Looking to Aggregates to Improve Pavement Sustainability
FHWA is conducting research, developing tools, and gathering data to improve how aggregate materials are sourced, planned, and used in sustainable pavement construction and maintenance.
Aggregate is a critical element in the construction, maintenance, and repair of roadways and concrete structures. However, the aggregate resources needed for the construction and maintenance of highways, local roads, and streets, as well as for basic materials for concrete structures and drainage applications, often are not given sufficient consideration early in the planning and design process. Planners and engineers need to consider: What are the local natural and recycled aggregate resources and are they of the quality needed for anticipated pavement layer construction, overlays, and preservation treatments?
|FHWA and its partners are aiming to make the use of aggregates in pavement more sustainable. Here, crushed stone is processed and shipped from a plant at a granite quarry in Virginia.|
Natural aggregate availability depends on the regional geologic resources and whether those resources can be mined and transported in a long-term, sustainable way to the major construction markets. There are significant challenges to managing resources for the reuse or recycling of reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA). In rural areas, the reclaiming and recycling of aggregates can be done on a project-by-project basis, ensuring more consistent quality, but it is expensive and inefficient to move these low-value materials long distances to other projects. Combined RAP stockpiles and urban rubble recycling of concrete, brick, and stone are common in many metropolitan areas, but create more variable quality for these resources.
To help address these challenges, researchers at the Federal Highway Administration's Turner-Fairbank Highway Research Center (TFHRC), along with other partners, have been researching the physical and chemical factors necessary to effectively deploy both natural aggregates and recycled materials in the major components of pavement construction and maintenance. These components include aggregate base courses for pavement foundation layers, compacted asphalt mixtures for bases and surface courses, and concrete pavements used extensively for heavy traffic and mainline highway pavements.
In addition to this research, FHWA has expanded its Sustainable Pavements Program, offering educational resources and tools to help project planners and highway engineers consider the impacts and costs of choosing pavement options in a holistic way. The program's vision is to increase the knowledge and practice of designing, constructing, and maintaining more sustainable pavements. To accomplish this vision, the Sustainable Pavements Program strives to educate and support stakeholder needs by producing technical resources, creating training opportunities, developing tools, and facilitating conversations through stakeholder meetings.
In particular, over the past 20 years, FHWA has partnered with the International Center for Aggregates Research and committees of the Transportation Research Board and the American Association of State Highway and Transportation Officials in shaping research and engaging with stakeholders to gather information and ideas on technical priorities and needed research approaches to improve the performance of pavements and materials, resulting in the Aggregate Research Roadmap. The roadmap is revisited and updated regularly to keep it current and match priorities to research needs.
Sustainability and the Triple Bottom Line
Sustainability is a quality that reflects the balance of three primary components: economic, environmental, and social impacts, which are often collectively referred to as the triple bottom line. Considering sustainability during the transportation decisionmaking process is not new. Typically, the environmental and social impacts are quantified during the conceptual design phases as part of the environmental review process, while cost-effectiveness is a consideration in pavement design. However, there is growing recognition in the industry that the pavement design, material selection, and construction phases, including aggregate mine sources, transportation, and aggregate recycling, contribute significantly to all three pillars, and thus opportunities for improvement exist.
The relative importance and consideration of each of the economic, social, and environmental impacts factors are context-sensitive and driven by the project goals and performance demands, characteristics, location, materials, and constraints of a given project, as well as the overarching goals of the sponsoring agency. By designing, constructing, restoring, preserving, and maintaining pavements that consider the triple bottom line over the project's life cycle, transportation agencies can develop a more resilient infrastructure system with increased performance and a high return on investment despite the growing constraints on economic and material resources, such as availability of local crushed stone, sand, and gravel.
"Aggregates are the most predominant material in highway and other transportation applications," says Matthew Beeson, Director of Materials and Tests, Indiana Department of Transportation and Chair of the Aggregates Technical Subcommittee of the AASHTO Committee on Materials and Pavements. "Improving the performance of aggregates used in current applications and expanding the use of and sources of natural and recycled aggregates for new applications will make a significant contribution toward creating a sustainable society and extending the performance of infrastructure containing aggregates."
