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|Federal Highway Administration > Publications > Public Roads > Vol. 64· No. 1 > National Research Projects on Recycling in Highway Construction|
National Research Projects on Recycling in Highway Construction
by Marcia J. Simon, Warren H. Chesner, T. Taylor Eighmy, and Howard Jongedyk
This article describes several recently completed or ongoing research projects pertaining to recycling in highway construction. These projects, which are national in scope, were sponsored by the Federal Highway Administration (FHWA) or the National Cooperative Highway Research Program (NCHRP), and they reflect a renewed interest in recycling, spurred by an increasing volume of waste or byproduct materials from industrial, domestic, and mining sources and a decreasing availability of landfill space for disposal.
FHWA encourages the use of recycled materials as part of its commitment to maintaining the quality of the natural environment. However, to ensure that the quality of the nation's infrastructure is maintained — or improved — FHWA insists that engineering and environmental performance using recycled materials must be equal to or better than the performance using conventional materials.
FHWA began programs in recycling in the early 1970s. Throughout that decade and into the early 1980s, several feasibility studies and demonstration projects were completed on various materials, including sulfate waste, coal combustion byproducts, incinerator ash, and mining waste. In 1991, the Intermodal Surface Transportation Efficiency Act (ISTEA) outlined some specific recycling-related activities, including a report to Congress by FHWA and the U.S. Environmental Protection Agency (EPA) on the technical, environmental, safety, and economic aspects of using crumb rubber in asphalt pavements and a compendium of experience with other recycled materials. These activities led to the initiation of several research projects on crumb rubber use, a symposium to assemble current knowledge and state-of-the-practice information, and the establishment of a high-priority research area in recycling.
The first project initiated in this high-priority area was the development of a guidance manual on the use of waste or byproduct materials in pavement construction. In 1997, four additional research projects were initiated through cooperative agreements with a consortium headed by the University of New Hampshire. These included an assessment of the residual alkali-silica reaction potential of recycled portland cement concrete used as aggregate, the development of mix design methods for cold in-place recycling of asphalt, the development of a framework for evaluating appropriate uses of recycled materials, and the prediction of long-term performance of recycled materials using accelerated aging.
During the same time frame, two NCHRP projects related to recycling were initiated. These were NCHRP 4-21, "Appropriate Uses for Waste Materials in Transportation," and NCHRP 25-9, "Environmental Impact of [Highway] Construction and Repair Materials on Surface and Ground Waters." A database of information about recycled materials was developed as part of NCHRP 4-21, and several recycled materials — such as reclaimed asphalt pavement, reclaimed concrete pavement, slags, coal ash, and rubber — were evaluated as part of NCHRP 25-9.
The development of the User Guidelines for Waste and Byproduct Materials in Pavement Construction began in 1996. The primary objective of the project was to assess the state of the practice of using waste or byproduct materials in pavement construction and to produce a guidance manual for a variety of interested parties, including state and local transportation agencies, environmental agencies, waste generators and recyclers, design engineers, and contractors.
At the time, two NCHRP syntheses, one on the use of recycled rubber and the other on waste and byproduct materials in highways, had just been published. Several states had also published reports on recycled material use. The User Guidelines project was designed to bring these reports and other information together into one guidance document that would provide information about materials and how to use them in appropriate pavement construction applications and about how to evaluate the suitability of a material with limited or no history of use in a given application. A related goal was to identify gaps in knowledge and unresolved issues related to the use of waste or byproduct materials in order to include them in the guidance document and to provide a basis for future research.
There are many recycled materials and many possible uses for them in the highway environment. To name just a few applications, recycled materials have been used in pavements, appurtenances, guardrails and lampposts, paints and signs, and landscaping. Because it was not feasible to cover all these areas in one project, the scope was limited to six pavement construction applications that require a large volume of materials: asphalt concrete, portland cement concrete, stabilized base, granular base, flowable fill, and embankments/backfill.
Within each of the six application areas, there are one or more possible uses or roles for a material. For example, in asphalt concrete, a material could be used as aggregate, mineral filler, or asphalt cement modifier. In granular base, a material could only be used as aggregate.
