportation asset management. In the State's three regions--northern, central, and southeastern--there are approximately 2,800 material sites on the road system, of which about one-third are active, Stanley says. Another 250 or so material sites are located at rural airports and have not been inventoried to date.
Alaska faces a number of challenges related to its aggregate supply, including limited transportation systems to deliver materials, material sites converted to other uses, and right-of-way and land use issues. The program will include development of a searchable database of material sites, an overview of available gravel sources, and justification to regulatory agencies for obtaining and retaining sites. Ultimately, Hardcastle says, the program "will help avoid planning conflicts such as with megaprojects and provide continuity despite personnel turnover. The system will be portable, easy to use, and designed to survive future program interruptions."
The Alaska Department of Transportation & Public Facilities has assigned each site an availability classification and documented detailed information about location and material quality and quantity for use in various applications. "The asset management data will be useful to help ensure sufficient material for the future and protect sources for materials mining operations and sharing sites with other agencies," Stanley says. "The data also will support better practices, including buffers between sites and adjacent private properties, correction of problems with land status plats and records, and meeting of environmental requirements such as storm water runoff rules."
Other objectives of Alaska's program include development of performance standards that the department can apply to material sites and to facilitate geotechnical asset management to drive long-term decision making concerning these material assets.
Public Issues Related to Supply and Transportation
In addition to efforts to reuse existing material and catalog the location of aggregate sites, States are faced with issues that arise at the crossroads of supply, materials transportation, and public policy. Highways provide reliable corridors for accessing natural resources, transporting products to markets, and facilitating convenient mobility for communities. Once constructed, highways in rural or remote areas that provide access to mines, agriculture, forests, and recreation areas generally require fewer aggregates for maintenance and upgrades. However, those that serve the Nation's urban and suburban markets and intermodal hubs require greater quantities of aggregates for maintenance and rehabilitation. But, often, aggregate mines and other sources are not available near these high-demand areas.
According to Mark Krumenacher, an industry consultant with GZA GeoEnvironmental, a number of issues surround permitting for aggregate sources. In addition to land use and environmental regulations, in more populated areas, continued development and population growth encroach on current and potential aggregate mining sources. "It is increasingly difficult to expand sources horizontally or open new sites unless there is an ample land buffer," Krumenacher says. "Aggregate producers can sometimes mine their deposits deeper if material of sufficient quality exists, but this is often expensive, with significant engineering challenges."
In Mineral Commodity Summaries 2011, the U.S. Department of the Interior and USGS point to the effect of public and permitting issues on the availability of crushed stone, sand, and gravel, stating that the "[m]ovement of sand and gravel operations away from densely populated centers was expected to continue where environmental, land development, and local zoning regulations discouraged them."
For crushed stone, the report says, "Shortages in some urban and industrialized areas are expected to continue to increase, owing to local zoning regulations and land development alternatives. These issues are expected to continue and to cause new crushed stone quarries to locate away from large population centers." In terms of recycled aggregates, the report acknowledges that, "Increasingly, recycled asphalt and Portland cement concretes are being substituted for virgin aggregate, although the percentage of total aggregate supplied by recycled materials remained very small in 2010."
Bottom line: Much of the natural aggregate needs for highways in more populated areas will need to come from further away with increased cost, congestion, and energy use. That is, unless State, local, and municipal organizations plan for the long term to optimize the use of existing closer-in aggregate resources and to facilitate rail and water movement of aggregates when available.
Uses of Aggregates and Relative Level of Quality Needed Lower Quality Backfill and Bedding Subbase, Select Material, and Subgrade Improvement Base Course (Unbound and Stabilized) * Stabilized (Asphalt, Portland Cement, and Lime-Fly Ash) * Dense Graded * Open Graded Aggregate Surfaced Roads (Gravel Roads) Chip Seal, Cover Material Portland Cement Concrete * Lean Concrete Base (Dense or Open Graded) * Structural Concrete * Concrete Pavement Hot-Mix Asphalt and Warm-Mix Asphalt * Dense Graded * Open Graded Higher Quality Drainage and Riprap Filter Aggregates
Aggregates and FHWA
Recognizing the importance of a sustainable supply of quality aggregates for road building and maintenance activities at the national level, FHWA is collaborating with the International Center for Aggregates Research at the University of Texas and Texas A&M University to sponsor research projects involving both concrete and asphalt. The partnership established a technical working group with Federal, State, university, and industry experts participating in a peer review of ongoing aggregate research and to examine research needs in the highway and transportation areas.
