U.S. Department of

Federal Highway
as specified in the
Intermodal Surface
Efficiency Act
of 1991
Section 1038(b)
    JUNE 1993

       The Intermodal Surface Transportation
       Efficiency Act of 1991 was enacted into law .
       on December 18. 1991. Section 1038(b).
STUDIES, requires the Department of Transportation
and the Environmental Protection Agency to perform
studies and rep'ort on the results of the studies to
Congress within  18 months after enactment.

The studies are to determine:      .               :

   • The threat to human health and the environment.
the ability to recycle, and the performance of asphalt
pavement containing recycled rubber.
   • The economic savings, technical performance.
and threats and benefits to human health and the envi-,
ron'ment of using recycled materials in highways.
   • The utilization and practices of all States relating
to the reuse and disposal of highway materials.

The Federal Highway Administration and the
Environmental Protection. Agency created a joint tech-
nical 1038(b) study coordination group to conduct-the
studyr. synthesize available information, and prepare
the report to Congress. A copy of the final research
study report, titled Engineering Aspects of Recycled
Materials for Highway Construction, is appended to
this report.

  This document is disseminated under the sponsorship of the Department of Transportation
  and the Environmental Protection Agency in the interest of information exchange. This
  report does not constitute a standard, specification, or regulation.  The U.S. Government
  does not endorse products or manufacturers.  Trade or manufacturers' names appear
  herein only because they are considered essential to  the object of this document.

                                                                        Technical Report Documentation Page
i 1. Report No.
j      FHWA-RD-93-147
I      EPA/600/R-93/095
 2. Government Accession No.
                                     { 3. Recipient's Catalog No.
• 4. Title and Subtitle
!                                         .                 •

|    A Study of the Use of Recycled Paving Material - Report to Congress
                                     i 5. Report Date
                                                  June 1993
                                     ] 6. Performing Organization Code
 7. Author(s)
                                      8. Performing Organization Report No.
I 9. Performing Organization Name and Address

 Federal Highway Administration, 400 Seventh Street,SW, Washington, DC
 Environmental Protection Agency, 401 M Street, SW, Washington, DC
 12. Sponsoring Agency Name and Address
                                      10. Work Unit No. (TRAIS)
                                      11 .Contract or Grant No.
                                      13. Type of Report and Period Covered

                                        :   .   .   Final .Report
                                                                           14. Sponsoring Agency Code
 1 5. Supplementary Notes
 16. Abstract                                                             "             ~~~	:	
 Section 1038(b) of the Intermodal Surface Transportation Efficiency Act of 1991  (Pub. L. 102-240) required the
 Department of Transportation and Environmental Protection Agency to conduct  a study of asphart pavements
 containing scrap tire rubber and synthesize the experience wi]th other recycled materials.

 Highway agencies have been evaluating crumb rubber modifier (CRM) technology applications at different levels of
 development since the 1970's. Ten CRM technologies were identified. The performance of asphalt pavements using
 CRM technology has been mixed. The amount of documented,research on recycling CRM paving materials is limited.
 An analysis, using the resulits of seven studies, was conducted to compare the relative threats/risks to human heath
 and the environment of conventional asphalt paving to CRM asphalt paving. The health/environmental comparison
 was influenced by numerous variables.  The data contained no obvious trends to indicate a significant increase  or
 decrease in emissions was attributed to the use of CRM.                                       •-..;'•

 The highway construction industry has a long history of using recycled products for highway construction. This report
 summarizes some of the industries' experiences and, where sufficient information exists, it provides documentation
 regarding the economic savings, technical performance, threats to human health and the environment.) and      :
 environmental benefits of using recycled.materials in highway devices and appurtenances and highway .projects.

 A supporting document to this study is a research synthesis report FHWA-RD-93-O88, titled "Engineering Aspects of
 Recycled Materials for Highway Construction."-                      ,                            .    ,      t
17. KayWords               .
scrap tires, crumb rubber modifier, recycling, hot mix
asphalt, envirnfnental assessment, comparative health
risk, waste materials, reclaimed asphalt pavement, glass,
plastic, disposal, ISTEA
                   18. Distribution Statement                          •    *
                   No restrictions;, This document is available to the public
                   from the National Technical Information Service,       '
                   Springfield, Virginia 22161                    ,     '
19. Security Classif. (of this report)

Form DOT F 1760.7 (8-72)
20. Security Classif. (of this page)

    ,.       Unclassified
21. No of Pages

..        50
22. Price
Reproduction of completed page authorized

                                                    APPROXIMATE CONVERSIONS FROM SI UNITS
              lrVn»n You Know
                                                                                     When You Know
             square inches
             square feet
             square yards
             square miles
                                            square millimeters
                                            square meters
                                            square meters
                                            square kilometers
 square millimeters
 square meters
 square meters
 square kilometers
               square inches
               square feet
               square yards
               square miles
             fluid ounces
             cubic feet
             cubic yards
                                           cubic meters
                                           cubic meters
NOTE: Volumes greater than 10001 shad be shown in m3
cubic meters
cubic meters
               fluid ounces
               cubic feet
               cubic yards
             ounces             28.35
             pounds             0.454
             short tons (2000 Ib)   0.907
                    ounces          02
                    pounds          Ib
                    short tons (2000 Ib) T
                     TEMPERATURE (exact)
                                                                                               TEMPERATURE (exact)

              FORCE and PRESSURE or STRESS
                                                          FORCE and PRESSURE or STRESS
            poundforce per
            square inch
                kilo pascals
                                 poundforce       Ibf
                                 poundforce per    Ibf/in
                                 square inch          !
1 SI is the symbol for the International System of Units. Appropriate
 rounding should be made to comply with Section 4 of ASTM E380.
                                                                                            (Revised June 1993)

                               TABLE OF CONTENTS
             A. Crumb Rubber Modifier
                   (1) Crumb Rubber Modifier Technology
                   (2) Summary of Experience,
                   (3) Discussion of Performance
             B. Recycled Crumb Rubber Modifier.
                   (1) Recycling Variables
                   (2) Summary of Experience
             A. Comparative Threats to Human Health and Environment.
             B. Crumb Rubber Modifier    	
             C. Recycled Crumb Rubber Modifier,
             D. Conclusions	
             A. Reclaimed Asphalt Pavement	
             B. Recycled Glass	
             C. Recycled Plastic.
             D. Blast Furnace Slag	
             E. Coal Flv Ash
             F. Roofing Shingle Waste.
             G. Mining Wastes	
             H. Municipal Waste Combustion Ash
             I. Steel Slags	
             J. Reclaimed Concrete Pavement.
             K. Sulfur	
            A. Health/Environmental Assessment
            B. Recycling	
            C. Performance
            A. Reclaimed Asphalt Pavement
            B. Recycled Glass    	
            C. Recycled Plastic	
            D. Other Recycled Material	

. 23


                                LIST OF FIGURES
FIGURE 1. Standard CRN! terminology
                                 LIST OF TABLES
TABLE   1. Crumb rubber modifier technologies
        2. Summary of experience.
       3. Crumb rubber modifier recycling variables
       4. Summary' of known waste applications	
       5. Summary of disposal practices	

 AR      -asphalt rubber
 ARPG   -Asphalt Rubber Producers Group
 ARRA   -American Recycling and Reclaiming Association
 CIPR    -cold in-place recycling
 CRM    -crumb rubber modifier
 DOT    -U.S. Department of Transportation
 EPA     -U.S. Environmental Protection Agency
 HOPE   -high-density polyethylene
 HIPR    -hot in-place recycling
 HMA    -hot mix asphalt
 IARC    -International Agency for Research on Cancer
 ISTEA   -Intermodal Surface Transportation Efficiency Act of 1991
 LDPE    -low-density polyethylene
 MIBK   -methyl isobutyl ketone
 MSW    -municipal solid waste
 MWC    -municipal waste combustion
 NAPA   -National Asphalt Pavement Association
 OSHA   -Occupational Safety and Health Administration
 PAH     -polycyclic aromatic hydrocarbons
 PEL     -Permissible Exposure Limit
 PET     -polyethylene terephthalate
 POM    -polycyclic organic matter
 PP       -polypropylene
 PS       -polystyrene
 PVC     -polyvinyl chloride
 RAP     -reclaimed asphalt pavement
 RCRA   -Resource Conservation and Recovery Act
 RUMAC -rubber modified hot mix asphalt
 SAM '    -stress absorbing membrane
 SAMI    -stress absorbing membrane interiayer
SEA     -sulfur extended asphalt
SHA     -State highway agency
SHRP    -Strategic Highway Research Program
VOC     -volatile organic compounds


                               CHAPTER 1 - INTRODUCTION
        The legislative history leading up to the devel-
        opment of this report includes both the
        Department of Transportation (DOT) appro-
 priations act for fiscal year 1992 (Pub.L.  102-143) and
 the surface transportation reauthorization bill
 (Pub.L. 102-240). titled the Intermodal Surface
 Transportation Efficiency Act of 1991 (ISTEA).  Both
 the appropriations act and ISTEA require the DOT to
 study the use of scrap tire rubber in asphalt pavements.
 The study required by the appropriations act was
 merged into the ISTEA study.

 ISTEA was enacted into law on December 18, 1991.
 Section 1038(b), STUDIES, requires the DOT and the
 Environmental Protection Agency (EPA) to perform
 studies and report on the results of the studies to
 Congress within 18 months after enactment.  The stud-
 ies are to determine:

   • The threat to human health and the environment.
       the ability to recycle, and the performance of
       asphalt pavement containing recycled rubber.

   • The economic savings, technical performance,
       and threats and benefits to human health and
       the environment of using recycled materials in

   • The utilization and practices of all States relating
       to the reuse and disposal of highway materials.

The Federal Highway Administration (FHWA) and
EPA created a joint technical 1038(b) study coordina-
tion group to conduct the study, to synthesize avail-
able information, and to prepare the report to be sent
to Congress. The  1038(b) research study was con-
 ducted in cooperation with the States to synthesize all
 available State and industry information and experi-
 ence.  A copy of the final research study report, titled
 Engineering Aspects of Recycled Materials for
 Highway Construction, is appended to this report.

 Other related concurrent activities through FHWA
 include seven national workshops on recycled rubber
 in asphalt technology, a symposium on other recycled
 materials, and direct technical support for  State high-
 way agencies and the paving industry. The seven
 workshops were held around the country in February
 and March of 1993. Over 1400 Federal. State, and
 local agency and industry representatives attended the
 2-day programs. The recycled materials symposium is
 scheduled for October 19-22, 1993, in Denver,

 The body, of this report is divided into two chapters:
 Chapter 2 - Scrap Tire Rubber and Chapter 3 - Other
 Recycled Materials. These chapters correspond with
 ISTEA Section 1038(b) subsections (1-2) and (3-4).
 Chapter 2 is further subdivided between FHWA's
 assessment of engineering and EPA's assessment of
 human health and the environment.  Both assessments
 address their respective technical issues as  they relate
 to asphalt pavement containing recycled rubber and to
 the recycling of those pavements.  Chapter 3 is subdi-
 vided by the three specific materials identified in sec-
 tion 1038(b)(3), a separate subdivision for all other
 recycled materials,  and a review of current  disposal
 practices.  In this chapter, environmental and engi-
neering assessments are given for each subsection.
Chapter 4 - Summary and Conclusions consolidates
the previous chapters and is formatted by the specific
issues raised in section 1038(b).

