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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction

EVALUATION GUIDANCE Environmental Issues

INTRODUCTION

The Resource Conservation and Recovery Act of 1976 (RCRA) and its subsequent amendments and regulations provide an environmental regulatory framework for the testing, reporting, storage, treatment, and disposal of waste materials in the United States. There is, however, no analogous regulatory framework for selecting, characterizing, recovering, and recycling of waste and by-product materials.

The absence of such a regulatory framework is, in most cases, an obstacle to recycling, since the prospective recycler is uncertain what target environmental criteria must be achieved in terms of material or product quality. At the same time, the absence of such formal regulatory testing, reporting, and management safeguards can result in the use of waste and by-product materials in applications that may be environmentally unsuitable.

Given the lack of formal procedures and criteria for recovery and use of waste or by-product materials, the following general guidance should be followed by all prospective recyclers of waste and by-product materials in pavement construction applications:

All wastes or by-product materials that are being considered for use, regardless of whether or not the material is exempt from regulation under RCRA, should be evaluated prior to use to fully assess the inherent hazard potential of the material, if used in the proposed application. Simply because a waste legally may not be subject to hazardous waste regulation is not necessarily an indicator that it is not potentially chemically hazardous or contains constituents that could pose threats to human health or the environment, nor is it necessarily an indicator of whether it may still be subject to other state or federal environmental laws. For example, RCRA allows for enforcement actions in cases where a solid (nonhazardous) waste may present an imminent or substantial endangerment to health or the environment. Under the Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA or "Superfund"), the release of hazardous substances from a RCRA-exempt or nonhazardous waste in a manner that poses a threat to human health or the environment can also lead to enforcement actions.

The remainder of this chapter is intended to provide the prospective user with general guidance pertaining to federal and state laws, regulations, and regulatory and evaluation requirements that he or she may face when proposing to use waste and by-product materials in pavement construction applications. It includes a description of:

  • Previous Federal legislative and regulatory activities and their influence on current waste and by-product material use regulations that are being adopted by the states.
  • Current regulatory practices, including both the general requirements that an applicant may face when proposing the use of a waste or by-product material, and specific procedures for obtaining approval to use a waste or by-product material reported in a survey of state environmental regulatory officials.
  • General information needed to perform an assessment of the environmental acceptability of a waste or by-product material in pavement applications.
  • Specific environmental issues associated with the materials and operations that are unique to pavement construction applications.

 

PREVIOUS REGULATORY AND LEGISLATIVE ACTIVITY

Historical Perspective

Prior to 1970, there was little if any environmental regulatory oversight regarding the use of waste and by-product materials in pavement construction applications. In general, those materials that exhibited acceptable engineering properties and were both cost-effective and not considered to be "harmful" to workers or the environment were often used. During that period, however, there were no specific procedures or criteria available to quantify potential environmental concerns or "harmful" impacts.

Three pieces of Federal legislation that were passed by Congress in a span of approximately 11 years from 1969 to 1980, and the implementing rules and regulations that followed, initiated a series of fundamental changes in the management of waste and by-product materials in this country. They presently affect the way in which regulatory agencies address waste and by-product material use.

These acts included:

  • The National Environmental Policy Act of 1969 (NEPA).
  • The Resource Conservation and Recovery Act of 1976 (RCRA).
  • The Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA or "Superfund").

NEPA introduced the requirement that environmental impact statements be prepared on all Federal actions significantly affecting the quality of the environment. This requirement was subsequently adopted by states and local governments to provide regulatory oversight over a broad range of environmentally related activities and also to provide mechanisms for public interface and review of these activities as part of a formal environmental permitting process.

RCRA introduced the concept of the separate management of hazardous and nonhazardous wastes, and defined procedures to identify whether a waste is hazardous or nonhazardous. Two types of hazardous wastes were identified. They are referred to as listed wastes or characteristic wastes. A listed waste is a waste that is classified as hazardous due to its source and the way it is produced. These types of wastes are "listed" by EPA in its regulations. A characteristic waste is a waste that must be tested to determine if it exhibits one of four properties: (1) ignitability, (2) corrosivity, (3) reactivity, or (4) toxicity. A waste exhibits the characteristic of toxicity (sometimes called the toxic characteristic or TC rule) if the concentration of any of 39 selected analytes in the Toxicity Characteristic Leaching Procedure (TCLP) extract (formerly the Extraction Procedure Toxicity Test (EP rule), which required the analysis of 14 analytes) exceed regulatory action levels. A hazardous or nonhazardous characterization of waste and by-product materials used in pavement-related applications will almost always be dependent on the results of the TC rule.

CERCLA was designed to address the release or imminent release of hazardous substances into the environment and established the mechanisms for responding to those releases and assessing liability. Regulations and procedures that evolved from CERCLA introduced the concept of human health risk assessments. The development of human exposure reference levels was accelerated as part of these assessment procedures. These reference levels represent the human intake dose below which adverse health effects are not expected to occur. CERCLA also provided the legal framework for assigning liability and assessing monetary damages for environmental impairment.