The scope was also limited in terms of materials. Materials within the general categories of agricultural, domestic, industrial, and mining wastes were considered, and selection was based on several criteria. Only materials that were suitable for the six application areas described above were considered. For this reason, agricultural wastes, although significant in volume, were not included. For materials meeting this requirement, sufficient information had to be available to allow guidelines to be developed for one or more of the selected application areas. Material availability (volume generated and stockpiled) was also considered.
Nineteen materials were selected. A matrix of the 19 materials and six applications was developed, giving 114 possible combinations. Each combination was evaluated, and 55 were determined to be existing or potential uses with sufficient information available to develop guidelines.
Because User Guidelines was intended to be a guidance manual rather than a standard research report, several factors had to be considered in designing a suitable format for the document. The first consideration was size. With the amount of information available on recycled materials, the document could easily have become too large to be practical. It took several iterations to determine what constituted "essential" information — enough to make the manual a valuable reference, but not so much that it became unwieldy. Second, the manual was intended to be user-friendly and consistent in organization so that a user could easily locate the information he or she needed without having to scan through unwanted information. Also, the document's format had to allow for periodic expansion and updates as new information became available and as new materials and uses became known. For this reason, the manual was designed to fit in a three-ring binder with tabbed section dividers.
Overall, the manual consists of four major sections:
The introduction summarizes the purpose, scope, format, and contents of the manual. The second and largest section, Material-Specific Guidelines, is divided into 19 sections that are organized alphabetically by material. Each section contains a description of the material and one or more user guidelines and is typically six to 12 pages long. Each material description is divided into five parts, providing information on the origin of the material and the quantities available, current management options, market sources, highway uses and processing requirements, and material properties.
For each material-application combination, a user guideline — containing information on previous use and performance, processing requirements for the specific application, engineering properties relevant to the application, design and construction considerations, and unresolved issues — is provided. The number of user guidelines varies for each material. For example, baghouse fines has one user guideline, while blast furnace slag has four. This was not a judgment that baghouse fines are only suitable for one application, but rather it is recognition that, at the time, there was sufficient information available to assemble a guideline for only one application.
The third major section consists of three Evaluation Guidance chapters, which are precursors to the more comprehensive Consensus Framework described later in this article.
The first of the three evaluation guidance chapters outlines a process for evaluating the suitability of a recycled material for use in a pavement application. The steps involved include identifying relevant technical issues, establishing appropriate tests and performance criteria, performing tests, considering implementation (nontechnical) factors, and possibly conducting field demonstrations.
The other two chapters provide additional guidance on environmental and cost issues. The environmental guidance chapter describes legislative and regulatory history related to the use of recycled materials in highways; summarizes current state regulatory practices; and outlines procedures for assessing health, environmental, and ecological impacts for pavement construction applications using recycled materials. The cost chapter describes the cost elements that must be considered to determine the initial and life-cycle costs of using a recycled material.
The fourth and final section of User Guidelines contains application descriptions that provide more detailed information about the applications (e.g., asphalt concrete, portland cement concrete, etc.) for users who may not have a civil engineering background. These sections provide a general introduction to the application and describe the conventional component materials, desirable properties of the component materials and the final product, and standard tests or specifications that are used to evaluate these conventional materials and products.
User Guidelines for Waste and Byproduct Materials in Pavement Construction was written by Dr. Warren Chesner, Chesner Engineering P.C.; Robert Collins, ISG Resources Inc.; and Michael MacKay, John Emery Geotechnical Engineering Ltd.
FHWA Research Projects (UNH Consortium)
In 1997, four recycling research projects were initiated through cooperative agreements with a consortium headed by the University of New Hampshire (UNH). The consortium included UNH, the University of Rhode Island, Cornell University, Rutgers University, and international consultants from Sweden and Japan. Each project is described briefly below.