                          CHAPTER 2 - SCRAP TIRE RUBBER
     Interest in developing alternative uses for scrap
     tires emerged in the mid-1980's after a number of
     major scrap tire stockpiles burned out of control.
 These .stockpile fires generate air pollutants, oils. soot.
 and other materials that can cause water and soil cont-
 amination. Additionally, tire piles present a potential
 haven for the breeding of mosquitoes and habitats for
 other vermin. The three principal categories of alter-
 native uses for scrap tires are whole tire applications.
 processed tire products, and combustion for energy
 recovery."1  As of 1990. the application of these alter-
 natives utilized approximately 17 percent of the annu-
 al scrap tire generation. Two-thirds of the scrap tires
 were consumed in combustion facilities, a very small
 fraction (less than 1 percent) was used in whole tire
 applications, and the balance was marketed by the
 processed tire products industry. The remaining 83
 percent were stockpiled, placed in landfills, illegally
 dumped, or exported as used tires.

 The potential alternative uses for scrap tires in the
 highway community include both whole tire applica-
 tions and processed tire products.12' Whole tire appli-
 cations, like impact attenuators (crash barriers) and
 retaining walls, have not developed into marketable
 products.  Several processed tire products are present-
 ly marketed in the highway industry. The types of
 processed tire products include shredded tires as
 embankment material (particularly  for engineered
 lightweight fills), molded rubber products for railroad
 grade crossings and safety hardware, and crumb rub-
 ber for asphalt paving. Some  of these highway appli-
 cations have the potential to use significant quantities
 of tires in particular regions of the country. The two
 main uses of tires that could have a significant impact
 on the scrap tire problem are the recycling of scrap tire
 rubber and the combustion of scrap tires for energy
recovery.'?' The remainder of this chapter will assess
the engineering and health/environmental issues
 regarding the use of scrap tire rubber as an additive to
asphalt paving materials.


A. Crumb  Rubber Modifier
 The history of adding recycled tire rubber to asphalt
 paving material can be traced back to the 1940's when
 U.S. Rubber Reclaiming Company began marketing a
 devulcanized recycled.rubber product, called
 Ramflex™. as a dry panicle additive to asphalt paving
 mixtures.  In the mid-1960's. Charles McDonald
 began developing a modified asphalt binder using
 crumb rubber.'4' This product was marketed by
 Sahuaro Petroleum and Asphalt Company as
 Overflex™.  The Arizona Refining Company, Inc..
 created a second modified binder in the mid-1970" s.
 replacing a portion of the crumb rubber with devulcan-
 ized recycled rubber and marketing it under the name
,Arm-R-Shield™. Both Overflex™ andArm-R-
 Shield™ were patented and eventually brought under
 single ownership. The companies marketing these two
 products founded a trade association known as the
 Asphalt Rubber Producers Group in the mid-1980's.
 Ramflex™ disappeared from the market when U.S.
 Rubber Reclaiming Company was sold by its parent

 The other half of the history originates in Sweden. In
 the 1960's. two Swedish companies began developing
 an asphalt paving surface mixture that would resist
 studded tire and chain wear.  The mixture included a
 small amount of crumb rubber as an aggregate and
 was called by the trade name Rubit™.  In the late
 1970's. this product was introduced and patented in
 the United States as PlusRide™  by AH Seasons
 Surfacing Corporation.  The design of PlusRide™
 evolved through a series of field projects in Alaska
 and other States  from 1979 through 1985.<5>
 PlusRide™ has been managed by a number of firnu
.and is presently marketed by EnvirOtire, Inc.

 With the environmental interest to  find alternative
 uses for scrap tires and the enactment of ISTEA in
 1991. asphalt technologists and rubber-recycling
 entrepreneurs began looking to modify or improve on
 the existing technologies available  to add crumb rub-
 ber to asphalt paving materials. Several new technolo-
 gies have emerged and are being evaluated. The ini-
 tial field test sections of crumb rubber asphalt mix-
 tures similar to PlusRide™ and McDonald technology

                                               Asphalt Rubber
                                         _^.   Rubber Modified
                                               Hot Mix Asphalt
                         FIGURE 1.  STANDARD CRM TERMINOLOGY
 were laid in 1989 and 1990. respectively. Additional
 technologies have been introduced since that time, but
 have not been widely evaluated.

 (1) Crumb Rubber Modifier Technology

 Highway agencies have been evaluating crumb rubber
 producing different types of modified paving mix-
 tures, but. to date, the technology and terminology do
 not separate the dry process construction method by
 facility type.

 An asphalt cement binder that has been modified with
 CRM is called asphait rubber (AR) and can be used
 in a number of asphalt paving products.  The binder
 modification is achieved through an interaction of the
 asphalt cement and the CRM. which is commonly
 referred to as a reaction. The degree of binder modi-
 fication depends on many factors, including size and
 texture of the CRM. the proportion of asphalt cement
 and CRM. compatibility with the asphalt cement, time
 and temperature of reaction, degree of mechanical
 energy during blending and reaction, and the use of
 other additives. Either a wet process or dry process
 can be used to  achieve an AR binder: however, the
 properties of the AR can be significantly different
 from one design to the next and may perform differ-

 A rubber modified hot mix asphalt (RUMAC) is
 defined as an HMA using a dry process where a domi-
 nant portion of the CRM particles retain their tire rub-
 ber characteristics in the final HMA paving mixture.
 The key to RLJMAC mixtures is to design the grada-
 tion of the stone aggregate and CRM "aggregate" to
 achieve the desired final mixture properties.
 Combining the basic concepts of the dry process and
 RUMAC implies that a significant portion of the CRM
 in the mixture is relatively coarse. Variations in
 RUMAC mixtures are characterized by the gradation
 of the stone aggregate. These mixtures are classified
 as dense-graded, gap-graded, and open-graded.

 There arc presently 10 known CRM technologies at
 different levels  of development in the United States.
 Table 1 provides a brief overview of each technology.
 As discussed above, only McDonald and  PlusRide™
 technologies have been evaluated for more than 5
 years. Some technologies have not been field-evaluat-
 ed to date. The wet process technologies are classified
 by the method of blending, and dry process technolo-
 gies are classified by the type of paving product.

 (2) Summary of Experience

The amount of experience in a'given State is primarily
 measured by the amount of documented research
reported by that highway agency, the State's response
  to surveys, and information supplied by industry
  sources. For this report, experience with CRM tech-
  nology falls into three categories:  extensive, limited.
  or none. Extensive experience describes those States
  or agencies that have made a significant effort to eval-
  uate one or more CRM technologies, placing a series
  of field-evaluation projects to measure the perfor-
  mance. Limited experience describes those States or
  agencies that have initiated field-evaluation in the last
  5 years or examined a CRM technology in the past.
  but did not put significant effort into the program.
  Table 2 summarizes the level of experience  that exists
  for each technology  based on the information avail-

 (3) Discussion of Performance

 Although a State may have a number of years of expe-
 rience with a particular CRM technology, the perfor-
 mance of that technology can only be measured by the
 product/application combination for which it is used.
 The three basic types of asphalt paving products are
 sealants, thiri surface treatments, and hot mix asphalt.
 Each of these product types can be further subdivided
 by the combination and proportion of materials used.
 A paving application is identified by the pavement dis-
 tress pattem(s) that are being addressed by the project

 Performance measurements are based on the degree of
 distress observed in the pavement and may include
 one or more different performance parameters.
 Typical parameters are ride, rutting, cracking, skid.
 splash/spray, fatigue, and aging. The four general cat-
 egories of variables that will affect pavement perfor-
 mance are: (1) pavement design/rehabilitation strate-
 gy. (2) materials. (3) mix design, and (4) construction.
 The strategy chosen for a specific project must coin-
 cide with the desired performance parameters and the
 expected climate/traffic conditions.  Proper selection
 of compatible, quality materials is essential.  The
 appropriate mix design procedure must be performed
 correctly to determine the optimum proportion of
 materials and related engineering property limits.
 Finally, the best precbnstruction design effort will not
guarantee an acceptable performing pavement unless
the pavement is properly constructed. Every  step of
the project must be accomplished with the correct
engineering decisions for the pavement to achieve its
intended performance. Pavements that do not perform
as expected can usually be traced back to an incorrect

i i i
McDonald ( 1 )
continuous blending
terminal blending
generic dr>' (RUMAC)
chunk rubber
generic dry (AR)
1960's - Arizona
1990 -Missouri
1989 - Florida
wet/continuous( terminal )/AR
1992 - Arizona
- Washington
wet/terminal/ AR
1992 - Canada
wet/termi nal/ AR
1986- France
1960's- Sweden
1989 - New York
dry/RUMAC-gap. dense
1992 -Kansas
patented (2)
extensive evaluation since 1970's
not patented
Dan Truax
has not been field-evaluated
not patented
Rouse Rubber Industries (4)
limited evaluations since 1989
not patented
U.S. Oil
limited evaluations since 1992
limited evaluations since 1992
BAS Recycling (Beugnet)
has not been field-evaluated in U.S.
extensive evaluations since 1978
not patented
limited evaluations since 1989
not patented
has not been field-evaluated
not patented
limited evaluations since 1992
( 1 ) McDonald Technology includes both Overflex™ and Arm-R-Shield™ products.
(2) There are numerous patents related to this technology.
Some of the patents have expired, but others have not.
(3) Prior to 1993. this technology was marketed through the Asphalt Rubber Producers Group and the licensed
applicators. Presently, the technology is marketed by individual applicators.
(4) individual highway agencies are developing their own products with this technology.

decision in the process. When new materials are intro-
duced into the mixture, each step of the process may
require modification to achieve optimum performance.
The performance of pavements built with CRM tech-
nology have had both successes and failures.  The suc-
cesses represent correct project selection, design engi-
neering, and construction decisions. The failures gen-
erally reflect inexperience with CRM technology in
project selection, design engineering, and construction
decisions. Reported successes in one region of the
country do not immediately substantiate success in  -
other regions since all the variables do  not remain the

The following paragraphs discuss the performance of
the different asphalt paving applications for AR binder
and RUMAC mixtures. The discussion does not dis-
tinguish between the various CRM technologies
because each technology is in a different level of
development. Provided two different CRM technolo-
gies can produce products with equal engineering
properties, they would be expected to achieve compa-
rable performance under the same application condi-
tions. This discussion of performance relies on the
available research reports and survey data to support
the findings. The findings do  not take into account
those projects that document failures that are traced to
improper design and/or construction practices. Those
failures do not represent an accurate measure of per-

Sealants - The  use of AR sealant is common across
the country. More than half the State highway agen-
cies include an AR sealant in their pavement mainte-
nance and rehabilitation programs. The material per-
forms better than most other asphalt sealants.1'''

Thin Surface Treatments - The performance of AR
binder in thin surface treatments has been extensively
evaluated.""1 Chip seals (stress absorbing membranes
- SAM) and slurry seals using  AR binder have per-
formed more effectively over certain pavement dis-
tress conditions than over others.  Stress absorbing
membrane interlayers (SAM!) used in two-layer and
three-layer rehabilitation strategies also performed
well in specific situations. Neither application appears
to improve the performance of all rehabilitation strate-
gies, particularly over pavements exhibiting dominant
transverse crack or joint patterns.
 Hot Mix Asphalt |T"g' - The performance of CRM in
 HMA is divided between hot mix asphalt with AR
 binder (HMA-AR) and rubber modified hot mix
 asphalt mixtures (RUMAC).  Each product must be
 further divided by the mixture type: dense, gap. or
 open-graded.  These distinctions are essential when
 discussing the performance of HMA applications.

 The performance of HMA-AR has not been extensive-
 ly evaluated across the entire  country.  A significant
 increase in field-evaluation activity has occurred in the
 last 5 years. Based on limited available data, the per-
 formance of dense-graded HMA-AR has been compa-
 rable to conventional dense-graded HMA.  Gap-grad-
 ed HMA-AR has shown improved performance over
 other conventional rehabilitation strategies for certain
 pavement distress conditions. An AR binder used in
 open-graded mixtures will improve the ability to con-
 struct this surface mixture and improve pavement
 aging, but will not improve its principle characteristics
 of skid resistance and reduced splash/spray.