Although none of the three laws or their implementing regulations directly addressed the use of waste and by-product materials in pavement-related construction applications, they provided the framework that was and is presently used by state environmental regulatory agencies as part of their evolving regulatory strategies.

Evolving Regulatory Strategies

In general, from the mid-1970s (the post-RCRA era) and on into the mid-1980s, RCRA provided the primary means for judging the acceptability of using or not using a waste or by-product material. If a material was found to be nonhazardous (almost exclusively by the EP or TC rule), then the material was considered suitable for use.

Since a hazardous waste designation under RCRA requires special manifest, storage and disposal procedures to ensure public health and safety, it is reasonable to expect that only nonhazardous materials or materials that are excluded from RCRA would be candidate materials for use in pavement construction applications. It is also reasonable to expect, however, that since the RCRA framework is based on the assumption that the waste or by-product material, if it is nonhazardous, will be managed by landfilling or incineration, introducing a waste or by-product material into the environment in recycling applications would require a revised (and perhaps more stringent) regulatory or evaluation framework by which to judge the suitability of such a strategy.

As a result, commencing in the mid-1980s, regulatory agencies began to incorporate additional environmental requirements, including preparation of environmental assessments (based on NEPA) and in some cases, human health risk assessments (based on CERCLA) to evaluate potential impacts associated with waste and by-product use in construction applications. These evaluations have been used as the basis for determining whether permits should be issued for material use.

The adoption of these more rigorous and comprehensive evaluation procedures was also catalyzed by CERCLA-related liability concerns and public involvement in the assessment and permitting process.

Recent Assessment Activities

Most of the initial environmental assessments, which were prepared during the late 1980s, were sponsored by the Electric Power Research Institute (EPRI) and local power companies and were designed to obtain field data and/or assess the suitability of using coal combustion residues (i.e., coal fly ash, bottom ash, boiler slag, and flue gas desulfurization sludges) in highway construction applications. (See references 1, 2, 3, 4, 5, 6, and 7.)

More recently, applications for use of municipal waste combustor ash have resulted in the preparation of a number of environmental and human health risk assessments. (See references 8, 9, 10, and 11.)

In November 1994, the EPA issued a risk and environmental assessment() that was used as the basis for supporting EPA’s rulemaking activity covering the use of high temperature metal recovery (HTMR) slags in pavement applications (i.e., as an antiskid or deicing material, as an aggregate substitute in asphalt pavements, as a road base/subbase material and as an additive in the manufacture of cement). HTMR slags are residues produced from high temperature metal recovery treatment of electric arc furnace dust.

 

CURRENT REGULATORY PRACTICES

General Requirements

Due primarily to the increased pressure to recover and use waste and by-product materials, in recent years, most state environmental regulatory agencies (especially those in industrial areas) have begun the process of formalizing their regulatory procedure for approving the use of waste and by-product materials. In many states this is referred to as a Beneficial Use Permit Process or Beneficial Use Determination (BUD). At the present time, however, there are no universally accepted environmental approval and permitting procedures.

Regulatory requirements in general can take one or more of the following forms:

  1. No approval is required — material is considered acceptable due to previous history of use (grandfather clause).
  2. RCRA approval is required — material must not exhibit the characteristics of a hazardous waste.
  3. Environmental or risk assessment is required — a field and/or desk-top evaluation must be provided to demonstrate that the material will have no adverse impact on human health and the environment.

Although the first two requirements are rather straightforward, the latter requirement can necessitate a series of evaluations that could include the preparation of an environmental assessment, a human health risk assessment, or an ecosystem risk assessment.

There are notable distinctions between the more traditional environmental assessment and human health risk assessments or ecosystem risk assessments; however, beneficial use regulations in general have not clearly delineated these differences or indicated when one or more of these assessments may be required as part of a permitting process.

Environmental assessments generally require a quantification of emissions or discharges from a proposed activity (e.g., construction of a pavement using a waste or by-product material) and a projection of the impact of this emission or discharge on the ambient environment. The magnitude of the impact is usually assessed by comparing the source discharge or the projected ambient impact to some source discharge standard (e.g., groundwater or surface water discharge limits) or some ambient air, water or soil quality standard (e.g., ambient air or water quality criteria). Projections of impacts to the ambient environment are normally estimated using environmental models (e.g., air and water quality models).

Human health assessments, which are an outgrowth of CERCLA and were originally intended to provide a framework for developing the risk information necessary to assist in decision making for remediation at hazardous waste disposal sites, provide for a linking of discharges and emissions from specific sources to vulnerable human receptors in an attempt to quantify risks (using reference doses for carcinogenic and noncarcinogenic effects) associated with a specific activity. They attempt to account for all potential contaminants and exposure routes (e.g., ingestion, inhalation, and dermal absorption) that might affect the identified receptor.

Ecosystem risk assessments are evaluations that focus on potential impacts to flora and fauna, usually in the immediate environment of the action. Like human health risk assessments, they tend to focus on specific transfer routes to identifiable flora and fauna and the impact on these organisms. They sometimes address long-term cumulative impacts that may result from the proposed action, such as bioaccumulation and potential food chain effects.