Residual ASR Potential in Existing PCC
Alkali-silica reaction (ASR) is a deleterious reaction between reactive silica in some aggregates and alkali present in the concrete (PCC) water. The alkali causes the reactive silica to go into solution, and it is deposited as a gel that expands when external water enters the concrete. This expansion leads to cracking and deterioration of structures and pavements over a period of years.
A significant portion of the nation's infrastructure is suffering from this dilemma. This makes it difficult to recycle concrete with ASR present as the reaction can continue within the new concrete matrix. It would be invaluable to have a better knowledge of how to mitigate ASR. In addition, to be able to successfully recycle ASR-affected structures as an aggregate source in new concrete would save our quickly declining supply of mineral aggregates.
Methods are being investigated to more quickly evaluate a given concrete for its ASR potential. Traditional methods such as ASTM 1260 and ASTM 1293 are being modified by using different sample geometries, electric fields, and microwaves to more quickly detect ASR in concrete made with recycled concrete aggregate. Electric fields and microwaves are also being used to accelerate the reaction in field cores obtained from pavements known to have ASR to determine their remaining ASR potential.
Dr. David Gress of the University of New Hampshire is the principal investigator for this project. The final report will be published in 2000, and it will also be available on the Recycled Materials Resource Center Web page (http://www.rmrc.unh.edu).
Mix-Design for Cold In-Place Recycling
Cold in-place recycling (CIR) offers a number of opportunities to reuse reclaimed asphalt pavement (RAP) from low-volume roads nationwide. However, performance-based mix-design procedures are needed. Presently, to develop a performance-based mix design, the effects of density and air voids must be assessed. Traditional mix designs are typically based on the modified Marshall method as described in the report from American Association of State Highway and Transportation Officials (AASHTO) Task Force #38.
An expert technical group (ETG) has been formed for this project. The group broadly represents the industry, including emulsion suppliers; emulsion chemists and contractors; state highway agencies; town, city, and county engineers; academia; and researchers. Based on input from the ETG, the Superpave mix design is being used to develop a performance-based mix design. Specimens are fabricated using the Superpave Gyratory Compactor (SGC). Experimental examination of important process variables such as moisture content, amount of emulsion, type of emulsion, asphalt content of the recycled material, and compactive effort have been undertaken. The specimens are prepared at densities similar to those found in the field. The new procedure will also include a performance analysis component using the incremental static-dynamic creep test (and fatigue test, if feasible) and the indirect tensile test to prevent rutting and cracking. Representative RAP samples from different regions of North America were obtained from Arizona, Connecticut, Kansas, and Ontario for use in the research.
Prof. K. Wayne Lee of the University of Rhode Island is the principal investigator for this project. He can be reached at (401) 874-2695 or firstname.lastname@example.org. Project information can be viewed on the Web (http://www.tserver.cve.uri.edu). The final report is expected in 2000 and will also be on the Web pages for the University of Rhode Island and the Recycled Materials Resource Center (http://www.rmrc.unh.edu).
Evaluation of the Use of Recycled Materials
While many recycled materials have historically been used in the highway environment, the use of recycled material is a relatively new concept in some states. Even among states with extensive experience in using recycled materials, considerable differences exist in the evaluation and permitting processes for the use of recycled materials under beneficial use determinations (BUDs) or similar mechanisms.
A logical and hierarchical evaluation process that all states could use to either develop or refine their BUD process would help reduce barriers to the use of recycled materials and allow for some measure of reciprocity between states. The goal of this project is to develop such a process.
Management and regulation of recycled materials use in the highway environment are jurisdictionally the responsibility of both the state departments of transportation (DOTs) and the state environmental regulatory agencies. One major goal of this project is to work with both state agencies to develop a consensus-based approach. Such an approach can facilitate and encourage the two agencies to work together in the BUD process. State DOTs and environmental agencies from Florida, Minnesota, New Hampshire, New Jersey, and New York are involved in this project.
The project used an ETG to help in the development of a hierarchical framework evaluation process. The framework process has a series of stages: issues definition, data evaluation, laboratory testing, and field testing. Each of these steps can lead to approval of the beneficial use application. Guidance on testing and specifications is also provided.