 RUMAC mixtures have only been extensively evaluat-
 ed in Alaska. These  mixtures are very sensitive to
 proper design and construction: and. therefore, many
 projects have failed prematurely. Provided the mix-
 ture was properly designed and constructed, gap-grad-
 ed RUMAC will perform comparably to conventional
 HMA and has been shown to perform more effectively
 for low-temperature skid resistance and rut resistance.
 There is insufficient development of dense-graded
 RUMAC to determine its performance.

 Whether various GRM applications enhance cost-
 effectiveness varies by project. Cost-effectiveness  is
 project specific.  A cost-effective analysis must
 account for variables such as safety, user costs, fre-
quency of reconstruction, and pavement performance.
 In the past, the initial construction cost for HMA with
 CRM on documented projects has generally ranged
 from a 50- to 100-percent increase over the conven-
 tional HMA product. Due to these high initial costs
 for CRM technology, most research evaluations have
 concluded that the specific project application has not
 been cost-effective. More recent projects show that
 the range of initial costs have been 20 to 100 percent
 more than the average cost experienced for conven-
 tional HMA. Given the added cost of CRM materials
 and processing and given the economies of scale, we
 would anticipate the future added initial cost would be
 at the lower end of this range.


pressure react.
cont. blending
terminal blend
generic dry-RUMAC
chunk rubber
generic dry-AR




Most of the 1970's and early 1980's
experience was with SAM and
SAMI applications. Most of the
research in the last 10 years has
focused on HMA applications.
Some routine use in the Southwest
Has not been field-evaluated.
Projects with low CRM contents
are not expected to exhibit
improved performance.
Designed to meet local binder
Very limited experience.
Has not been field-evaluated in U.S.
Projects constructed prior to 1 985
do not represent existing PlusRide™
design guidelines.
Projects represent early
technology development.
Has not been field-evaluated.
Very limited experience.
Tim table does not reflect the use of crack/joint sealant and does not distinguish between various types of applications
for each technologs . vv
B. Recycled Crumb Rubber Modifier

(1) Recycling Variables

There are three major variables that describe the type
of CRM recycling that is being evaluated. They are
the materials, design, and construction technique. The
CRM product being recycled may be either AR. a
modified binder with CRM that has reacted with the
asphalt cement, or RUMAC. a HMA with particles of
CRM in the aggregate matrix. The reclaimed asphalt
pavement (RAP) containing CRM (CRM RAP) may
be added back into a conventional asphalt paving
product or the CRM RAP may be added back into a
CRM paving product.  The design of the recycled
CRM product will determine the proportion of CRM
RAP in the mixture and the type of application (base,
surface, shoulder) the mixture will be placed in.  There

 are three basic construction techniques used to incor-
 porate RAP into the mixture. They are plant-recycled
 HMA. hot in-place recycling, and cold in-place recy-
 cling. Just as some of these variables have been
 demonstrated to be unacceptable for conventional
 RAP in certain pans of the country, they may also be
 limited for CRM RAP.

 (2) Summary of Experience

 A matrix of the recycling variables and known CRM
 experience is shown in table 3. Only two projects
 have documented the use of CRM RAP in North
 America. This amount of documented research is
 insufficient to draw any conclusions relevant to the
 ability to use CRM RAP on a routine basis.
 Furthermore, the projects were all constructed in the
 last 4 years, so performance evaluations are not com-
 plete. Each project is summarized below.

 Ontario. Canada — As pan of a planned research pro-
 gram, the Ontario Ministry of Transportation recycled
 a RUM AC [18 kg (40 Ib) of CRM per ton of mix] in
 1991 after  the mix was in service for 1 year. The
 CRM RAP was added back at 30 percent into a
 RUMAC and placed as a surface mix. No engineering
 problems were noted during mixture production and
 placement.' '2|

 New Jersey - In  1992. the New Jersey Department of
 Transportation recycled a 1988 RUMAC [27 kg (60
 Ib) of CRM per ton of mix] back into a conventional
 surface mix.  The CRM RAP was introduced through
 the normal  RAP feeder of the drum plant as 20 percent
 of the total  mix and no problems were noted during
 construction operation.""

A. Comparative Threats to Human Health and

An important starting point in the comparison of
threats to human health and the environment from
conventional asphalt paving to asphalt paving modi-
fied with CRM is an understanding of the complexity
and variability of the compositions found in asphalt
cements (bitumens) used in the U.S. paving industry.
Asphalt cement is not a singularly defined material
with a specified or known chemical composition.
  Almost all asphalt cement used today is obtained by
  processing crude oils. Crude petroleums van1 in com-
  position from source to source. They yield different
  amounts of residual asphalt cement and other distill-
  able fractions. The amount of residual  asphalt cement
  refined from various crude oil sources can range from
  1 percent to over 50 percent, depending on whether
  the crude oil is light crude or heavy crude. Just as the
  residual asphalt cement content of the crude oils varies
  greatly, so does the chemical composition of the crude
  oils and the residual asphalt cement.  The International
  Agency for Research on Cancer (IARC) described
  asphalt cements as "complex mixtures containing a
  large number of different chemical compounds of rela-
  tively high molecular weight: typically. 82-859r com-
  bined carbon. 12-15% hydrogen, 2-8% sulphur. 0-3%
  nitrogen and 0-2% oxygen."' '•«> IARC further chemi-
  cally characterized asphalts into four broad classes of
  compounds: asphaltenes (5 to 25 percent by weight).
.  resins (15 to 25 percent by weight), cyclics (45 to 60
  percent by weight), and saturates (5 to 20 percent by
  weight). The main point is that asphalt cements are
 chemically undefinable mixtures that are extremely
 variable, so determining definite quantitative risks
 from asphalts or modified  asphalts will be extremely
 difficult or impossible at this time.  But. determining
 the relative comparative threats/risks of conventional
 asphalt pavements with those of CRM asphalt pave-
 ments can be done in a qualitative  sense and primarily
 on a comparative risk basis.

 Hot mix asphalt facilities are comprised  of "any com-
 bination of the following: dryers; systems for screen-
 ing, handling, storing, and  weighing hot  aggregate:
 systems for loading, transferring, and storing mineral
 filler; systems for mixing hot mix asphalt; and the
 loading, transfer, and storage systems associated with
 emission control systems.""5"  Hot mix asphalt is pro-
 duced by heating and drying aggregate and mixing
 them with asphalt cement and modifiers. There are
two  general types of HMA production  processes:
batch mix and drum mix. Each HMA mix process has
numerous plant configurations and contractor modifi-
cations for materials flow and mixing.

Emissions from an  HMA plant consist  of steam from
aggregate drying, combustion products (such as car-
bon dioxide and nitrogen oxides), excess combustion
air, and leaks from  the system (fugitive emissions).
The  magnitude of the relative components of emis-
sions can vary depending on a number  of factors, for

. Type of CRM RAP
Percent RAP
Recycled HMA
(plant mixed)
Conventional mix
CRM mix
Hot in-place recycling
Cold in-place recycling
Asphalt Rubber


Rubber Modified
New Jersey

 example, the plant type and age (including combustion
 and emission collection systems); operating conditions
 (including ambient and operating temperature, mois-
 ture, and type of fuel); and materials (including aggre-
 gates, mineral fillers, modifiers, and asphalt cement).
 There is little or no control of fugitive emissions from
 HMA plants.  Fugitive emissions usually originate
 from the dryer unit, mixing chamber, and storage
 silos.  Stack exhaust emissions are commonly con-
 trolled at HMA plants with primary and secondary
 control devices. Primary control devices, such as
 knockout boxes or cyclones, remove large dust parti-
 cles.  Secondary' devices, such as wet scrubbers or
 baghouses. remove smaller panicles from the exhaust
 stream. The proper operation and-maintenance of the
 pollution control equipment are key factors affecting
 the air emissions from the production of HMA.

 In addition, differences in the wet and dry processes
 for CRM asphalt paving material production may
 impact the composition and magnitude of emissions
 from HMA plants. In the dry process, if the CRM is
 added with the aggregate into the system, the potential
 exists for the interaction of crumb rubber and the
 flame or heat from the burner to impact emissions
 from the asphalt plant.  In both processes, the interac-
 tion between CRM and the heated asphalt binder may
 influence emissions from HMA production.

Asphalt cements are known to contain and emit many
hazardous constituents.114' Polycyclic organic matter
(POM) and. in particular, the polycyclic aromatic
hydrocarbons (PAH's) are commonly mentioned as
 groups of hazardous constituents of asphalts. The
 PAH's have been researched to the greatest degree,
 looking for possible carcinogenic responses in test ani-
 mals and for association of carcinogenic outcomes in
 exposed workers. Many of these PAH's are rnutagens
 and have been reported to cause skin cancer in treated
 animals and have been associated with skin and lung
 cancers in exposed workers.'I6> < 17)< 18> Many of the
 known carcinogenic PAH's. in particular,
 benzo(a)pyrene, have been reported in asphalt cement
 itself and in its emissions.*I4) Other classes of haz-
 ardous constituents of asphalt cements are the volatile
 organic compounds (VOC's). which contain such
 chemicals as benzene, benzaldehyde, alkylated ben-
 zenes, naphthalene, and alkylated naphthalenes, etc.
 Each of these VOC's has its own critical toxic effects
 after mammalian exposure. Benzene in particular is a
 known human carcinogen with an EPA group A can-
 cer classification."91

 EPA has not classified asphalt cements as to carcino-
 genicity.  However, IARC has divided the substances
 via categories. In 1985 and 1987, IARC evaluated the
 available data on human exposures to bitumens and
 classified asphalt cement (bitumen) as a mixture of
 ingredients in IARC Group 3, inadequate evidence of
 carcinogenicity to humans.'201 IARC further evaluated
 the available animal data as limited evidence for car-
cinogenicity to animals for undiluted steam-refined
and cracking-residue bitumens and as inadequate evi-
dence of carcinogenicity to animals for undiluted air-
refined bitumens.  Applications of various extracts of
steam-refined and air-refined bitumens to the skin of

  mice have resulted in tumors at the site of application.
  This finding has lead IARC to classify only those con-
  stituent extracts of steam-refined and air-refined bitu-
  mens in Group 2B. possibly carcinogenic to humans.
  and is based upon sufficient evidence of carcinogenic-
  ity in those animals.1-01

  In June 1992. the Occupational Safety and Health
  Administration fOSHA) critiqued the available animal
  and asphalt worker studies in their proposed rulemak-
  ing concerning the occupational exposure hazards
  from workers in close proximity to asphalt fumes.1:''
  OSHA is revisiting the Permissible Exposure Limit
  (PEL) for asphalt worker exposure to asphalt fumes.
  In their presentation of the data. OSHA evaluated how
  they could use available epidemiology data to deter-
  mine the possibility of excess lung cancer deaths in
 asphalt paving workers due to occupational lifetime
 exposure to asphalt fumes. OSHA has not reached
 any final conclusions at this time. EPA has not,  at this
 time, sufficiently studied the OSHA approach to the
 evaluation of the epidemiological studies.

 B. Crumb Rubber Modifier

 Currently. EPA has found seven studies that can  be
 used in a qualitative sense for a weight-of-evidence
 comparison of the relative threats/risks of convention-
 al asphalt pavement materials with CRM asphalt  pave-
 ment materials. Six of the studies were made avail-
 able to EPA as currently available emissions data from
 HM A production plants, recycling of asphalt pave-
 ments, or a.s a pilot worker exposure study rather than
 as final reports with specified conclusions.  Six of the
 studies have not been available to the general public as
 published studies and none has been extensively peer
 reviewed. These studies represent a very limited  data
 base for making this type of qualitative risk compari-
son and the quality control and quality assurance  of
the data collected have not been confirmed. Each
study was conducted using:

   • Unspecified asphalt cement chemical composi-
   • Different percentage of asphalt binder in the con-
      trol and CRM mixes.
   • Different types of asphalt paving mixtures were
      compared (e.g., surface treatments, open-grad-
      ed, dense-graded, etc.).
   • Different operating conditions existed during the
     • Varied plant configurations and emissions con-
     • Varied analytical criteria and procedures.