The group provides updates to the TRB Committee on Mineral Aggregates and is working on a roadmap for aggregate research to identify technological and sustainability innovations needed for aggregate granular bases, concrete technology (especially use of manufactured sand), and asphalt pavement mixtures. Matching future regional needs with availability is an important element of that discussion. Balancing land use and resource availability is part of a complex matrix that involves the public at many levels, including consideration at the State and metropolitan planning organization levels.
European Experience And Perspectives
The Europeans too are concerned about the sustainability of local aggregate supplies, as reported by Andrew Dawson, associate professor at the University of Nottingham in the United Kingdom, who studies European aggregate supply issues. Since 1987, the European Aggregates Association has promoted the interests of the European aggregates industry by representing member associations on economic, technical, environmental, and health and safety policies. The association's Annual Review 2009-2010 highlights production and use data from 2008. According to the report, Europe extracts approximately 3.3 billion tons (3 billion metric tons) per year overall, which exceeds current U.S. aggregate production. Of this total, 2 percent is natural sand and gravel dredged from marine seabed sources, and 6 percent is supplied by recycling.
Over the next 5 to 10 years, European production could rise to as much as 4.4 billion tons (4 billion metric tons). The top three countries in terms of recycling percentage (with about 20 percent of total production coming from recycled sources) are Belgium, the Netherlands, and the United Kingdom, which now recycle nearly all available construction and demolition materials. Citing a report by the University of Leoben, Austria, Dawson notes that across Europe a value of 15 percent would represent total recycling, and that in the medium term recycling is unlikely to grow beyond 10 percent of production due to demolition material limitations and the economics of transport.
Dawson reports that permitted aggregate reserves are dropping in Europe due to competing land use, lack of strategic policy and planning, a political drive toward localization of decisionmaking, environmental restrictions, and the complexity and uncertainty of the permissions system. As an example of the lack of planning, Dawson says that data collection on aggregates in Europe is inconsistent and incomplete. "Much of it is industry collected, and many governments do not evaluate aggregate resources. It is therefore difficult to establish policy.
Planning authorities need to conduct minerals mapping. In addition, planning is seldom strategic and often reactionary. In many cases, land use decisions are pushed to local authorities who do not have a broad enough view, thus hindering national and regional policy development."
But, he says, the European Commission's EU Raw Materials Initiative, launched in 2008, could be a step in the right direction. The initiative aims to build a strategy for dealing with raw materials issues and underpin the strategy with legislation. "Aggregates are well represented in the plans," Dawson says, "which is critical, because the availability of aggregates from regional and local sources is essential for economic development in view of logistical constraints and transport costs."
Looking to the Future
The future of public roads depends on a reliable, sustainable supply of aggregates with the quality levels needed to build and maintain long-lasting, durable pavements and transportation structures. State and local DOTs need access to good-quality sources of virgin aggregates--sand, gravel, crushed gravel, and crushed stone--reclaimed asphalt pavement, recycled concrete aggregate, crushed rubble, reworked/rebound aggregates from pavement rehabilitation and full-depth reconstruction, and other alternative byproduct materials to support their highway programs.
Although the use of recycled aggregate is growing, many industry experts doubt the supply will meet the demand. Aggregate mining, therefore, remains a necessity, and needs to be done in an environmentally sound and sustainable way. As individual quarries and mines are depleted and no longer able to supply aggregates, agencies and landowners will need to follow through with reclamation plans to reuse the land for other purposes approved by planning agencies, such as lakes, fish habitat, parks, greenways, groundwater recharge, mixed-use residential and commercial sites, recreation, and wildlife preserves.