Dr. Taylor Eighmy of the University of New Hampshire and Dr. Warren Chesner, Chesner Engineering P.C., are the principal investigators. The final framework document will be published in 2000. The document will also be available on the Recycled Materials Resource Center Web page (http://www.rmrc.unh.edu).
Predicting Long-Term Environmental Performance
The use of recycled materials in the highway environment frequently leads to several questions: How will this road with recycled materials perform physically and environmentally in 10 or 20 years? Will there be deleterious physical deterioration? Will there be a release of environmentally deleterious constituents if such deterioration occurs?
At this time, there is a clear lack of predictive strategies to identify future physical or environmental behavior of recycled materials used in highway construction. Traditionally, real-time, 20-year field-scale demonstrations are used to evaluate aging and subsequent physical and environmental performance. A predictive approach, however, would allow more timely evaluation of recycled material and pavement compatibility issues and expected future performance. Ideally, such a method could either replace or complement field trials. The purpose of this project is to develop such a predictive approach using accelerated aging.
A common and accepted highway application of a recycled material — portland cement concrete pavements made with coal fly ash — was selected for development of an accelerated aging methodology. A pavement from U.S. Route 20 in Fort Dodge, Iowa, which failed after 10 years of service, is being used as the model system. The pavement concrete contained Class C fly ash. A pavement slab, obtained with the assistance of Iowa DOT, has been analyzed for a number of experimental response variables, including:
The approach chosen to accelerate the aging of concrete pavements used three principal aging variables. The concrete materials and mix designs for the Iowa pavement were used to cast 400 laboratory prism specimens, which were temperature aged, cyclical stress aged, and freeze-thaw aged. The prisms were analyzed for the same response variables as the slab, and models relating the physical and environmental performance of the concrete to the levels of aging are being developed.
Preliminary results suggest that this aging regimen was able to produce samples that are similar to the pavement slab that failed in the field for some of the response variables that were tested (strength, mineralogy, and leaching behavior). Over time, further work will lead to a predictive strategy based on laboratory-accelerated aging methods and on predictive models that would be available for adoption by state and federal transportation officials.
This interdisciplinary project is being conducted by the Environmental Research Group and the Civil Engineering Department at the University of New Hampshire, the U.S. Army Corps of Engineers' Cold Regions Research and Engineering Laboratory (CRREL), Vanderbilt University, Cornell University, and Michigan Technological University (MTU). The principal investigators are Dr. Taylor Eighmy, Dr. Ray Cook, and Dr. David Gress, all from the University of New Hampshire; Dr. Ken Hover from Cornell University; Dr. David Kosson from Vanderbilt University; and Chuck Korhonen of CRREL. Dr. Tom Van Dam of Michigan Technological University is a subcontractor. A literature review on accelerated aging and the final report are expected in 2000. These documents will also be available on the Recycled Materials Resource Center Web page (http://www.rmrc.unh.edu).
Recycled Materials Information Database (NCHRP 4-21)
The development of the Recycled Materials Information Database began in 1998. The primary objective of the project was to provide a tool that could be used to store data about recycled material properties, applications, testing procedures, and reference information. The database was designed to be an interactive tool that would provide users with the means to update and print information, edit the text and tables within the database, update the information with new tables and maps to respond to new data inputs, or delete existing information.
Preparation of the database was sponsored by AASHTO in conjunction with FHWA and was conducted under the auspices of the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council.
The Recycled Materials Information Database is a Windows-based program containing information on 21 waste and recycled materials (WRMs) for use in transportation-related applications. The 21 materials are baghouse fines, blast furnace slag, cement kiln dust, coal boiler slag, coal bottom ash, coal fly ash, flue gas desulfurization sludge, foundry sand, lime kiln dust, mill tailings, municipal waste combustor ash, non-ferrous slag, phosphogypsum, quarry waste, reclaimed asphalt pavement, reclaimed concrete material, roofing shingle scrap, scrap tires, sewage sludge ash, steel slag, and waste glass.