  All the variables in these studies should alert the read-
  er that the reported data should be viewed as relative
  air quality determinations and not definitive values
  that can be replicated with precision. There was a
  great amount of variability observed in  most of the
  studies"  chemical analytes. both within  each study and
  even greater variability was observed when trying to
  compare between studies. With all these caveats, the
  seven studies will be described briefly to provide a
  sense of the relative comparison of the conventional
  asphalt paving materials with the CRM  asphalt paving
  materials.  A more complete description of the studies
  and the data can be found in the referenced reports.

  The Asphalt Rubber Producers Group (ARPG) con-
  ducted a worker exposure study of conventional and
  CRM asphalt paving (using the CRM wet process).'--*
  Their 2'/:-year study monitored workers who came
  into direct contact with the highest potential exposure
  to asphalt paving fumes, such as aggregate spreader
 operators, paver operators, screedmen, rakers, and
 bootmen. They monitored the workers for the stan-
 dard OSHA contaminants of asphalt cement and com-
 pared their results to the applicable OSHA PEL's.
 The original authors found that exposures to both the
 conventional and CRM asphalt paving materials were
 well under the OSHA PEL's for VOC's,  benzene, and
 PAH's. They identified a methodological problem
 with the determination of coal tar pitch volatiles,
 which they rectified by measuring individual  PAH's.
 Their final study concluded that the "Emission expo-
 sures in Asphalt-Rubber operations did not differ from
 those of conventional asphalt operations." These find-
 ings have been published and  released to  the public.
 Additional analysis concerning the details of this
 information are provided in the research report

 The Ontario Ministries of Transportation  and the
 Environment have provided data on two studies that
 they conducted.  The first study, and the most com-
 pletely reported, deals with the determination of the
 effects of CRM (dry-process) on the stack emissions
 from a drum-mix HMA plant located in Thamesville,
Ontario. The asphalt binder content was 5.3 percent
for the conventional HMA mixes and 6.1  percent for
the CRM HMA mixes.  Changes in stack emission due

 to the addition of CRM were difficult to assess
 because of this variation in binder contents between
 the two mixtures.  It is our understanding that the"
 Ontario Ministries are analyzing the data from this
 study for conclusions in the near future. Regardless.
 the results presented below should be looked at as pre-
 liminary until a more complete study analysis can be
 obtained and evaluated. The currently available
 results show small increases in most PAH emissions in
 the CRM asphalt paving data compared to the conven-
 tional asphalt  paving data. The confidence intervals of
 the mean PAH emissions overlap and have not been
 assessed for binder content effects. The emissions of
 most VOC's were reduced in the CRM asphalt mix-
 tures as compared to the conventional HMA. Other
 monitored emissions were mixed for metals and other
 organics. One finding among the volatile organics
 was the emission of methyl ispbutyl ketone (MIBK) in
 only the CRM asphalt paving mixes.

 Contained in one of thie reports of the Thamesville.
 .Ontario, study were the results of a second study of
 stack emissions from a batch mixing plant during mix-
 ing of conventional and CRM HMA. The study was
 conducted by the Regional Municipality of
 Haldimand-Norfolk within the Province of Ontario.
 Few details were available regarding the trials con-
 ducted at the plant. The currently available results
 show lower emission rates for most of the elements
 and inorganic compounds in the CRM mixes com-
 pared to the conventional mixes. Many of the individ-
 ual PAH emissions were higher in the CRM mixes
 compared to the conventional mixes. Although emis-
 sions were higher for many PAH's. the total semi-
 volatile emissions were lower in the CRM mixes com-
 pared to conventional mixes. The VOC emissions
 were slightly higher in the CRM mixes in this study
 compared to the other Ontario study. These data are
 illustrative  of the variability observed in these studies.
 Emissions of MIBK were found only in the CRM  mix-

 It is hard to draw any firm conclusions from the two
 Ontario studies because of the many apparent study
 variables that have not been controlled and the vari-
 able data results that question whether any trends can
 be found. One exception is the finding of MIBK in
the CRM mixes.  Although there were no specific con-
clusions reported from this  study, a researcher, in his
letter to the Library of Congress concerning his work
on the Ontario  studies, stated. "Based on our experi-
   ence to date, we are of the opinion that there is no sig-
   nificant difference in the air emission profiles  associ-
.   ated with the .production of rubberized and conven-
   tional asphalt."'-4'

   Texas recently completed two studies comparing the
   stack emissions from CRM HMA to emissions from
   conventional HMA. One study conducted in Farmer
   County. Texas, involved the monitoring of stack emis-
   sions from a drum-mix plant.'--"' The-CRM was added
   to the asphalt binder using the wet process, resultins in
   18 percent of the binder being CRM. The mix temper-
  atures were varied, with the conventional mixes run at
  340~F and the CRM mixes run at 340°F and 305°F.
  respectively. Current available results show that the
  paniculate emissions from the 340°F CRM HMA  mix-
  ture were slightly higher than the conventional HMA
  mixture emissions.  Emissions from the 305°F  CRM
  mix were approximately equal to the emissions from
  the conventional mix at 340°F. The results for the
  semi-volatiles were mixed, with some compounds
  being higher for the CRM mixes and some lower
  when compared to the conventional mixes at the same
  temperature. Even though most of the semi-volatiles
  were generally lower in the 305°F CRM mixes com-
  pared to the 340°F CRM mixes, a few semi-volatiles
  emissions were actually higher at the lower tempera-
  ture. The monitored VOC's were slightly lower in the
  CRM mixes compared to the conventional mixes at
  the same temperature, but the 305°F CRM mixes were
  slightly higher in VOC emissions.  1,3-Butadiene was
  only detected in the low temperature CRM mixes.
  Although Texas has not reached a conclusion at this
  time, the data variability for the compared chemicals
  seems to indicate that there is little difference between
  the conventional and CRM asphalt mixes in this study.

  The other Texas study was conducted at a drum-mix
  plant in  San Antonio. Texas.'26' The CRM was added
  to the asphalt cement using the wet process, resulting
  in 18 percent of the binder being CRM.  The HMA
  mix design called for 7.5- to 9-percent binder content.
  The emissions tests were conducted  with the HMA
  plant operating at 325°F for conventional and some
 CRM mixes. Additional tests for other CRM mixtures
  were conducted at 300°F.  Currently available results
 showed that the conventional  mixes were higher in
 paniculate emissions than either CRM mix.  For the
 most pan. the semi-volatiles and PAH's were compa-
 rable for both CRM mixes and conventional mixes.
 The VOC's were mixed with some CRM emissions

 being higher than the conventional HMA mixes and
 some CRM emissions being lower than the conven-
 tional mixes. 1.3-Butadiene was only detected in the
 conventional mixes in this study. In this study, the
 presence of MIBK was noted in only the 325=F CRM

 The National Asphalt Paving Association (XAPA) has
 just completed a pilot study comparing asphalt cement
 fumes from the HMA plant asphalt tank headspace
 and from personal and area monitors during two
 paving operations in Valencia. California.1-1 The
 CRM asphalt binder was prepared using the wet
 process containing 20 percent rubber.  The HMA was
 placed at the paving site at temperatures between
 270°F to 350°F. The conventional asphalt tank fumes .
 contained greater levels of PAH's than the CRM
 asphalt binder tank fumes. The VOC's and some of
 the nitrosamines in the asphalt cement tanks were
 higher with the CRM asphalt binder than with the con-
 ventional asphalt cement. Asphalt fume, as total par-
 ticulate. was reported as the only contaminant detected
 above the California OSHA PEL at the paving site.
 Confounding factors that were mentioned in this study
 which could potentially influence the personal and
 field sampling were automobile traffic, diesel exhaust.
 and tobacco smoke. At this time, very few conclu-
 sions can be drawn from this pilot study.

 C. Recycled Crumb Rubber Modifier

 The New Jersey Department of Transportation con-
 ducted a study incorporating recycled CRM asphalt
 pavement into a paving project in 1992. This was
 done to assess the concerns of the asphalt paving
 industry regarding the recyclability of asphalt pave-
 ments containing ground tire rubber. The project
 involved materials testing of the recycled CRM
 asphalt paving mix and monitoring the drum-mix
 HMA plant for air emissions. The RAP containing 3
 percent CRM was introduced as 20 percent of the new
 HMA. "No modifications were required to the drum
 plant and all production procedures were normal from
 producing the recycled mixtures."'-81 "An analysis of
 air quality testing performed for this project shows
 that PlusRide™  can be recycled within  current air
quality standards."1-*' The air emissions study ana-
lyzed paniculate, carbon monoxide, total hydrocarbon
(as methane),  oxygen, stack opacity, and odor.129' The
  New Jersey Department of Transportation studs' was
  the only one of this type identified and is limited to the
  study of the recyclability of one dr\ process CRM
  asphalt pavement in a drum-mix HMA plant.

  D. Conclusions

  The weight-of-evidence from these seven studies.
  along with using the emissions data from other con-
  ventional HMA plants, show that the emissions from
  any HMA plant can van- widely, both in emissions
  profiles of contaminants and in the level  of contami-
  nants emitted. The currently available data collective-
  ly indicate that no obvious trends of significantly
  increased or decreased emissions can be attributed to
  the use of CRM in HMA pavement production. One
  exception is the observation of MIBK in CRM mix
  stack emissions in three out of seven studies. Great
  variability was observed within each study's chemical
  emission analyses and even greater variability was
  observed between the studies' chemical emission
  analyses.  The emissions levels for each chemical
  found in these studies are within the broad range of
  emissions levels that have been previously reported
  from HMA plant operations, except for the finding of
  MIBK in CRM mix stack emissions in three of the
  seven studies.

 The source of the  MIBK in the three CRM mix stack
 emissions is not known at this time. Since MIBK was
 not evaluated in other asphalt studies, we  cannot say
 that MIBK will not be found in conventional asphalt
 mixes and. therefore, the impact of this finding  is
 unclear. The stack emissions of MIBK were fairly
 low in the three studies compared to the level of other
 VOC's. Studies have not found MIBK to be a car-
 cinogen and the toxicity of MIBK is relatively simile
 to other VOC's found in asphalt.  These findings of
 MIBK may warrant further investigations.

.In summary, using the currently available  information.
 we find there is no compelling evidence that the use of
 asphalt pavement containing recycled rubber substan-
 tially increases the threat to human health  or the envi-
 ronment as compared to the threats associated with
 conventional asphalt pavements.  These findings are
 based on the limited available data from a  few studies.
 These conclusions are subject to revision as additional
 information is obtained and evaluated.

         Today, the United States is experiencing a dra-
         matic increase in the amount and types of
         materials being discarded. This increase, cou-
  pled with the concern of society regarding environ-
  mentally safe and efficient disposal of these materials.
  has placed a tremendous burden on the Nation's land-
  fills and disposal sites.  In  1960. 82 million metric  tons
  (90 million tons) of municipal solid wastes (MSW)
  were produced per year in the United States.  This  rose
  to 146 million metric tons (161 million tons) in 1986,
  and 164 million metric tons (181 million tons) in
  1988.(30> In addition, other solid waste materials from
  agricultural, industrial, building and construction, and
  mining add to the solid waste stream. When added
  together, the total amount of solid waste produced in
  the United States annually is  4.1 billion metric tons
  (4.5 billion tons).131 >

  The highway  construction industry has a long history
  of using recycled products for highway construction.
  From the use of asphalt cement, a waste product from
 oil refinement, to the current usage of fly-ash in
 Portland cement concrete, the industry has used waste
 products to further the quality and durability of the
 highway infrastructure.