State Regulatory Approaches

In January 1996, a survey was conducted by the New York State Department of Environmental Conservation on behalf of the Association of State and Territorial Solid Waste Management Officials (ASTSWMO), located in Washington, D.C. A questionnaire was sent to all 50 states to obtain information regarding the environmental regulatory approaches presently used to manage waste and by-product material recycling applications in the United States.

The survey form consisted of questions requesting information pertaining to the existence and nature of any regulatory review or permit process facing a prospective waste or by-product material user or applicant prior to proceeding with the use of the material in a recycling application. It also included a request for a description of the general provisions of the process.(13)

A total of 39 states and the District of Columbia provided sufficiently informative responses to allow characterization of their regulatory processes. Of the 40 respondents, 17, or 42 percent, indicated that they required that the applicant receive a formal permit prior to using such materials. Nineteen, or 48 percent, indicated that they had an informal review process with no permit required. Four states, or approximately 10 percent, indicated that they had no process.

Table 22-1 presents a summary of the information received from each of the responding state environmental regulatory agencies. Included in Table 22-1 is a listing of the type of procedures required and specific comments regarding details associated with the review or permit process. A key to Table 22-1 provides clarification to the comments presented in the table.

Although most states report the availability of either a review or permit process, specific evaluation procedures that would be expected of an applicant were unclear in almost all cases. Many states reported that evaluations were done on a case-by-case (CbC) basis or that an environmental assessment (EA) was required. No specific assessment approach or evaluation criteria were available. Some states required that a market be available, while others simply required that the material be nonhazardous (NH). One state reported that periodic monitoring of any new application would be required.

The results of the survey suggest that, until such time as more definitive formal criteria are established, in most cases applicants will be required to define the procedures with the appropriate regulatory agency on a case-by-case basis.

Table 22-1. Summary of state regulatory procedures.

table1.gif - 11.41 K

 

GENERAL ASSESSMENT REQUIREMENTS

Although there are few formal procedures or criteria for establishing the environmental suitability of using waste and by-product materials in pavement construction applications, there are common elements to all environmental assessments that form the basis for determining the potential impacts associated with a proposed application. These common elements include the following:

  • Identification of potential hazards posed by the use of the material.
  • Identification of persons or media (e.g., air, water, soils) likely to be impacted by the identified hazard.
  • Identification of the magnitude of the potential impact.

Identification of Potential Hazards

Some waste and by-product materials may contain concentrations of trace metals or trace organics that are higher in concentration and/or more environmentally mobile than those found in conventional materials. Others may contain highly alkaline materials (e.g., free lime), high concentrations of soluble salts, very fine particles that may be susceptible to dusting and may also be respirable. Still others may contain volatile organic or inorganic material that could be released in high-temperature environments. In general, the identification and magnitude of these properties can be assessed by examining the parameters listed in Table 22-2. Table 22-2 also outlines potential concerns associated with each of the listed properties.

Impacted Persons or Media

The pavement construction process comprises numerous operations including material storage, handling, production, placement, demolition, excavation, and disposal or recycling operations. These operations are all part of the pavement production, construction, service life, and postservice life activities. Potential dust or volatile emissions or liquid discharges from these operations could have an impact on ambient air, surface or groundwaters, soils, or the worker environment.

The identification of each of these operations is important when identifying impacted persons or media.

Table 22-3 presents a listing of common operations, environmental release mechanisms, and impacted media associated with most pavement-related applications where waste and by-product materials may be used. A more detailed examination of these operations, release mechanisms, and transport media is presented in the next section entitled, "Environmental Issues — Pavement Applications."

Table 22-2. Chemical and physical properties of environmental concern.
Parameters Potential Hazardous Property
Leachable (or soluble) trace metals Presence of extractable and mobile metals such as As, Cd, Cu, Cr, Hg, Pb, Zn, etc., that could impact groundwater and surface water quality.
Leachable (or soluble) trace organics Presence of extractable trace organic compounds such as benzenes, phenols, vinyl chloride, etc., that could impact groundwater and surface water quality. Leachable corrosivity (highly acidic or alkaline materials) Presence of extractable and mobile alkalinity or acidity that could impact the pH of groundwater or surface water.
Soluble solids Presence of soluble and mobile salts that could impact groundwater quality and sensitive freshwater environments.
Total and respirable dust Presence of fine particulate matter that is respirable or is susceptible to airborne migration.
Trace metals present in total and respirable dust Presence of trace metals in fine mobile particulate that could be inhaled or deposited at secondary locations.
Trace organics present in total and respirable dust Presence of trace organics in fine mobile particulates that could be inhaled or deposited at secondary locations.
Volatile metals Volatile metals such as As, Hg, Cd, Pb, and Zn, which could be released at high temperature (mostly a worker health issue).
Volatile organics Volatile organics such as chlorinated hydrocarbons which could be released at high temperatures (mostly a worker health issue).
Table 22-3. Pavement application exposure pathways.
Source Operations Release Mechanisms(a) Impacted Media(b)
Stockpiles
Screening
Crushing
Blending
Conveying
Transport
Drying
Placement
Demolition
Recycling
Primary
Dispersion of:
Fugitive Dust, Particulate Abrasion, and Point Source Particulate Emissions
Dispersion Into the Worker Air Environment and into the Ambient Air Environment
Secondary
Deposition of Air Emissions
Other Media (Land, Water)
Drying Primary
Dispersion of Volatile Emissions
The Worker Air Environment and Ambient Air Environment
Secondary
Condensation and Deposition
Other Media (Land, Water)
Stockpiles
Service Life
Disposal
Primary
Discharges of Surface Runoff Containing Soluble Components or Particulates
Surface Waters and Groundwater
Secondary
Deposition and Absorption
Other Media (Soils, Sediments)
Stockpiles
Service Life
Disposal
Primary
Leaching of Soluble Components
Groundwater and Surface Waters
Secondary
Deposition and Absorption
Other Media (Soils)
(a) Primary mechanisms refer to those transport processes that result in "direct" transport from the source operation to the impact media. Secondary mechanisms refer to additional processes, after the primary process, that result in transport to a second or third media.
(b) The Worker Air Environment represents the airspace of the worker and is subject to OSHA regulations; the Ambient Air Environment is the greater airspace that would be regulated by ambient air quality regulations.