Some areas and regions of the country have experienced aggregate shortages for certain highway pavement applications that have necessitated longer transportation distances for aggregates that meet project requirements. Some of these inefficient and unsustainable practices have been prompted by method (rather than performance) specifications or pavement design practices that have not considered the best uses of certain local natural aggregate characteristics, nor considered whether local recycled or reclaimed aggregate products can be used instead to construct or rehabilitate quality, long-lasting pavement layers.
|An aggregate stockpile in Estes Park, CO.|
A Roadmap to Research Success
For more than 20 years, FHWA has participated with highway agency and industry stakeholders, as well as university researchers and the TRB Aggregates Committee (AKM80), in developing and maintaining an Aggregate Research Roadmap. The Aggregate Research Roadmap was originally developed by an FHWA technical working group that included university researchers, State departments of transportation, and aggregate industry representatives. In cooperation with the International Center for Aggregates Research, and later, in collaboration with TRB and AASHTO aggregate committees and subcommittees, these stakeholders helped shape aggregate research conducted over the past two decades.
Discussions about the needs for pavement technology research and the periodic review of the roadmap have resulted in many research needs statements, such as those vetted by TRB and AASHTO, and aggregate research projects funded by the States, industry, and FHWA. Roadmap priorities have led to completed research projects and syntheses recommended through TRB and conducted by the National Cooperative Highway Research Program, universities, and agencies. These include completed projects on compacted aggregate base course properties and aggregate-specific gravity research, and syntheses on granular base course construction and on aggregate quality criteria.
Resources for Sustainable Pavement Systems
FHWA's Sustainable Pavements Program provides webinars, tools, and resources developed and presented to the pavement community that consider the elements of the triple bottom line of sustainability that are impacted by pavement design, aggregate choices, and pavement life cycle performance. Through the program, FHWA has developed tools to aid in choosing sustainable options for aggregate sources, transportation, and recycling, as well as showing how better performance and timely preservation are significant inputs into life-cycle analysis and pavement management concepts.
In 2015, the Sustainable Pavements Program published Towards Sustainable Pavement Systems: A Reference Document (FHWA-HIF-15-002), which walks through the pavement life cycle and highlights potential strategies for improvement. The publication identifies context-specific, potential strategies for sustainability improvements, including strategies for aggregate production and use. Strategies for aggregates include reducing virgin aggregate use by using recycled waste materials, using more durable aggregates to maximize pavement life, reducing hauling distances by selecting local materials, and improving aggregate acquisition and processing. Building on the publication, FHWA offered a 10-part webinar series in 2019 and 2020 on the key concepts and practices related to pavement sustainability to help engineers begin identifying potential environmental, economic, and social impacts of decisions. The webinar recordings are available at www.fhwa.dot.gov/pavement/sustainability/webinars.cfm.
Tools exist to quantify and assess the sustainability performance and to help agencies review sustainability strategies and take a holistic life cycle approach to balance tradeoffs. For example, FHWA's RealCost tool, available at www.fhwa.dot.gov/infrastructure/asstmgmt/lccasoft.cfm, can help with life-cycle cost analysis. For pavements, life-cycle cost analysis provides a way of measuring the economic consequences of choices in design, materials, construction techniques, construction windows, maintenance schemes, and end-of-life treatments over a prescribed analysis period. Many State agencies have life-cycle cost analysis policies and procedures in place.
Life-cycle assessment (LCA) is a methodology used to evaluate a range of environmental impacts, such as potential for climate change, ozone depletion, acidification, eutrophication, and smog formation. Over the last decade, FHWA, along with its partners from transportation agencies and industry, have conducted substantial work to incorporate LCA into the pavement investment decisionmaking process. One resource is FHWA's LCAPave, a new tool that is currently going through FHWA's publication process after a successful beta test, which can help agencies understand the environmental impacts of material and design choices. LCAPave can incorporate aggregate producers' specific data, via environmental product declarations, or can help make generic assessments of different mixture and pavement designs, such as inverted pavements compared to conventional designs. FHWA has a pooled fund project for States interested in trying LCAPave in a demonstration project; for more information, visit www.pooledfund.org/Details/Solicitation/1542.