 State highway agencies (SHA's) and private organiza-
 tions and individuals have completed or are in the
 process of completing numerous studies and research
 projects concerning the feasibility, cost-effectiveness,
 and performance of pavements constructed using vari-
 ous waste products.'-1" These studies attempt to mesh
 the need of society to safely and economically manage
 the increasing  amount  of waste materials with the con-
 tinuing needs of the highway industry for better and
 more cost-effective construction materials.

 EPA and FHWA have  existing policy and technical
 guidance supporting the use  or reuse of waste materi-
 als where technically and economically feasi-
 b!e«2KJ3x34KWi This Kpon summarizes some of the
 industries' experiences and.  where sufficient informa-
tion exists, it provides documentation regarding the
economic savings, technical  performance qualities.
threats to human health and the environment, and
  environmental benefits of using these materials in high-
  way devices and appurtenances and highway projects.


  Waste materials for the purpose of this report will be
  divided into broad categories of wastes. Table 4 is a list
  of the major waste categories with a specific breakdown
  under each major heading. The annual quantity gener-
  ated by each broad category is also includ-

  Research into the use of waste materials is ongoing and
  new research findings and recommendations are being
  developed. To keep abreast of the current usage of
  waste materials in highway construction. FHWA and
  EPA will be conducting a symposium on "Recovery
 and Effective Reuse  of Discarded Materials and By-
 Products for Construction of Highway Facilities." The
 primary objective of this symposium is to gather and
 disseminate current, state-of-the-art information on new
 and innovative methods for recycling discarded materi-
 als and by-products in the construction of highway
 facilities.  AH sources of information on this  subject
 will be represented to give a broad perspective on the
 many ways in which  recycling can benefit the highway
 construction industry. The 3-day symposium will be
 held in Denver. Colorado, on October 19 through 22.

 Many of the materials appearing in table 4 have had
 some use in the construction of highways. These use*
 range from a very limited experimental basis  to wide
 use and acceptance. Included in table 4 is a summary
 of some of the experiences highway agencies have had
 with the use and performance of these materials.
 However, although it  can be concluded that there are
 many varied uses for waste materials in highway con-
 struction, this report focuses only on those uses of
 waste materials  that have been or may be combined into
 asphalt concrete paving mixtures. The reason for limit-
ing the scope of this report is to focus on those materi-
als that may be substituted for CRM in asphalt concrete
pavements as allowed in section I038(d)(2) of ISTEA.


 A. Reclaimed Asphalt Pavement

 {1) General

 Over 80 percent of the asphalt pavement removed is
 reused in highway applications and less than 20 per-
 cent is disposed.'7' Most SHA's specifications permit
 the contractor to retain ownership of RAP. This poli-
 cy permits contractor flexibility in managing equip-
 ment capabilities and material inventories in order to
 compete in the competitive bidding process.

 There are several ways to categorize pavement recy-
 cling methods, depending on how and where the recy-
 cling is accomplished. However, the most frequently
 used methods for recycling asphalt pavement materials
 falls into three categories: plant (off-site) recycled
 HMA, hoi in-place recycling, and cold in-place recy-

 (2) Recycled  Hoi Mix Asphalt

 Plant-recycled HMA is a process where the existing
 asphalt pavement is removed, usually by a cold
 milling machine, hauled to an HMA plant, and
 processed and stockpiled at the plant yard for future
 use as RAP. The RAP is remixed as a component of
 an HMA. The percentage of RAP in a recycled mix is
 determined by an engineering analysis usually requir-
 ing the recycled mix to meet conventional HMA mate-
 rials and mixture design properties. Experience has
 shown that when recycled HMA is designed to meet
 the same materials properties as conventional HMA,
 its performance has been as good as conventional

a. Recycled Hot Mix Asphalt (Conventional HMA

There are two basic types of conventional HMA
plants: batch  and drum dryer mixers. Conventional
plants super-heat virgin or new aggregates to transfer
heat to the RAP and obtain the final recycled mixture
temperature.  Direct flame heating of the RAP was
found to further age the RAP and cause air emission
problems.  FHWA Demonstration Project 39 showed
that "heat transfer" was the easiest method to retrofit
existing plants to produce a recycled mix and meet
 existing air quality requirements. Batch plants usually
 are limited to producing mixes with a maximum RAP
 content of 50 percent. Dryer drum mixers are usually
 limited to a maximum RAP content of 50 to 70 per-
 cent.'38' NAPA reported that the production of recy-
 cled HMA was 26 percent of the total HMA produc-
 tion in 1985. and 23 percent in 1986.'4'  NAPA also
 reported that the average RAP content in a recycled
 mix was 24 percent in 1985 and  22 percent in  1986.(4("

 The SHA's that routinely permit RAP as a component
 in quality HMA production report cost  savings.  The
 Florida Department of Transportation has found that
 the initial construction cost of a recycled HMA project
 is 15 to 30 percent less than that  of a conventional
 paving approach.'4" This range of initial cost savings
 is consistent with those reported  and predicted by
 FHWA.'391 The actual savings on individual projects
 will be dependent on project location, plant location,
 materials availability and location, and asphalt cement
 prices, etc.

 b. Recycled Hot Mix Asphalt Containing Greater Than
 80 Percent RAP

 Cyclean™. a proprietary hot mix plant,  is a recent
 innovation in the HMA industry. Cyclean™ plants
 can recycle HMA  with RAP contents in excess of 80
 percent. The RAP is fed to a counter-flow dryer drum
 to preheat the RAP and remove moisture.  Virgin
 aggregate, if required by  mix design  and evaluation, is
 added by a separate feed  bin to the dryer drum. The
 RAP is heated to approximately 135°C (275°F) in the
 dryer drum, the RAP is fed to a microwave tunnel
 where the RAP is heated  to 155°C (311°F). A rejuve-
 nating agent is added and the RAP is remixed, stored,
 and then loaded for placement.  The advantage of this
 plant is that the RAP is not further aged  and oxidized
 during the reheating process.'4^ One disadvantage of
 this process is the high cost of microwave energy.'43'

Cyclean™ has been producing recycled  HMA, con-
taining at least 80 percent RAP, for the city of Los
Angeles since 1987.  Performance of these recycled
 HMA pavements has not been documented. Recycled
 HMA specifications for the city of Los Angeles are
different from those of most SHA's and  it cannot  be
determined, based on documentation, whether these
recycled mixtures would  have met conventional HMA
 specifications.  The Georgia Department of
Transportation used the Cyclean™ process to recycle

Animal Manure
Crop Wastes
Logging and Wood Wastes
Miscellaneous Organics
Paper and Paperboard
Yard Waste
Municipal Waste Ash*
Sewage Sludge\Ash
Scrap Tires*
Reclaimed Asphalt Pavement
Coal Fly Ash
Demolition Debris
Cement & Lime Kiln Dust
Sulfate Waste
Coal Bottom Ash/Bottom Slag
Blast Furnace Slag
Non-Ferrous Slags*
Foundry Waste*
Roofing Shingles
Steel Slag
Reclaimed Concrete Pavement
Lime Waste
Waste Rock*
Mine Tailings*
Coal Refuse
Washery Rejects
360 ~"~:~
66.7 j 16.4
31.9 ! 3.8
14.7 I 0.3
12.0 2.4
2.3 0.4
273 30
91 73
45 1 11
21 13
16 5

T ER _" ER
1 NA NA~
"UN" " NA""

' NA • NA • AA
: LA : LA i LR
1 LA i NA i UN
i ER • LR : LR
: ER : AR i AR

UN : UN.
NA ! NA '
• AA ' LR 'UN
i ER ER , UN
! ER ' ER •: LR
1 LR ' ER ^ UN
I— LR..-L._LR.._.i_ AA
UN ! ER ! ER
UN i AR 1 'UN
, 	 LL ! AA T LA
AA | AA i AA
ER ' ER j ER
i i
..LR i LR i UN
AR i ' AL i UN
ER f ER [ UN
NA 1 ER i UN
Kr\ n> Abbreviations
AA - Accepted use: No further research suggested NA . Unacceptable use
AR - Accepted use: Design & performance research suggested UN - Unknown use
LA - Limited use: No further research suggested
LR - Limned use: Design and performance research suggested • There are environmental concerns with this
ER - Experimental: Des.gn and performance research suggested material that may require further research.

State-owned RAP into recycled HMA for pavement
shoulders. The RAP content of this mix was 90 per-
cent with 10 percent natural sand added. Testing of
the recycled mixture showed that it would not have
met air-void criteria in conventional HMA specifica-
tions.'44' Michigan. Pennsylvania, and Texas have
also experimented with this process. These States  are
using the recycled mix. containing at least 80 percent
RAP. in the pavement structure. These projects have
been in service less than 2 years and thus, long-term
performance cannot be reported.

Life-cycle costs cannot be reported without substantial
performance data, however, initial construction cost
savings have been reported. According to bid infor-
mation from the Michigan  project, the mix produced
by the Cyclean™ process provided an initial cost sav-
ings of roughly SI I/metric ton (SlO/ton) over the engi-
neer's estimated cost for conventional HMA.
Although the recycled HMA was not bid as an alterna-
tive to conventional HMA, the engineer's estimate is
usually an average of statewide or areawide bid prices
as can be used to measure cost savings. The engi-
neer's estimate for conventional HMA was 532/ton
and the combined cost of the Cyclean™ recycled
HMA was S21.29/ton. Bid tabulations from the Texas
1-35 project showed that the Cyclean™ recycled HMA
provided an initial cost savings of S6.60/metric ton
(S6.00/ton) compared with bid items for conventional
HMA on that  project.'451 It should be noted that the
quantity for conventional HMA [4.166 metric tons
(4.593 tons)] is substantially less than the quantity for
Cyclean™ recycled HMA [ 113.54-7 metric tons
(125.190 tons)], which somewhat inflates the initial
cost savings.

 (3) Cold In-Place Recycling

 Cold in-place recycling (CIPR) is another recycling
 technique that is used to rehabilitate existing pave-
 ments. Production takes place at the site of the exist-
 ing pavement surface and  involves milling, mixing.
 and placing of pavement material in the absence of
 heat.  After placement, the material is cured so that
 water from the asphalt emulsion evaporates.  The  layer
 is then compacted. Further curing is necessary before
 placing a wearing surface  and opening to heavy truck
 traffic. Because curing is  necessary and relies on high
 temperatures  with low moisture, this rehabilitation
 technique is limited to certain climates and roadway
The American Recycling and Reclaiming Association
(ARRA) estimates that approximately 2.060.000 met-
ric tons (2.270.000 tons) of pavement were processed
as CIPR in 1991. This equates to 9.300 lane-km
(5.800 lane-mi).14^ The depth of treatment is usually
50 to  100 mm (2 to 4 in).  California. Kansas. New
Mexico, and Oregon are frequent users of CIPR. It
has been used mainly on medium to lower traffic vol-
ume roadways.  New Mexico uses CIPR on Interstate
highways: however. 75  to 125 mm (3 to 5 in) of hot
mix asphalt are required to be placed on top of the
CIPR layer to accommodate anticipated truck traffic.