The database is divided into nine major categories that are intended to provide the user with both general and detailed engineering and environmental information on each WRM. Included in the database are recommended laboratory engineering tests that can be used to assess the suitability of each WRM for use in transportation-related applications and recommendations for monitoring WRM field trials.
Format and Content
The nine major sections within the database for each material are:
General Information contains material descriptions, including photographs and graphical displays of potential uses and production processes. Production and Use contains a series of maps that illustrate the geographic distribution of recycled material production and use within the United States. Engineering Properties provides descriptions and tabular listings of the physical, mechanical, and chemical properties of the selected materials. Environmental Properties provides descriptions and tabular listings of trace metals, trace organics, leachate properties, and environmental issues associated with the selected material. Applications provides information on design, specifications, processing, construction, performance, and research needs for the selected material in a given application. Laboratory Testing provides a listing of test methods and test criteria to evaluate selected properties. Field Testing provides recommended field-testing procedures to evaluate selected properties. The References section provides a listing of database references, and The Contacts section provides a list of agencies or organizations that can be contacted to obtain additional information on a specific material.
The NCHRP Materials Database was created by Dr. Warren Chesner and Christopher Stein of Chesner Engineering P.C.; Robert Collins, ISG Resources Inc.; and Michael MacKay, John Emery Geotechnical Engineering Ltd. The NCHRP program officer is Dr. Edward Harrigan; his telephone number is (202) 334-3232.
Environmental Impact on Surface and Ground Waters (NCHRP 25-9)
The recently completed NCHRP study "Environmental Impact of [Highway] Construction and Repair Materials on Surface and Ground Waters" was directed to determine whether commonly used construction and repair materials might affect the quality of surface and ground waters. The objective of the study was to evaluate the persistence of any toxic leachates of the selected materials and to estimate the possible impacts of these materials on surface and ground waters. The materials were evaluated considering the removal, retardation, and remediation that occurs as leached substances migrate from the material used in pavements, sub-bases, or fills toward surface or ground water. The study did not include materials deposited on the pavements, pavement wear particles, and vehicle wear and exhaust particles.
Following evaluations based on product literature reviews, the materials tested were selected from the most common construction and repair materials considered likely to impact water quality. Due to the current interest in considering the use of waste and byproduct materials in highway construction, a number of widely used waste and byproduct materials were evaluated as part of this project, including municipal solid waste (MSW) ash, coal fly ash, foundry sand, phosphogypsum, blast furnace slag, steel slag, non-ferrous slags, roofing shingles, scrap tire rubber, reclaimed asphalt, mine tailings, and mining/smelter slags from various locations in the United States and Canada.
Materials in their pure form were mixed with deionized water to extract leachates. The leachates were then used in bioassay tests on algae and Daphnia (representing plant and animal life). Along with these tests, concurrent chemical analyses were made to determine the level of toxicity and, when it did occur, why.
Many of the materials were tested in the form in which they might be used in the highway environment — for example, as a component of pavement. Also, if materials were found to be toxic from the initial bioassay tests, the leachates were mixed with common soils, and the resultant supernatant liquids were given the same bioassay and chemistry tests. These tests gave an indication of the inherent remediation and removal processes that would be at work when the materials are used under field conditions. The fate of leachates in the surface and subsurface soil environments was determined by testing the persistence of contaminant concentrations after the removal processes of volatilization, photolysis, biodegradation, and sorption had occurred. Other observed changes due to dilution, ionic changes, and so forth were also considered.
While leachates by distilled water from the "pure" materials were, in many cases, found to be toxic to the fresh water alga Selanastrum capricornutum and/or to the macro-invertebrate Daphnia magna (water flea), the toxic impacts were reduced when these materials were incorporated into pavement materials. In almost all cases, the toxicity was removed by mixing the leachates with soils. Only a few waste or byproduct materials produced a toxic impact on nearby surface or ground water when they were placed into a pavement mixture, perhaps as part of aggregate. Also, leachates were generally non-toxic when passed through soils next to their placement in the highway materials, provided there was at least a nominal thickness (several centimeters) of a typical soil. Three Oregon soils of low to relatively high organic and silt and clay content, representative of large areas of the United States, were used in the laboratory sorption tests.