Social indicators are an important pillar of sustainability, but harder to assess. Social impacts from pavement and material decisions can include both short-term construction and long-term operations factors, such as noise, traffic, safety, materials hauling, resource conservation, and ride quality. To stay abreast on the latest topics of pavement sustainability, join the FHWA Sustainable Pavements Friends list by signing up at https://public.govdelivery.com/accounts/USDOTFHWA/subscriber/new?topic_id=USDOTFHWA_146.
Pavement Sustainability Sectors
Recently, FHWA refined and restated the core objectives of the pavement program to emphasize the importance of constructing and managing quality pavements through the full life cycle, while also meeting community objectives. These objectives and categories were developed through the collaboration of the pavement and materials team leaders within FHWA's Office of Infrastructure, Office of Infrastructure Research and Development, and Resource Center.
All phases of the life cycle are important in meeting the core sustainability mission, and since aggregates are the predominant constituent both by volume and mass in the various pavement materials and layers (bases, asphalt, and concrete), the selection and quality assurance of aggregates play a major role in all segments of the pavement life cycle. These sustainability sectors are pavement design, materials, quality assurance, pavement construction, pavement management, and pavement preservation and rehabilitation.
|Because aggregates are the predominant constituent, both by volume and mass in the various pavement materials and layers (bases, asphalt, and concrete), the selection and quality assurance of aggregates play a major role in all segments of the pavement life cycle wheel.|
Pavement design and pavement type are often influenced by local and regional aggregate sources and quality, as well as by the properties of the subgrade soil.
Materials used in pavements and structures can be much more successful and durable when specifications and test methods for the individual aggregates, and for the performance of mixtures containing aggregates, have been developed to properly characterize materials in the laboratory and in mixture designs that will be durable in service, and to facilitate efficient production and placement in the field.
Quality assurance is the backbone for durable, long-lasting highways and pavements. Owners and engineers have the responsibility to provide inspection, sampling, testing, and verification that construction using aggregates is in conformance with the plans, specifications, and materials designs. This includes ensuring that laboratory and field quality assurance personnel are qualified in the specialized sampling techniques for aggregate from stockpiles, bins, and roadway placements.
Pavement construction involving the placement and optimum compaction of granular aggregate bases can be challenging in that segregation needs to be avoided in all phases of recovery from stockpiles and bins, truck delivery, and spreading aggregate on the grade.
Pavement management depends on the acquisition of periodic data on rideability, friction, rutting, cracking, and profile and drainage issues. Aggregates exposed at the wearing surface contribute to the duration of sufficient macro- and micro-texture to provide needed traction in wet weather. Aggregate selection and performance can also contribute to the likelihood of rutting, drainage, and durability problems causing spalling, raveling, and water accumulation.
Pavement preservation and rehabilitation treatments and methods often require special attention to aggregate grading and quality. Even for pavements that have deteriorated, the aggregate bases can often be considered a valuable asset that can be reshaped, rehabilitated, or stabilized through processes such as full-depth reclamation and drainage improvements to provide a quality foundation for new pavement layers, overlays, and wearing course treatments.
|Photomicrograph of thin sections of recycled concrete aggregate particles for analysis, viewed through a petrographic microscope. The top images use cross-polarized light for mineral identification. The bottom images use fluorescent light to study porosity.|
Relative Supplies of Natural Aggregates by State
To help highway agencies understand the available supply of natural aggregates (crushed stone, sand, and gravel) in each State, FHWA recently compiled key statistical indicators relating annual natural aggregate production expressed as a ratio to lane-miles of pavement in each State for 2017. This was done to normalize potential resources based on the total amount of constructed and managed lane-miles of roadways in each State. Total aggregate production includes current United States Geological Survey (USGS) data of annual supplies of construction crushed stone and sand and gravel available in each State. Natural aggregate availability regions for sand and gravel, and for the different types of crushed stone, often do not follow State boundaries, but are important considerations in local pavement material selection.