Magnitude of Impact

Techniques for determining the magnitude of the impact will depend in great part on the type of evaluation that is required (i.e., traditional environmental assessment, human health risk assessment, or ecosystem risk assessment). In all cases the use of source emission, ambient air, surface water, and groundwater models will probably be required. Guidance on the selection of these models and methods for determining the magnitude of potential impacts can be obtained from several sources. The first source to check is the agency requiring the assessment, which may have specific requirements for models and criteria to use in determining the extent of estimated impacts. A second source is previously completed assessments, such as references 1-11, which may contain guidance on previously used and accepted models. Finally, the EPA and EPRI have published several guidance documents that may be useful for determining potential impacts. (See references 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27.)

The suggested sources of guidance presented in this section are not intended to be exhaustive, but are intended as guidance and a possible starting point in the impact assessment process. In preparing an assessment it may be determined that adequate information is not available to satisfactorily perform a comprehensive evaluation. In such cases field demonstration programs may be required as part of the assessment process to assist in the development of source emission data.

 

ENVIRONMENTAL ISSUES — PAVEMENT APPLICATIONS

Material Characteristics

During the preparation of these guidelines, insufficient data were found to be available to adequately define whether most of the waste or by-product materials included in the guidelines pose significant environmental risk if used in pavement applications. Inasmuch as previous practices did not require testing of such materials for their environmental properties, future efforts will be required to obtain this type of data.

Despite the lack of material-specific environmental data, it is possible, based on an examination of pavement materials and their applications, to provide generic insight into potential environmental issues associated with waste and by-product materials that may be used in pavement applications. This can be accomplished by considering the characteristics of the material (e.g., is it a dust susceptible to airborne emission?), how much of the material is actually introduced into the pavement (e.g., relative quantity), and the activities and operations to which the material and pavement will be subjected during its production and processing, construction, service life, and postservice life.

Pavement Applications

Pavement applications can be subdivided into five major categories. They include: (1) asphalt paving applications, (2) Portland cement concrete applications, (3) flowable fill applications, (4) stabilized base applications, and (5) unbound aggregate and fill applications.

Waste and by-products that are used in these pavement applications are generally used as a replacement material for one or more of the following:

  • Mineral filler
  • Aggregates in concrete (asphalt and Portland cement) applications
  • Asphalt cement modifiers
  • Portland cement concrete mineral admixtures.
  • Flowable fill aggregates
  • Flowable fill pozzolans or initiators
  • Granular aggregates
  • Embankment and fill materials

From a relative environmental perspective, it is reasonable to expect that the higher the percentage of recycled material that is incorporated into a pavement structure, the greater the potential concern that might arise regarding the use of the material. The magnitude of this concern will ultimately depend on the nature and level of contamination associated with the subject material.

A list of these material categories, including the general pavement application, physical properties, purpose and approximate percentage of material used in each of the respective applications, are presented in Table 22-4.

The information presented in Table 22-4 suggests that the major uses of recovered materials in paving can be expected to occur in the aggregate and fill replacement applications. Asphalt modifiers, Portland cement concrete admixtures and flowable fill pozzolans or initiators make up relatively small percentages of the pavement product (e.g., almost always less than 5percent). There would only be a small likelihood that these materials could result in significant environmental impacts when used in these relatively low percentages. In addition, while aggregates and fill materials are generally stored, processed, transported, blended, and placed in the open environment, asphalt modifiers, Portland cement concrete admixtures, and flowable fill pozzolans and initiators are generally stored and handled in closed containers until they are applied to the product.

Finally, incorporating waste and by-product materials into asphalt or Portland cement concrete pavements, flowable fill, or stabilized bases, or placing these materials in bases or subbases below asphalt or Portland cement concrete pavements, affords varying degrees of protection from exposure of the material(s) to the elements. The e into the adjacent environment.