Performance studies have shown that CIPR retards or
eliminates the reoccurrence of reflection cracking
from environmental  distresses, depending on the depth
of treatment versus the depth of crack.'48"49"50'
However, research has shown that CIPR does not
structurally improve the existing pavement.'481
Comprehensive nationwide information on perfor-
mance of CIPR is not available and thus life-cycle
costs cannot be determined. However, first cost sav-
ings of 6 to 67 percent have been reported over com-
parable rehabilitation strategies.'511

(4) Hot In-Place Recycling

Hot in-place recycling (HIPR) is a third recycling
technique that is used to rehabilitate an asphalt pave-
ment.  Production takes place at the paving site and
involves:  (1) heating the existing pavement.
(2) milling, (3) adding new aggregate, asphalt cement.
and/or rejuvenating  agent, and (4) mixing, placing,
and compacting in one  pass of the recycling train.
Currently, HIPR is limited to depths of 60 mm (2 in)
or less. This technique is used mostly by maintenance
forces to address pavement distresses confined to the
surface course of the pavement [top 50 mm (2 in)].

The ARRA reported that its contractors  used HIPR to
recycle approximately 545;000 metric tons (601,000
tons) of existing pavement in 1991. This is roughly
equivalent to 3,900  lane-km  (2,400 lane-mi).'47>
Performance of this technique is not widely reported.
Thus far. HIPR has been'used on pavements that are
structurally adequate and do not require any structural
 improvement. The cost of this technique has varied
 greatly. As much as a  16-percent increase in cost has
 been reported and as much as 40-percent cost savings
 have been reported  when compared to milling and
 replacing with conventional  hot mix asphalt. Recent

  reports show that cost savings of less than 10 percent
  have been realized.1-18"5-"-*-*1

  B. Recycled Glass

  (1) Material Availability

  Glass makes up approximately 7 percent of the total
  weight of the MS W discarded annually or approxi-
  mately 12 million metric tons (13 million tons). Of
  this, approximately" 20 percent is being recycled, pri-
  marily forcullet in glass manufacturing.1-1"1 The avail-
  ability of glass for use as a highway construction
  material is dependent upon the type and availability of
  collection methods used, costs, and public factors. In
  general, large quantities of waste glass are only found
  in major metropolitan areas.

  (2) Experience

  Many SHA's have experimented with the use of glass
  in asphalt pavements. Some SHA's have only per-
  formed laboratory testing while others have actual
  field experience. Studies indicate that at least 10
  States have some experience with the use of glass in
 asphalt pavements.'-1"'-16^! Based on the experiences
 of the States and research completed by Hughes,
 Larsen. and others, the addition of glass into asphalt
 pavements can be accomplished successfully when
 limited to the following conditions:'31x36x56x57)

    •  The amount of glass is limited to 15 percent (by
        weight of total aggregate).
    •  The glass is crushed so that 100 percent
       passes the 9.5-mm (3/8-in) sieve with no more
       than 8 percent passing the 75-m  m (No. 200)
   •  An anti-strip additive is added to improve resis-
       tance to moisture damage.
   •  HMA with crushed glass is limited to binder or
       base course mixes and is not  used in a surface
       or friction course.

(3) Economics

The highway construction industry has an ongoing
need for high quality aggregates. Research studies
indicate that the current cost of fine aggregate for use
in asphalt paving mixes is approximately SI to
S4/metric ton (SI to S4/ton).'7' These costs include
crushing and transportation  to the construction site.
  However, in some metropolitan areas, fine aggregates
  can be as much as SB/metric ton (S 12/ton).'54'

  Glass disposal costs vary depending upon location.
  Disposal costs range from S22 to S55/metric ton (S20
  to S50/ton).<54> The purchase price for sorted
  uncrushed glass varies by region, but is generally S44
  to S55/metric ton (S40 to S50/ton) for clear glass. S28
  to S55/metric ton,(S25 toS50/ton) for brown glass.
  and SO to S55/metric ton (SO to S50/ton) for »reen •
  glass.'5*1  In major metropolitan centers in the
  Northeast, unsorted uncrushed glass can sometimes be
  obtained at no cost.'7'159' The costs of crushing and
  sizing the glass for use as a highway construction
  aggregate will add to the purchase cost.

 (4) Health and Environmental Effects

 The health or environmental effects of incorporating
 glass into asphalt paving mixtures have not been stud-
 ied. However, it is reasonable to conclude that addi-
 tional stack emissions or leachate would not be a prob-
 lem due to the inert nature of glass. Possible risks to
 human health may be in the handling and transporting
 of the crushed glass. This risk could be minimized by
 taking precautions during crushing, handling, and

 C. Recycled Plastic

 (1) Material Availability

 Plastics comprise over 8 percent of the total weight of
 municipal waste stream or approximately 12 percent
 to 20 percent of the volume.'7'™' Approximately 14.7
 million metric tons (16.2 million tons) of plastics arc
disposed of each year with only 2.2 percent being
recycled. Based on available information, the follow-
ing list identifies the primary resins used to make plas-
tic and their respective uses:'3'««»

   • Low-density polyethylene (LDPE) - film and
     trash bags.
   • High-density polyethylene (HOPE) - 1-gal milk
   • Polypropylene (PP) - luggage and battery tasings.
   • Polystyrene (PS) - egg cartons, plates, and cups.
   • Polyvinyl chloride (PVC) - siding, flooring, and
   • Polyethylene terephthalate (PET) - 2-L soda bot-

 Some of these materials, most notably those contain-
 ing PET resins, have been successfully recycled.
 However, the amount of plastic that is currently recy-
 cled is limited and there is a growing need to decrease
 the amount of plastics that must be disposed of in

 (2) Experience ..

 The use of polyethylene as an additive to asphalt pave-
 ments is noi a new technology. These additives are
 generally made from virgin plastics. The only two
 known processes that use recycled plastic as an asphalt
 cement additive are Novophalt™ and
 Polyphalt™.l7ll?lMft(l1 These two processes, although
 somewhat different, use recycled LDPE resin (gener-
 ally made from trash and sandwich bags) as an addi-
 tive to asphalt cement.  The recycled plastic is made
 into pellets and added to the asphalt cement at 4 per-
 cent to 7 percent of the binder by weight of the asphalt
 cement (0.25 percent to 0.50 percent of the total mix
 by weight).'7*60"

 There is limited long-term experience with the use of
 recycled plastics in polymer modified asphalt cement
 in the United States.  However, there has been a
 greater amount of experience with other types of vir-
 gin polymer modifiers. The success or failure of these
 other polymer modified asphalt cements is dependent
 upon  a number of factors, including their compatibility
 with the  virgin asphalts and the environment into
 which they are placed.'7"6"' FHWA. as part of the
 S150  million Strategic Highway Research Program
 (SHRP). is progressing toward  specifications that can
 be directly related to performance.  Once these specifi-
 cations have been finalized, asphalt binders modified
 with recycled plastic may provide the properties nec-
 essary to conform with these specifications.

 (3) Economics

 Although plastic comprises about 8 percent of the total
 weight of the municipal waste stream, it accounts for
 up to 20 percent of the volume.'30*-"*60' Thus, a small
reduction by weight can produce a significant reduc-
 tion in landfill volume.

Data on the cost associated with the use of recycled
plastic in polymer modified asphalt cement is limited
to information from the two known producers. Based
on their data, incorporation of recycled plastic modifi-
 er into conventional hot mix asphalt concrete will
 increase the initial cost by approximately S8/metric
 ton (S7/ton) of mix.'6"1

 (4) Health and Environmental Effects

 There-is limited research in human health and environ-
 mental effects associated with asphalt cement modi-
 fied by recycled plastics.'60' Research, performed by
 Novophalt™. indicates that there is no substantial dif-
 ference between the HMA containing recycled  plas-
 tics and conventional HMA. Further research is nec-
 essarv to substantiate their findings.

 D. Blast Furnace Slag

 (1) Material Availability

 Blast furnace slag is ah industrial by-product generat-
 ed in the production of iron in a blast furnace.  This
 slag consists primarily of silicates and aluminosilicates
 of lime and other bases.'-16'  Approximately 14 million
 metric tons (16 million tons) of blast furnace slag is
 produced annually.'-1" Large accumulations of this
 material have been stockpiled, primarily in those
 States with extensive iron production plants.
 Although no specific environmental concerns with the
 production and accumulation of blast furnace slag has
 been identified, studies indicate that blast furnace slag
 should not present significant environmental problems
 in the form of leaching.

 (2) Experience

 Air-cooled blast furnace slag is an all-purpose con-
 struction aggregate. It is commonly used in concrete.
 HMA. aggregate bases, and as a fill material.'-11 >  Air-
cooled blast furnace slag has a number of desirable
aggregate properties, including hardness, angularity.
high durability, wear resistance, and low specific grav-

Research studies indicate that at least 10 States have
experience  using air-cooled blast furnace slag as an
aggregate in asphalt pavements.'31"36"55"6" The  per-
formance of these pavements has generally been good
with a number of States routinely using blast furnace
slag as an aggregate in HMA. Some reports indicate

 limited use of air-cooled blast furnace slag in asphalt
 pavement due to higher than normal asphalt cement
 content requirements.1:1.6'

 (3) Economics

 Information on the cost of disposing or stockpiling
 blast furnace  slag was not available. Limited data on
 the cost-effectiveness of using blast furnace slag as an
 aggregate in highway construction indicates its use is
 either cost-effective 6r equal to conventional aggre-
 gates.1-'6' Exact cost data is not available.

 (4) Health and Environmental Effects

 There is limited research in human health and environ-
 mental effects associated with the use of blast furnace
 slag as an aggregate in HMA. Blast furnace slag has
 been exempted from hazardous waste status because it
 is classified as a mineral processing waste.'62'

 E. Coal Fly Ash

 (1) Material Availability

 Coal fly ash. commonly referred to as "fly ash," is a
 by-product of coal combustion for power generation.
 Fly ash is generated in 720 plants in 44 States.1-1" The
 chemical content of the fly ash varies depending on
 the type of coal burned. Fly ash generally  contains sil-
 icon, aluminum, iron oxide, and calcium oxide.
 Approximately 45 million metric tons (50 million
 tons) of fly ash is produced annually, with  34 million
 metric tons (37.5 million tons) being disposed of
 either onsite or in State-regulated disposal  areas and
 11  million metric tons (12.5 million tons) being

 Environmental concerns with the continued disposal
 and stockpiling of coal fly ash include possible leach-
 ing of metals (such as cadmium, lead, and arsenic)
 into the ground water. Also, because most fly ash par-
 ticles are smaller than 0.1 mm in diameter (No. 20
 sieve), the waste is susceptible to erosion.'64'

 (2) Experience
There has been a wide variety of experience with the
 use of fly ash  in highway construction.  In  1991. about
 15  percent of DOT funds spent for concrete was spent
 for Portland cement concrete containing fly ash.165'
 EPA's guideline for purchasing cement and fly ash
 requires all Federal agencies, all State and local gov-
 ernment agencies, and contractors that use Federal
 funds to purchase cement and concrete to implement a
 preference program favoring the purchase cement and
 concrete containing fly ash.1-^1 However, its use in
 HMA is limited to use as a mineral filler. A mineral
 filler consists of the material that passes the 75--  m
 (No. 200)  sieve and is typically between 3 percent to 6
 percent of the mix by weight.1661 Mineral fillers are
 readily available by-products of aggregate production
 and the operation of baghouses in hot mix asphalt

 States that have used fly ash as the dust portion of a
 mineral filler generally have been successful.1-1"
 However, the performance of asphalt concrete mixes
 is sensitive to proper aggregate gradation. To obtain
 proper material mix design, limits must be placed on
 the amount of material that passes the 75-m m
 (No. 200) sieve.'67'  Because many aggregates contain
 sufficient quantities of this material, the use of fly ash
 as a mineral filler will be in limited amounts.