Some States and regions, such as the High Plains and the Gulf Coast, have limited availability of quality natural aggregates. Therefore, aggregates have to be transported long distances at greater economic and environmental costs. Alternatively, project planners may consider other options, such as tailoring pavement base and pavement layer construction methods to available local or marginal materials, greater use of stabilized local soils and materials for subbases and bases, and more aggressive pavement preservation techniques using surface treatments and thin overlays to maintain pavement serviceability.
Many States east of the continental divide and along the Gulf Coast have lesser amounts of natural aggregates (sand, gravel, and crushed stone) available. Rocky Mountain and Appalachian States, and those closer to the mountains, tend to have more natural aggregate resources as a ratio per lane mile in these roadway systems. States in the upper Midwest also typically have higher amounts of resources per lane mile because of glacial sand and gravel deposits, as well as ample limestone and dolomite bedrock sources in the region.
The top States for carbonate crushed stone as a percentage of the total natural aggregate resources include West Virginia, Kentucky, Missouri, Iowa, Tennessee, Florida, Texas, Indiana, Ohio, Michigan, and Illinois. States like these with high percentages of carbonate crushed stone, which is relatively soft, may face challenges in locating hard mineral aggregates for pavement wearing surfaces and surface treatments that provide required friction and texture performance.
|Amounts of annual aggregate resources expressed as a ratio of tons of production per lane-mile of roadways in each State, including total natural construction aggregates, sand and gravel, and crushed stone.|
|This chart shows the percent of annual crushed stone production that is carbonate aggregates (limestone, dolomite, and marble) in each State. Carbonate aggregates are relatively soft, limiting use in pavement surfaces because they can polish under traffic, creating slippery-when-wet conditions.|
"Understanding the local availability of various natural aggregates can help project planners determine the best methods and solutions to meet the needs of each project," says Bob Younie, Director of Maintenance for the Iowa Department of Transportation.
Recent FHWA Aggregates Research
FHWA's research has helped to advance the understanding of aggregates and pavement materials.
For example, an ongoing TFHRC study using FHWA's Pavement Test Facility (PTF) examined different percentages of RAP and recycled asphalt shingles in both conventional and warm-mix asphalt (WMA) pavements to quantify the cracking resistance of high RAP mixtures and the effect of lower WMA mixing and compaction temperatures on RAP performance. Lanes containing recycled shingles and high RAP using the stiffer binder were more susceptible to fatigue damage. The warm mix and hot mix materials provided similar performance. While this study focused on binder issues, the research advanced a method for determining the workability of loose hot mix based on deformation conducted at a fixed deformation rate. This can be used as a quality control test for RAP mixes and RAP materials.
|FHWA's Pavement Test Facility at TFHRC.|
An earlier study at the PTF looked at the use of a 4.75-millimeter nominal maximum aggregate size mixture containing 20 percent RAP to study effects on delaying top-down cracking. This was a less permeable mix and provided good performance prior to its aging.
FHWA plans additional research at the PTF to include research topics such as recycled materials and the development of data on performance of asphalt mixtures and overlays as well as the incorporation of preservation strategies, which ultimately affects pavement life-cycle cost analysis and performance.
Other research from FHWA's Chemistry, Concrete, and Petrographic Laboratories has explored aggregate durability and performance issues, as well as characterizing the variability and quality of recycled concrete aggregates. FHWA developed a chemistry test called T-FAST to identify the potential alkali silica reactivity of coarse and fine aggregates. The test specification for coarse aggregates was recently approved by AASHTO. Research to date has shown that this test gives 100 percent agreement with block farm data and can be run quite rapidly and more efficiently than traditional methods.
FHWA has also conducted a field study to research the concept of inverted pavement in cooperation with the Virginia Department of Transportation and the aggregates industry. An inverted pavement uses the same materials as a conventional pavement, but the layers are arranged differently. In conventional designs, the layer above protects the layer below (stiffer layers higher up). The inverted pavement uses a stiff base and essentially flips the middle layer. The field study found that the performance of the inverted pavement sections was comparable to standard Virginia pavement sections.