Table 22-4. Waste and by-product pavement materials, applications, properties, and uses.
General Application Material Substitutes Physical Properties Purpose Amount Used1
Asphalt Pavements Mineral Filler Silt-size, less than 75 micron material Fill voids in pavement structure Constitutes 5 percent or less by weight when used in pavements
Asphalt Aggregates Gravel-sand size material, generally less than 19 mm (3/4 in) in size Provide structural and bearing capacity for pavement Generally constitutes 90 to 95 percent of the asphalt concrete structure by weight
Asphalt Cement Modifiers Silt-size or liquefied material Introduced into asphalt cement to modify cement characteristics Generally less than 25 percent of the asphalt cement, which constitutes about 1 percent of the asphalt concrete structure by weight
Portland Cement Concrete Portland Cement Concrete Admixtures Silt-size, less than 75 micron material Introduced as additive or to replace portion of Portland cement; acts as pozzolan or cement Generally replaces 15 to 50 percent by weight of cement, which constitutes about 1.5 to 5 percent of the Portland cement concrete structure by weight
Portland Cement Concrete Aggregates Gravel-sand size material, generally less than 19 mm (3/4 in) in size Provide structural and bearing capacity for pavement Generally constitutes 80 to 90 percent of the Portland cement concrete structure by weight
Flowable Fill Flowable Fill Aggregates Fine-grained sand-silt material, generally less than 4.4 mm (1/4 in) in size Provide structural and bearing capacity for fill Generally constitutes 90 to 95 percent by weight of fill
Flowable Fill Pozzolans and Initiators Silt-sized, less than 75 micron material Introduced as additive or replacement for Portland cement; acts as pozzolan or cement Generally replaces 15 to 50 percent by weight of cement which constitutes about 1.5 to 5 percent of the fill structure
Stabilized Base Stabilized Base Pozzolan or Initiator Silt-sized, less than 75 micron material Introduced as additive or replacement for Portland cement; acts as pozzolan or cement Generally replaces 15 to 50 percent by weight of cement, which constitutes about 1.5 to 5 percent of the fill structure
Stabilized Base Aggregate Gravel-sand sized material, generally less than 19 mm (3/4 in) in size Provide structural or bearing capacity for overlying burden Generally comprises 80 to 90 percent of the base by weight
Unbound Aggregate and Fill Granular Aggregates Gravel-sand sized material, generally less than 19 mm (3/4 in) in size Provide structural or bearing capacity for overlying burden Can constitute up to 100 percent by weight of the granular base, subbase, or fill structure
Embankment and Fill Materials Soil-like, sandy-silty material, generally less than 4.4 mm (1/4 in) in size Provide structural or bearing capacity for overlying burden Can constitute up to 100 percent by weight of structure of the embankment or fill structure
1. Note: The amounts presented represent the quantity of natural materials used in the specific application. The fraction of waste and by-product material use will be dependent on the mix design.

Aggregate and Fill Applications and Operations

The pavement applications addressed by these guidelines, including the use of waste and by-product materials as aggregates or fill material, can be divided into five major phases: (1) aggregate storage and processing, (2) material production (e.g., asphalt concrete), (3) construction, (4) service life, and (5) postservice life

Depending on the specific application, each of these phases involves one or more operations. When waste materials are used in an application, these operations could initiate the transfer of soluble metals, soluble organics, soluble solids, dust, or volatile constituents into the environment.

The remainder of this section provides illustrated summaries of the source operations, release mechanisms, and transfer media associated with the general pavement applications listed in Table 22-4, when a waste or by-product material is used as an aggregate substitute in that application. A list of measures that could be used to mitigate many of the release mechanisms is also included.

Aggregate Storage and Processing

Aggregate storage and processing operations are common elements to almost all pavement applications considered herein. Figure 22-1 presents an illustration of potential sources and transfer mechanisms that can be expected to result from operations associated with aggregate storage and processing activities.

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Figure 22-1. Storage and processing facility: operations, sources, transfer mechanisms, receptors, and mitigating measures.

Operations and sources of potential concern include unloading, loading, on-site transport, temporary storage, screening, and conveying. Fugitive dust emissions could be generated in all of these operations, but can be mitigated to a great extent by the use of dust suppressants (e.g., moisture control) or by enclosing the operations.

Leachate and surface runoff discharges can be expected from outdoor stockpile operations. These discharges can be mitigated by covering the stockpiles and/or enclosing the operations.

Material Production, Construction, Service Life, and Postservice Life Operations

Hot Mix Asphalt Pavement

There are two types of hot mix asphalt production facilities that are commonly used to produce hot mix asphalt concrete: (1) batch plants and (2) drum-mix plants. Figure 22-2 depicts potential sources and transfer mechanisms that may result from operations at an asphalt production facility (batch plant). A drum-mix plant will have similar pathways.

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Figure 22-2. Hot mix asphalt: production operations, sources, transfer mechanisms, receptors, and mitigating measures.

In batch plants, fine and coarse aggregate are first dried and then screened. The screened aggregates are deposited into storage or hot bins. Hot bins are used to temporarily store the heated and screened aggregates, which are subsequently withdrawn, in predetermined proportions, to a mixer where the aggregates are combined and mixed with the asphalt cement. Hot screening and hot aggregate storage is provided in an enclosed structure. Emissions from the aggregate dryer are routed to an air pollution control system (typically a baghouse).