As is the case with energy resources, viable solutions for aggregate supplies will vary by location and local circumstances. The TRB workshop presentations and ongoing discussions among industry experts underscore the need for attention to this critical issue: ensuring sustainable sources of mineral aggregates and recycled aggregate materials for tomorrow's transportation system.
For more information, contact Richard Meininger at 202-493-3191 or email@example.com, or Steven Stokowski at 202-493-3403 or steven.stokowski.CTR@dot.gov. See also Aggregate Resource Availability in the Conterminous United States, Including Suggestions for Addressing Shortages, Quality, and Environmental Concerns (Open-File Report 2011-1119), available at http://pubs. usgs.gov/of/2011/1119).
The authors would like to acknowledge the contributions of the following TRB committees that organized the workshop: Low-Volume Roads (AFB30), Exploration and Classification of Earth Materials (AFP20), and Mineral Aggregates (AFP70).
RELETED ARTICLE: Selected solutions to ensure a sustainable supply of aggregates.
1. Compile geologic knowledge of where potential aggregate resources are located and their characteristics. This effort will help in strategic planning and project development to optimize use of regional resources.
2. Develop project designs to best use local marginal and recycled materials for appropriate base layers, and reserve higher quality materials for pavement wearing courses.
3. Recognize that some high-spec materials might have to be imported to meet project objectives. For example, Delaware has abundant natural sand sources, but crushed stone must be imported from other States.
4. Consider expanding specification options and whether the agency can employ blended materials or performance specifications.
5. Use recycled materials where available and consider stockpiling surplus materials for use on future projects.
6. Consider backhaul trucking options, such as hauling com from Nebraska to Colorado for feedlots and backhauling crushed stone aggregate. About 90 percent of aggregate transport is by truck, and, generally, transporting aggregate with haul distances of 30 to 50 miles (48 to 80 kilometers) can double the cost of the aggregate, as reported by Gilpin R. Robinson, Jr., and William M. Brown in the USGS publication Sociocultural Dimensions of Supply and Demand for Natural Aggregate--Examples from the Mid-Atlantic Region, United States.
7. Consider rail and waterway transportation options. Some States have sufficient rail networks or access to major rivers, the Great Lakes, canals, and seaports. Truck transportation costs are rising because of higher fuel prices and are higher in congested traffic or on mountainous hauls. Efficient unit trains can reduce the cost per ton-mile significantly; barge waterway transportation is less; and ocean bulk carrier is even less. Materials suppliers are moving aggregates to coastal areas, such as in California and along the Gulf of Mexico and Florida, because coarse aggregates are in short supply. For example, the majority of aggregates used in Louisiana are shipped via barge from Arkansas, Illinois, Kentucky, and Missouri, and via bulk ship from Mexico.
8. Plan strategically for aggregate resources in growth areas. Aggregate extraction is often a transitional land use, and the ultimate use of the land can be planned for implementation in later development phases.
--Bill Langer USGS
Richard C. Meininger, P.E., is a highway research civil engineer on the Pavement Materials Team at the FHWA Office of Infrastructure Research and Development. Based at the Turner-Fairbank Highway Research Center in McLean, VA, Meininger's primary responsibilities include managing research projects related to concrete and aggregates in the center's laboratories and projects by outside researchers as well. He has M.S. and B.S. degrees in civil engineering from the University of Maryland, College Park.
Steven J. Stokowski, P.G., is an aggregate technologist and petro-graphic laboratory expert with SES Group & Associates, LLC, a contractor for FHWA at the Turner-Fairbank Highway Research Center. He has an M.S. in geology from the South Dakota School of Mines & Technology in Rapid City, SD, and a B.S. in geology from The George Washington University in Washington, DC.
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|Author:||Meininger, Richard C.; Stokowski, Steven J.|
|Date:||Sep 1, 2011|
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