 (3) Economics

 Disposal costs for coal fly ash can vary substantially
 with the size of the power plant, the rate of operation.
 and the type of coal used (some coals have a higher
 ash content than others).  In 1986, total landfill costs
 ranged from $2 to $7/metric ton ($2 to $6/ton) at
 3.000-MW plants to $10 to $20/metric ton ($9 to
 $18/ton) at 100-MW plants.'64'

 The cost of fly ash varies based on the location of the
 source. The average cost is approximately $22/metric
 ton ($20/ton) with a variance of $13/metric ton
 ($12/ton) in the Southwest to $77/metric ton (S70/ton)
 in the Northwest.(7>  As was previously reported, aver-
 age costs of fine aggregates are between $1 to $4/met-
 ric ton ($1 to $4/toh).  However, coal fly ash may
 prove cost-effective as a mineral filler in asphalt con-
crete if there is a limited supply of natural aggregates
that contain the desired amount of material passing the
75-m m (No. 200) sieve.

(4) Health and Environmental Effects

No information could be located that specifically
addresses health and environmental effects when using
coal fly ash as a mineral filler in hot mix asphalt. Of
26 States reporting on the environmental and health

 risks for all uses of coal fly ash (which includes:
 asphalt pavement. Portland cement concrete, aggre-
 gate base coarse, subbase. or embankment), only 1
 State had concerns with environmental acceptability.
 This State's concern was primarily due to leachate
 problems. The remaining States reported either "good"
 or "satisfactory" environmental acceptability .'-'6|

 Fly ash is a relatively inert material that will be used
 as an aggregate  and encapsulated in the HMA.
 Therefore, it could be expected that there will be no
 significant difference in health or environmental risks
 over conventional HMA.

 F. Roofing Shingle Waste

 ( 1 ) Material Availability

 Approximately  8.6 million metric  tons (9.5 million
 tons) of roofing shingles are manufactured each year.
 Approximately 65 percent of these shingles are used
 for reroofing, producing 5.6 million metric tons (6.2
 million tons) of old waste shingles.1681 In addition, up
 to 800.000 metric tons (900,000 tons) of waste are
 produced from the manufacturing of roofing shingles
 annually. Typical roofing waste products, including
 old shingles, consist mainly of asphalt cement (36 per-
 cent). hard rock granules (22 percent), and rock filler
 (8 percent). There are also smaller amounts of larger
 [25-mm (1-in) diameter or greater]  aggregates, fiber
 felt, glass fiber felt, asbestos felt, and polyester
Disposal of the waste from the manufacturing process
can pose a difficult problem for shingle manufacturers.
Some plants are required to transport the scraps up to
500 km (300 mi) for disposal.168" Roofing shingles, as
a component of construction and demolition debris,
are generally landfilled in either MSW landfills or spe-
cial construction and demolition landfills.

(2) Experience

There is limited field experience in the use of roofing
shingles in HMA. Currently, no long-term pavement
performance data exist. A report documenting the
technical feasibility of using recycled roofing shingles
in asphalt pavement came to the following conclu-

   •  "Acceptable paving mixtures that contain 20%
        by volume of roofing wastes can be produced.
        With proper selection of binder type, binder
        quantities, and aggregate gradations acceptable
        mixtures containing roofing waste quantities
        to. and perhaps beyond, the 309r level can
        probably be prepared."
    • -"The type of binder selection for use in a mixture
        containing roofing waste should be based on
        the stiffness (penetration and viscosity) of the
        asphalt cement in the roofing waste."
    • "Improved asphalt extraction and recovery
        processes need to be developed for roofing
        waste in order to effectively determine the
        properties of the asphalt cement in the roofing
    • "Gradation of conventional aggregates and roof-
        ing waste should be considered when design-
        ing the paving mixtures."

 The Minnesota Department of Transportation com-
 pleted a project in 1991 that used from 5 percent to 7
 percent asphalt shingles by weight of mix.'70' The
 shingles were ground to a uniform consistency resem-
 bling coffee grounds and were then added to a drum
 mix plant as if they were RAP. No construction prob-
 lems were noted. After less than 2 years, there have
 been no reported problems with pavement perfor-
 mance. Other pavement sections have also been con-
 structed in Florida with good results.'68*

 (3) Economics

 Based on information provided by the Minnesota
 Department of Transportation, shingles used for the
 test project were being disposed of by the manufactur-
 er in landfills at a cost of $21 /metric ton ($ 19/ton).
 For this project, the shingle producer paid the proces-
 sor the same $2I/metric ton ($19/ton) to take owner-
 ship of the roofing shingle waste.  Based on an esti-
 mate provided by the contractor, it cost $9.55/metric
 ton ($8.65/ton) for processing and transportation of the
 shingles to the project site. Assuming the shingles
contained 30 percent asphalt, a savings was also real-
ized in a reduction in the amount of asphalt required
for the mix. Overall, adding roofing shingles to the
asphalt pavement increased the cost by $23/metric ton
($21/ton). This was due primarily to the additional
negotiated costs associated with changing the mix
design after award of the project.

Other data indicates that roofing shingle waste can

 cost up to S66/metric ton (S60/ton) for disposal.
 Landfills are charging between S20 to S50/metric ton
 (SI8 to S45/ton) to accept old roofing shingles.'68'
 Based on these figures, an asphalt cement cost of
 S130/metric ton (S120/ton). and an aggregate cost of
 58/metric ton (S7/ton). using 5 percent roofing shin-
 gles by weight in an asphalt mix can save up to
 S3.08/metric ton (S2.79/toncover conventional HMA.

 (4) Health and Environmental Effects
 mixes depends upon the type of mineral waste used.
 Research indicates that four States ha%re used mine
 tailings in asphalt pavements, primarily to improve
 skid resistance, with good to excellent results.1-"1 The
 burning of coal refuse produces a material called "red
 dog" that has also been used successfully  in asphalt
 pavements.  The major deterrent to using these materi-
 als in highway construction projects is the increased
 cost associated with transporting them to the construc-
 tion  site.lbh
 Research on the human health and environmental
 effects of using roofing shingle waste in asphalt pave-
 ments is not available. Since tHese'wastes contain the
 same basic materials as conventional asphalt pave-
 ments, there should be no significant difference in any
 health or environmental risks, provided the recycled
 shingles do not contain asbestos.

 G. Mining  Wastes

 (1) Material Availability

 Approximately 1.6 billion metric tons (1.8 billion
 tons) of mineral processing wastes are produced annu-
 ally in the United States.01)  The three types of min-
 eral processing wastes that have been used in asphalt
 pavements are waste rock [0.9 billion metric tons/year
 (1 billion tons/year)], mine tailings [450 million metric
 tons/year (500 million tons/year)], and coal refuse
 [110 million metric tons/year (120 million tons/year)].
 Past mining activities have accumulated mountainous
 stockpiles of these materials.  Each of these  materials
 has its own specific environmental problems, but can
 generally be summarized as follows:*61*7"

   • Acidic drainage from both coal and metal mining
       waste that in turn promotes leaching  of heavy
       metals into surface and ground water.

   • Radiation hazards from uranium mill tailings.

The availability of these materials is dependent upon
the location of the mining activity, which is typically
located in remote geographical areas.16"

(2) Experience

There has been a wide range of experience with the
use of the various mining wastes in highway construc-
tion.131'  Their use as an aggregate in asphalt concrete
 (3) Economic Concerns

 Information on the costs associated with the disposal
 of stockpiling mining waste was not available. The
 cost of incorporating mining .waste into an HMA pave-
 ment will depend on a number of factors, including
 selling price, transportation costs, and processing
 costs.*61' Experience has shown that when economi-
 cally viable, these products have been used in asphalt
 concrete pavement projects.

 (4) Health and Environmental Concerns

 Research on the health and environmental effects of
 using mining waste in asphalt pavements was not

 H. Municipal Waste Combustion Ash

 (1) Material Availability

 In  1980, 2.4 million metric tons (2.7 millions tons) of
 MSW was burned,  resulting in approximately 800.000
 metric tons (900,000 tons) of municipal waste com-
 bustion (MWC) ash or residue.'30)  In 1990, this figure
 jumped to 29 million metric tons (32 million tons)
 burned and approximately 7 million metric tons (8
 million tons) of MSW ash or residue.*30' Between 80
 percent and 99 percent of this ash is bottom ash with
.the remainder being fly ash.(36> The requirements for
 disposal of MSW ash will vary by State with some
 States classifying it as a hazardous waste. At the pre-
 sent time, EPA officials estimate that less than 10 per-
 cent of the MWC ash produced is being used in a lim-
 ited number of beneficial-use projects.

 (2) Experience

 A study was done in 1978 by Teague and Ledbetter on
 the performance of using incinerator residue in an

   asphalt concrete base course.'7-' The project was con-
   structed in Houston in 1974 and performance data
   were collected after 3 years of use.  The results indi-
   cated the asphalt base course performed in an excel-
   lent manner, almost identical to the conventional
   asphalt pavement section. The mix design used 89
   percent incinerator ash. 9 percent asphalt, and 2 per-
   cent lime (as an ami-stripping agent) by weight of
   mix. A project in Washington. DC] was constructed
   with 50 percent incinerator ash and 50 percent natural
   aggregates and showed promising results.173' Other
   test sections have also been placed with satisfactory
   performance results.1-1'1

   (3) Economics

  In 1979. FHWA published a report that evaluated the
  economic and environmental feasibility of using incin-
  erator residue in highway construction.'73' The report
  analyzed data from five Standard Metropolitan
  Statistical Areas and included costs associated with
  purchasing the materials, transporting the materials.
  processing the materials, if necessary, and any savings
  in landfill costs.  As a result of this study, the follow-
  ing was concluded:

  "When landfill cost savings associated with incinera-
  tor residue used for highway construction are taken
  into account, economic analysis shows that unfused
  incinerator residue is strongly viable as a bituminous
  highway construction material."

 (4) Health and Environmental Effects

 Currently, the health and environmental effects of ben-
 eficial use of M WC ash are being researched. No con-
 clusions have been reached.
 I. Steel Slags

 {1) Material Availability

 In 1989. approximately 7 million metric tons (8 mil-
 lion tons) of air-cooled steel slag were produced in the
 United States.'-'" Steel slag is a by-product from pro-
 ducing steel and the amount of slag can vary consider-
ably .based on the different types of steel furnaces
used.161' The basic constituents of steel slag are fused
mixtures of oxides and silicates, primarily calcium,
iron, unslaked lime, and magnesium.'31' Research
   indicates that steel slag should not present significant
   environmental problems in the form of leaching.1^'

   (2) Experience

   Steel slags are highly variable materials that have been
   shown to have a potentially expansive nature.13*"'74'
   Steel slag is a fairly well-graded material with a top
   size of about 20 mm (3/4 in), with from 3 to 10 per-
   cent passing the 75-- m (No. 200) sieve: however, for
   use as an aggregate in asphalt pavements, it will need
   to be regraded or blended with natural aggresates.|74'
  The Collins survey reports that eight States have expe-
  rience with steel slag in asphalt concrete.(31) Though
  Collins reports mixed success with steel slags, it
  should be emphasized that different steel plants will
  produce slags with different properties.161'

  One of the major problems associated with the perfor-
  mance of steel slags is their expansive nature.'74' The
  above-referenced reports lead to the conclusion that
  some steel slags may be acceptable for use as an
  aggregate in asphalt concrete pavement, provided care
  is taken to ensure that the slag is subjected to a con-
  trolled curing process of about 6 to 12 months.