In drum-mix plants, cold aggregates of predetermined size are metered on a conveyor directly into a drum dryer where the aggregates are heated and mixed with the asphalt cement. There are no hot screens or hot bins in a drum-mix plant. Emissions from the drum dryer are routed to an air pollution control system, as in a batch plant.

Operations and sources of potential concern include stockpile storage, on-site truck and front-end loader traffic, loading, unloading, and conveying. The remaining operations (e.g., drying, screening, storage, mixing) occur in enclosed structures. Emissions from these latter operations (particularly drying) are typically routed to the air pollution control system. Air pollution control requirements (e.g., particulate, hydrocarbon, and carbon monoxide emissions) are defined by state air pollution control regulations.

Most of the release mechanisms at an asphalt production facility are dust related, resulting in air emissions that could affect local air quality or result in particulate deposition onto adjacent soils. Fugitive dust emissions will in most cases be a local worker environment issue, and can be mitigated using standard dust suppressant measures (e.g., moisture control).

Leachate or runoff discharges can be expected from outdoor stockpile operations, but can be mitigated by covering material stockpiles.

At a hot mix plant, there are no direct remedial methods for mitigating the release of highly volatile emissions without incorporating additional air pollution control equipment such as wet scrubbers. In most cases this may not be a practical solution. An alternative approach includes introducing materials into low-temperature zones within the facility to control the temperature of the material. In any event, the introduction of untried wastes or by-product materials that contain volatile constituents into an asphalt production facility will in most cases require testing to ensure that asphalt plant emissions do not exceed local air pollution control criteria.

Figure 22-3 depicts construction, service life, and postservice life sources and transfer mechanisms for hot mix asphalt pavement. Particle abrasion, runoff, and leaching are the primary release mechanisms that can be expected to occur during the pavement service life. Fugitive dust releases can be expected to occur during demolition, excavation, and pavement recycling operations; however, these activities are very short-lived and in most cases will have minimal impact.

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Figure 22-3. Hot mix asphalt: construction, service life, and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

During the service life of the pavement, the magnitude of any releases will depend almost exclusively on the mobility factors (e.g., solubility and transfer mechanisms) associated with pavement weathering and service wear. These factors will be much more prominent in a wearing surface, which is in direct contact with the wheel load and the elements, than a base course pavement.

Cold Mix Asphalt Pavement

Cold mix production involves the storage, metering, conveying, and mixing of aggregates with a cold asphalt emulsion. Figure 22-4 depicts the potential sources and transfer mechanisms that could be expected to result from cold mix production operations.

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Figure 22-4. Cold mix asphalt: production operations, sources, transfer mechanisms, receptors, and mitigating measures.

Operations and sources of potential concern include on-site transport, loading, unloading, and blending. These operations could result in fugitive dust emissions. Outdoor stockpiles could generate leachate, runoff discharges, and fugitive dust emissions. Potential impacts and mitigating measures are similar to those for hot mix asphalt production operations. In cold mix operations, where aggregate drying is not included in the production process, there is reduced concern over volatile emissions.

Construction, service life, and postservice life operations and exposure pathways, and specific mobility issues, are similar to the pathways illustrated in Figure 22-3 for the hot mix pavement application.

Portland Cement Concrete Pavement

The production of Portland cement (ready mix) involves operations that include temporary storage, aggregate blending, conveying, mixing, and product loading. Figure 22-5 depicts the potential sources and transfer mechanisms that could be expected to result from operations associated with a Portland cement concrete production facility.

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Figure 22-5. Portland cement concrete pavement: production facility operations, sources, transfer mechanisms, receptors, and mitigating measures.

Most of the operations at the Portland cement production facilities and their release mechanisms and mitigating measures are similar to those at asphalt production and the aggregate storage and processing facilities. Local occupational dust exposure represents the primary potential environmental concern.

Figure 22-6 depicts construction, service life, and postservice life exposure pathways. Particle abrasion, runoff, and leaching are the primary release mechanisms that could be expected to occur during the pavement service life. Construction, service life, and postservice life mobility issues are similar to those issues addressed under the hotmix pavement review.

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Figure 22-6. Portland cement concrete pavement: construction, service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

Fugitive dust releases can be expected during demolition, excavation, and pavement recycling operations, but once again, these are short-term activities from which minimal impact is expected.

Flowable Fill

Operations associated with the production of a flowable fill product and potential exposure pathways are very similar to those operations identified in Figure 22-5 for the production of a ready mix concrete. Figure 22-7 depicts construction, service life, and postservice life operations and potential sources and release mechanisms associated with these operations.

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Figure 22-7. Flowable fill: construction, service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

Since flowable fill is normally used as a backfill material, leaching would be the primary release mechanism during the product’s service life. Mobility issues will primarily depend on solubility and transport mechanisms associated with the flowable fill matrix, which should exhibit relatively low permeabilities.