Field observations and experiences from numerous states suggest that these laboratory results were correct. Under both laboratory and field conditions, the rates of leaching of toxic substances decrease with time as the ingredients are more tightly sorbed, modified, degraded, or removed.
A model that can be applied to each successive media the leachates may pass through has been developed to evaluate these overall processes. Leachates are generated from common highway configurations (pavement, pilings, aggregate, etc.) and transported in the near-highway surface and subsurface environment (a few meters). When the concentrations of toxic materials fall below toxic or detection limits or background concentrations, it is then determined that there are no toxic impacts from the highway construction and repair materials on nearby surface or ground water.
Bioassay and other procedures have been developed under this project to test waste (recycled) materials for environmental impacts on surface and ground waters. Although many example materials were tested and the results were archived, it is not possible to make industry-wide generalizations based on the limited number of samples tested and the variability in the properties of materials — such as foundry sands, coal ashes, or slags — depending on their origin. Hence, more tests will be required for specific candidate materials for use in highways.
However, it should be emphasized that for every material tested except a deck sealer (methacrylate), toxicity to the algae and daphnia was low when the material was used in an assemblage form (e.g., pavement or fill) and drastically reduced or totally eliminated following passage of the leachate through soils.
Even though the study included no field tests to confirm models of toxicity changes, numerous field tests provide guidance and conceptual qualitative confirmation. For example, laboratory results showed relatively rapid biodegradation of organic toxic leachate from rubber. Field tests from Virginia where scrap rubber was put in a fill section and nearby soils and ground water were observed showed no toxic impact, suggesting that toxicity of the leachates was quickly removed by biodegradation and sorption.
The principal investigators for NCHRP 25-9 are Dr. Neal Eldin and Dr. Wayne Huber from Oregon State University. The NCHRP program officer is Dr. Edward Harrigan; his telephone number is (202) 334-3232.
The projects described in this article illustrate a commitment at the national level to promote the appropriate uses of recycled materials in the highway environment. The goal is to conserve resources while maintaining a safe, efficient, high-quality highway system. The results of these projects are helping states and municipal governments effectively use recycled materials in road construction.
Marcia J. Simon is a research materials engineer for the Federal Highway Administration's Office of Infrastructure Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. For the past nine years, she has been involved in research and problem-solving in the area of portland cement concrete (PCC) materials, as well as waste and byproduct materials and their uses in highways. As a member of the PCC Team, she oversees staff research on various aspects of concrete materials, such as mixture optimization and freeze-thaw durability. In the recycling area, she served as technical manager for the FHWA research study that produced the User Guidelines for Waste and Byproduct Materials in Pavement Construction. Simon is currently involved with two secondary materials projects at the University of New Hampshire; the projects deal with development of a consensus framework for evaluation of recycled materials in highways and with the prediction of long-term performance using accelerated aging for concrete pavements containing fly ash. She received her bachelor's degree in civil engineering from the Massachusetts Institute of Technology in 1984 and a master's degree in civil engineering from Cornell University in 1989.
Warren H. Chesner is the president of Chesner Engineering P.C., an enginering firm in Commack, N.Y. He has more than 25 years of experience in stabilizing waste materials and using byproduct materials in construction applications. Dr. Chesner is the primary author of the Federal Highway Administration's User Guidelines on Waste and Byproduct Materials in Highway Construction Applications, the Transportation Research Board's computerized database on waste and recycled materials, and co-author of the University of New Hampshire's framework document. He is currently involved in the development of AASHTO specifications for recycled material use in highway applications.
Taylor Eighmy 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).
Howard Jongedyk is a research engineer in FHWA's Office of Infrastructure Research and Development. Since 1969, he has been managing and conducting research related to highway and traffic operations effects on the environment and has participated in numerous Transportation Research Board activities and projects. He has worked on materials problems, hydraulic engineering problems, and soils problems for private industry and for other government agencies before coming to FHWA. He has a master's degree in soil science from the University of Illinois and a master's degree and doctorate from the University of Minnesota.
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