Looking to the Future: Aggregate Research Needs
FHWA and its partners continue to update the Aggregates Research Roadmap. Currently, the roadmap indicates research needs for aggregate granular bases, aggregates in asphalt paving mixtures and in concrete, and designs and methods of using recycled aggregates and excess supplies of some natural aggregate sizes.
For aggregate granular bases, inverted pavements provide one option for road foundations based on compaction of quality crushed aggregate base against a more rigid treated subbase. Research needs include achieving more uniform quality compaction of granular aggregate layers that will provide stiffer, long-lasting foundations for roads that will resist permanent deformation under heavy truck traffic.
Ongoing research is examining the quality of RAP and how much can be used in asphalt mixtures, as well as exploring how well a coating of aged asphalt on the RAP blends with the new materials. FHWA has also initiated research to assess the role that aggregate gradation plays in pavement macrotexture. Other research needs indicated by the roadmap are finding methods for balanced design of performance asphalt mixtures, including aggregate grading optimization and the interaction of binder types with aggregate surface properties to resist moisture and freezing exposures and to provide longer-lasting ride quality.
Total Annual Construction Aggregate Resources Available in the U.S.
FHWA and its partners are developing and refining test methods for use with the new AASHTO standard for performance-engineered mixtures for concrete pavements. These methods address both the workability and placeability of the mixture for slip-form paving, but also the important durability criteria for aggregates in concrete to resist freeze-thaw, deicing chemical, and alkali aggregate reaction exposures that can shorten concrete pavement service.
All pavements require aggregates at the wearing surface with hard, wear-resistant minerals that provide consistent friction and texture. Research related to improving the measurement of friction and texture on pavements at traffic speeds on a more continuous basis is ongoing, so that these performance data can improve traction and safety during wet weather.
|A large power shovel loads blasted granite rock from a crushed-stone quarry in Virginia into a hauling vehicle for transportation to the crushing and screening plant nearby.|
The roadmap indicates that research is needed to better characterize recycled and reclaimed aggregates and develop pavement designs for using and blending these materials in new pavement layers, as well as to determine treatments to extend or renew service life. In addition, in many areas, crushed stone quarries have high demand for clean coarse aggregate sizes, but that leads to the challenge of how to best use the excess crushed fine aggregate and oversize rock materials on a sustainable basis so that aggregate resources can be used more effectively locally for highway construction and improvements.
"Achieving sustainable pavements isn't simply a matter of maximizing the use of recycled and locally available aggregates," says Dr. Cheryl Richter, Director of FHWA's Office of Infrastructure Research and Development. "To be sustainable, the pavement must be durable. FHWA research is providing information and guidelines needed to support use of RAP, reclaimed asphalt shingles, and other materials to achieve long-lasting, truly sustainable pavements."
Richard Meininger is the manager of FHWA's Aggregates and Petrography Lab at TFHRC and has been an FHWA highway materials researcher at TFHRC for 20 years. He holds a B.S. and an M.S. in civil engineering from the University of Maryland and is a registered professional engineer.
Heather Dylla is the program manager for FHWA's Pavement Sustainability Program. She holds a Ph.D. and an M.S. in engineering science from Louisiana State University and a B.S. in civil engineering from Bradley University.
Jack Youtcheff is the leader of FHWA's Infrastructure Materials Team in the Office of Infrastructure Research and Development at TFHRC. He has a Ph.D. in fuel science and a bachelor's degree in chemistry from Pennsylvania State University.
For more information, see https://highways.dot.gov/research/turner-fairbank-highway-research-center/laboratories/laboratories-overview, and www.fhwa.dot.gov/pavement/sustainability, or contact Richard Meininger at 202–493–3191 or Richard.Meininger@dot.gov, Heather Dylla at 202–366–0120 or Heather.Dylla@dot.gov, or Jack Youtcheff at 202–493–3090 or Jack.Youtcheff@dot.gov.