Fugitive dust and leachate and runoff releases could be expected to occur during postservice life removal operations; however, these operations will be short-lived and minimal impact is expected from such operations.

Stabilized Base or Subbase

Production of a stabilized base material could occur at a central mixing facility, which would have operations similar to those outlined in Figure 22-4, for the cold mix production operation. Stabilized base production could also occur in the field where the aggregate or soil and stabilizing agent would be mixed, spread, and compacted. Figure 22-8 depicts the potential sources and transfer mechanisms that could be expected to result from field operations associated with stabilized base or subbase construction, service life, and postservice life operations.

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Figure 22-8. Stabilized base: field construction service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

Operations and sources of interest include field transport, loading, unloading, spreading, and compacting operations, all of which could generate fugitive dust emissions. Most of these fugitive dust emissions would be expected to result in localized impacts, primarily to the worker environment. Both construction and postservice life excavation operations are short-lived events and would not be expected to produce significant impacts.

Leachate discharges could occur during the service life of the material. Due to the presence of the overlying pavement, the mobility of the material or potential soluble components will be greatly reduced during the service life of the structure.

Stabilized Surface

Surface treatments involve the use of aggregates and a binder (usually asphalt cement) to assist in providing an improved pavement surface. Operations involved in the production of a surface treatment are similar to paving operations, except the aggregate would be directly exposed to the surface during the service life of the pavement structure. As a result mobility considerations take on greater importance since the product will be directly exposed to the elements and traffic.

Figure 22-9 depicts the sources and potential release mechanisms that could be expected to result from construction, service life, and postservice life operation, respectively.

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Figure 22-9. Stabilized surface: field construction, service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

Operations during the pavement service life could be expected to be a source of particle abrasion and fugitive dust, leachate, and runoff discharges.

Granular Base

Granular base construction includes aggregate unloading, placement or spreading, and compacting operations. These operations are similar to those associated with a stabilized base as shown in Figure 22-8. In the construction of a granular base, however, no stabilizing agent is added to the aggregate or aggregate blend.

Figure 22-10 depicts the sources and potential release mechanisms that could be expected to result from operations associated with granular base construction, service life, and postservice life operations, respectively.

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Figure 22-10. Granular base: construction, service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

Operations during the short-lived construction period could generate fugitive dust emissions. During the service life of the base, leachate discharges could potentially occur, but in most cases will be mitigated by the overlying pavement. A granular base could be more readily exposed in high groundwater locations. Postservice life excavation operations of interest include excavation, temporary storage, and transport activities where fugitive dust releases and runoff and leachate discharges could occur; however, once again these short-lived operations would not be expected to produce significant impacts.

Embankments and Fills

Embankment or fill construction, service life and postservice life operations, and sources and release mechanisms that could be expected to occur from these operations, are presented in Figure 22-11.

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Figure 22-11. Embankment or fill: construction, service life and postservice life operations, sources, transfer mechanisms, receptors, and mitigating measures.

During short-lived construction operations, sources of interest include transport, unloading, loading, temporary storage, spreading, and fill compaction. Fugitive dust releases could be expected in almost all of these operations. Once again the primary concern in almost all cases would be the local worker environment. Leachate and surface runoff discharges could be expected from the temporary storage operations.

During the service life of the embankment or fill, runoff, leaching, and particle abrasion could be expected. Short-lived excavation, storage, transport, and loading operations represent sources of dust, runoff, and leachate discharges. Postservice Life Recyclability Issues

Assessments of the environmental suitability of using waste and by-product material in pavement construction applications are complicated by the fact that most pavement products become new waste materials that must be disposed of or recycled after their initial service life, which generally lasts from approximately 5 to 20 years.

As a result, to examine the life cycle of a waste and by-product material in the environment, an examination must be undertaken of the potential disposition of the material after its initial service life. The present materials management strategy in most jurisdictions is to recycle as much of the excess pavement material as is economically practical. From an economic perspective, the maximum economic benefit of pavement material reuse is achieved when the pavement material is recycled into a product that will take maximum advantage of its inherent economic value. For example, concrete and hot mix aggregates are more "valuable" than granular base aggregates, which are more "valuable" than subbase aggregates, etc. As an example, the most cost-effective use for reclaimed asphalt pavement (RAP) may be to utilize the pavement in the production of new asphalt pavement, where the old aggregates and asphalt cement can reduce the quantity of new aggregates and asphalt cement required. Alternative uses, such as granular base, may not take advantage of the old asphalt cement present in the RAP, and therefore, this option may not represent the most cost-effective use, despite still being an important beneficial option.

In addition to this hierarchy, technical requirements (e.g., material specifications) will route excess materials to certain applications. For example, due to the asphalt present in RAP, its use in Portland cement concrete would not be advisable, and the use of embankment or fill material as an aggregate material in hot mix asphalt would probably not meet the more stringent specifications for aggregate to be used in hot mix asphalt.

Table 22-5 contains a matrix that indicates potential applications of excess materials from a demolished pavement in subsequent pavement construction. This matrix can help identify future options for evaluation.

Table 22-5. Waste and By-Product Material Recycling/Matrix.
table5a.gif - 9.49 K
table5b.gif - 4.24 K

 

REFERENCES

  1. Use of Coal Ash in Highway Construction; Georgia Demonstration Project, GS-6175. EPRI Research Project 2422-4, Prepared by Southern Company Services, Georgia Power Company and Electric Power Research Institute, February, 1989.

  2. Ash Utilization in Highways, Pennsylvania Demonstration Project, GS-6431. EPRI Research Project 2422-1, Prepared by GAI Consultants for Duquesne Light Company, June, 1989.

  3. Ash Utilization in Highways: Delaware Demonstration Project, GS-6481. EPRI Research Project 2422-3, Prepared by Delaware Power and Light Company and VFL Technology Corporation, August, 1989.

  4. Use of Coal Ash in Highway Construction: Kansas Demonstration Project, GS-6460. EPRI Project 2422-15, Prepared by Kansas Electric Utilities Research Program and Terracon Consultants, September, 1989.

  5. Environmental Performance Assessment of Coal Ash Use Sites: Little Canada Structural Ash Fill, EN-6532. EPRI Research Project 2796-1, Prepared by Radian Corporation, May, 1990.

  6. Environmental Performance Assessment of Coal Ash Use Sites: Waukegan Ash Embankment, EN-6533. EPRI Research Project 2796-1, Prepared by Radian Corporation, December, 1990.

  7. Wastes from the Combustion of Coal by Electric Utility Power Plants. Report to Congress, Office of Solid Waste, U.S. Environmental Protection Agency, EPA/530-SW-88-002, February, 1988.

  8. Risk Assessment of SEMASS Boiler Aggregates. Prepared by ENSR Consulting and Engineering for Energy Answers Corporation, Albany, New York, March, 1993.

  9. The Laconia, New Hampshire Bottom Ash Paving Project. Prepared by Environmental Research Group, University of New Hampshire, National Renewable Energy Laboratory, U.S. Department of Energy, March, 1996.

  10. Municipal Waste Combustor Bottom Ash: Stockpile Runoff and Fugitive Dust Evaluation Report. Prepared by Long Island Regional Planning Board and Chesner Engineering, P.C., National Renewable Energy Laboratory, U.S. Department of Energy, September, 1996.

  11. Health Risk Assessment of Minnesota MSW - Ash Utilization Demonstration Project. Prepared by AWD Technologies, Inc. for USPCI Corporation, 1992.

  12. Assessment of Potential Risks to Human Health and the Environment from Management and Uses of HTMR Slags. U.S. Environmental Protection Agency, November, 1994.

  13. Compilation of Questionnaire Regarding States’ Use of Waste and By-Product Materials: Regulatory Requirements for Recycling. Solid Waste Subcommittee, Association of State Territorial Solid Waste Management Officials, Washington, DC, Prepared by the Division of Solid and Hazardous Materials, New York State Department of Environmental Conservation, Draft Report, March, 1996.

  14. Chemical, Physical and Biological Properties of Compounds Present at Hazardous Waste Sites: Final Report. EPA/530-SW-89-010, September, 1985.

  15. Chemical, Physical and Biological Properties of Compounds Present at Hazardous Waste Sites: Final Report. EPA/530-SW-89-010, September, 1985.

  16. "Compilation of Air Pollution Emission Factors," Stationary Point and Area Sources, Volume 1, Fourth Ed., EPA/PB86-124-906, September, 1985.

  17. "Compilation of Air Pollution Emission Factors." Vol. I, Stationary Point and Area Sources, Fourth Ed., EPA Supplement B, September, 1988.

  18. Exposure Factors Handbook. EPA/600/8-89-043, July, 1989.

  19. Integrated Risk Information System (IRIS), 1993.

  20. Methodology for Assessing Environmental Releases of an Exposure to Municipal Solid Waste Combustor Residuals. Office of Health and Environmental Assessment Exposure Group, EPA-68-02-4199, April, 1991.

  21. Quality Criteria for Water. 1986, EPA/440/5-86-001, May, 1986.

  22. Rapid Assessment of Exposure to Particulate Emissions from Surface Contamination Sites. Office of Health and Environmental Assessment, EPA-600-8-85-002, February, 1985.

  23. "Risk Assessment Guidance for Superfund." Volume I, Human Health Evaluation Manual (Part A), Interim Final, Office of Emergency and Remedial Response, EPA/540/1-89/002, December, 1989.

  24. Superfund Assessment Manual. Office of Remedial Response, EPA/540/1-88/001, April, 1988.

  25. Risk Assessment Guidance for Superfund. Volume I, Human Health Evaluation Manual Supplemental Guidance, Standard Default Exposure Factors. Interim Report, U.S.Environmental Protection Agency, OSWER Directive 9285-03, March, 1991.

  26. Water Quality Assessment: A Screening Procedure for Toxic and Conventional Pollutants in Surface and Ground Water, Part 2. EPA/600/6-85/002a, September 1985.

  27. Water Quality Assessment: A Screening Procedure for Toxic and Conventional Pollutants in Surface and Ground Water, Part 2. EPA/600/6-85/002b, September 1985.

 

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