  (3) Economics

 Research on the cost to dispose or stockpile steel slags
 was not available. Current information on the exact
 costs associated with incorporating steel slags into
 asphalt concrete pavements was not available.  Studies
 on the use of steel slag as  an aggregate in highway
 construction indicate that of the limited number of
 States indicating usage, the initial cost of steel slags
 are comparable with other aggregate sources.1361

 (4) Health and Environmental Concerns

 Limited research is available on the health and envi-
 ronmental effects of using steel slags in asphalt pave-
 ments.  Of the five States reporting on the environ-
 mental and health risks for all uses of steel slag (which
 includes HMA pavement, Portland cement concrete.
 aggregate base course, subbase. or embankment), one
 State had concerns over possible leachate problems.
The remaining States reported either "good" or "satis-
factory" environmental acceptability.'36'

 J. Reclaimed Concrete Pavement

 (I) Material Availability

 It is estimated that approximately 3 million metric tons
 (3 million tons) of concrete pavement is being recy-
 cled annually.'"?1 The remaining amount of concrete
 pavement rubble that is not recycled is generally con-
 sidered a waste material and is disposed of in landfills
 or other disposal sites.  However, at least one State
 (Florida) does not allow the disposal of construction
 debris in its landfills.'•"'

 (2) Experience

 Reclaimed concrete can be crushed or mbblized and
 used as an aggregate source. The recycled aggregate
 can be used in a subbase. base, stabilized base,
 Portland cement concrete, or asphalt concrete.
 Concrete recycling basically consists of breaking up
 the pavement, hauling broken pieces to a crushing
 plant, crushing and processing the broken concrete to
 appropriate sizes, and stockpiling the processed mater-
 ial for use in its end product. Recycled concrete
 aggregate will usually meet specification requirements
 for conventional aggregates, although its widespread
 usage is not documented.  Florida and Illinois have
 been reported as using recycled concrete aggregate in
 hot mix asphalt. Two States have performed research
 on the use of reclaimed concrete as an aggregate in hot
 mix asphalt and one State is planning on conducting
 research.1-"1 Collin's survey indicated that  its most
 common usage was as an aggregate subbase or base

 (3) Economics

 The first cost savings for using reclaimed concrete as
 aggregate is dependent on: availability and  haul length
 of virgin aggregates,  location of existing pavement.
 haul length to crushing plant, and haul length to end
 user. Typical crushing costs average approximately
S3.30/metric ton (S3.00/ton). while hauling costs an
average of approximately SO.lO/metric ton-km
($0.15/ton-mi). The first cost savings from using
recycled concrete aggregate is offset a little by the
increase in asphalt cement that is  required by the high-
ly absorptive material.1761 Recycled concrete aggre-
gate normally requires 0.5 to 1.0 percent more asphalt
cement than most conventional aggregates.
 (4) Health and Environmental Concerns

 The health or environmental effects of incorporating
 reclaimed concrete into asphalt pavements have not
 been studied. However, it is reasonable to conclude
 that additional stack emissions or leachate would not
 be a problem due to the inert nature of the concrete.

 K. Sulfur

 (1) Material Availability

 Sulfur is an important industrial raw material. Though
 elemental sulfur has been mined, the current major
 sources of sulfur are now a by-product of natural gas
 "sweetening" and refinement of petroleum and tar
 sands.'771 The availability of sulfur is greatly depen-
 dent on the world market and estimates regarding the
 amount of sulfur stockpiled at any one time can vary
 from very little to millions of metric tons.

 (2) Experience

 From 1977 to 1982. 26 projects in 18 States were con-
 structed using sulfur as an extender to asphalt cement
 [sulfur extended asphalt (SEA)] in asphalt paving mix-
 tures. Sulfur was substituted for asphalt binder in
 these mixes at a rate of 20 percent to 40 percent by
 weight. In 1987. a field study was undertaken by
 FHWA to determine the performance of these pave- .
 ment sections.178) Based on the results from this
 report, it was concluded that the overall performance
 and susceptibility to distress are not significantly dif-
 ferent for SEA pavements than for closely matched
 control sections of conventional asphalt pavements. It
 also stated that, as a group, the  SEA pavements show a
 significantly smaller incidence  of transverse cracking
 than the AC pavement control group.

 (3) Economics

The cost associated with the use of sulfur as an addi-
tive to asphalt pavements will depend on the market
cost of the sulfur. Based on results from a study com-
pleted by the Washington State Department of
Transportation, sulfur is a cost-effective substitute for
asphalt when the market price of asphalt is greater
than 1.7 to 1.8 times the market price for sulfur.177'
Due to the high variability in the cost of sulfur, it is
not typically substituted for asphalt cement.

 (4)  Health and Environmental Concerns

 In 1980. a study was undertaken by the Arizona
 Department of Transportation in cooperation with
 FHWA and the U.S. Bureau of Mines to determine if
 SEA concrete could be safely and efficiently produced
 in a drum mix plant.'79'  The study examined both
 stack emissions and worker health and safety.  Results
 from this study indicated that for health and worker
 safety, no harmful emissions of either H2S or SO-,
 were reported.

 Results from stack emission testing results indicated
 that the sulfur gaseous emissions were far in excess of
 those for conventional asphalt (78 to 84 ppm vs. 469
 ppm).179' The emissions were similar to those emitted
 by power plants burning low-sulfur coal without sulfur
 emissions control. Without some type of emission
 control system, such as a wet scrubber, this amount of
 emissions may exceed air quality standards down-

 State legislatures throughout the Nation have
 expressed concern over the increasing amounts of
 waste materials that are being produced. This concern
 has resulted in several types of legislation aimed at
 reducing the generation of waste and promoting recy-

 By 1992.39 States had some form of statewide law
 encouraging or mandating recycling.180' Every State
 has passed legislation promoting the procurement of
 cenain products (often paper products) using recycled
 materials by State agencies and their contractors.  At
 least two States require highway construction projects
 to use recycled materials. Thirteen States have legis-
 lation requiring minimum contents of recycled materi-
als in certain products (often newspaper), while an
 additional 11 States have voluntary agreements with
 the same goal.

 Though there are many possible waste products pro-
 duced during the construction of highways, the great-
 est quantity comes from the removal or replacement of
 the existing pavement structure. Not surprisingly.
 these are also the materials that are most often recy-
 cled or reused.

 The appended research synthesis report contains a sur-
 vey of SHA's on their current reuse/recycle/disposal
 practices.17' A summary of this survey is provided in
 table 5.  Based on this survey, the most commonly
 recycled or reused material is RAP. Of the 29 SHA's
 responding to the survey, only Minnesota reported that
 it disposed of all RAP.  Although Minnesota reported.
 that 100 percent of the material was disposed of. this
 value is misleading. Minnesota, as is the case with
 many SHA's, makes this material the property of the
 contractor, who may reuse, recycle, or dispose of the
 RAP. The Minnesota highway construction specifica-
 tions allow the contractor to reuse this material (at a
 rate of up to 60 percent by  weight) in a State-approved
 recycled asphalt pavement  or other recycled pavement

 As was previously reported, many States are recycling
 or reusing reclaimed concrete pavements. The uses
 for reclaimed concrete pavement include aggregate for
 reuse in: asphalt or concrete pavement, subbase or
 unbound base courses, or as a slope stabilization mate-
 rial (i.e., rip-rap).

 Aggregates, including base courses, subbases. and
 shoulders are also primarily reused or recycled. Many
 States reuse or recycle old guardrail systems including
the rail itself or the steel posts. The refurbishing of
sign faces for reuse is a common practice among many
States.  Most States surveyed also reported experience
with the reuse or recycling of steel girders removed
from reconstructed bridges.

Material/ Appurtenance Type
Asphalt Concrete: Surface Course
Base Course
Stabilized Base
Crushed Stone
Crushed Gravel
. ~
Granular Subbase
Stabilized Subbase
Shoulders, Asphalt
Concrete Culverts
Corrugated Steel Pipe Culverts
Wood Culvert
Multiplate Underpass or Culvert
Guard Rail
Guard Rail Posts (Steel & Wood)
Signs - Advisory and Regulatory
Sign Posts
Sign or Signal Pole and Structures
Bridges: Aluminum or Steel Railing
Steel Superstructure & Deck .
Concrete Beams
Concrete Deck
Average Percent:
Disposed ( 1 )
. 83
ige of Material
Reused/Recycled (2)
(11 The-* materiaK may be buried on the project, landfilled. sold as scrap material, and/or disposed of as contractor property. These
materials may be reused or recycled in non-highway applications.
(2) These materials are functionally reused or recycled,in highway projects.

       Section I038(b) of ISTEA calls for the Secretary
       of DOT and the Administrator of EPA to con-
       duct studies to determine:  (A) the threat to
 human health and the environment. (B) the recyclabili-
 ty. and tCl the performance of asphalt pavement con-
 taining CRM. The study also directs the examination
 of the use of other waste materials in highways.
 Section 1038fd) requires the minimum utilization of
 tire rubber in asphalt paving materials beginning in

 This study evaluates available data regarding the vari-
 ous engineering, health, and environmental aspects of
 working with asphalt pavement containing recycled
 rubber. In addition, other recycled materials applica-
 tions were specifically evaluated:  reclaimed asphalt
 pavement, asphalt pavements containing recycled
 glass, asphalt pavements containing recycled plastics.
 and others. The initial phase of the studies required by
 section 1038 is complete and we have developed a
 synthesis of all available information.

A, Health/Environmental Assessment

The weight-of-evidence from the currently available
information shows that the emissions from any asphalt
plant, either producing conventional HM A or CRM
HMA. can vary widely, both in the profile of emis-
sions observed and in the levels of each contaminant
released. Based on the findings from seven projects in
the United States and Canada, the currently available
data collectively indicate that no obvious trends of sig-
nificantly increased or decreased emissions can be
attributed to the use of CRM in HMA pavement pro-

The finding of MIBK in CRM asphalt pavement mix-
tures in three out of seven studies may warrant further
investigation. An evaluation of the most exposed
human population, workers involved in the production
and construction of asphalt pavements containing
 CRM. indicates no obvious basis for concern of
 increased risk to this population, based principally on
 an analysis of emission data.

 In summary, using the currently available information.
 we find there is no compelling evidence that the use of
 asphalt pavement containing recycled rubber substan-
 tially increases the threat to human health or the envi-
 ronment as compared to the threats associated with
 conventional asphalt pavements. These findings are
 based on the limited available data from a few studies.
 These conclusions are subject to revisions as addition-
 al information is obtained and evaluated.

 B. Recycling

 Based on the results of two projects where asphalt
 pavements containing CRM were recycled, the avail-
 able literature, and an evaluation of variability in plant
 configurations and operations, this technology appears
 to be constructible as a recycled pavement. To date.
 these two recycled pavements are performing compa-
 rably to existing hot mix asphalt pavement. However,
 sufficient information regarding long-term perfor-
 mance and economics is not available. These two pro-
jects represent an extremely limited perspective of the
 variability of in-service pavement properties, environ-
 mental conditions, varying asphalt cements and mix-
 tures, and asphalt plant configurations and operations.
 However, there is no reliable evidence that asphalt
 pavements containing recycled rubber cannot be recy-
cled to substantially the same degree as conventional
 HMA pavements.

Additional evaluations are contemplated and  will be
required to develop further criteria for recycling CRM
asphalt pavements. A national pooled-funds study has
been initiated. Thirty-three States will participate with
FHWA and EPA to further evaluate recycling of CRM
pavements. Requests for proposals for this pooled-
fund research effort will be solicited this fiscal year

C. Performance

While pavements containing CRM have been con-
structed and have been in sen-ice for as many as 20
years in Arizona. California, and a few other States
and based on an extensive review of available litera-
ture and project data, only limited information on
engineering and economic performance is available.
This is due  to limited documentation, experimental
evaluation,  and a resulting incomplete data base upon
which to conduct long-term performance evaluations.
While other States have conducted limited experimen-
tal research with CRM technologies, the performance
of asphalt pavements containing recycled rubber has
received only limited evaluations under varied climat-
ic and use conditions.

In order to develop a reliable cost and economic eval-
uation of pavements containing CRM. comparable
information must be developed on the construction of
CRM asphalt paving projects of typical size rather
than experimental applications. The performance to
date on the  CRM projects has been mixed, some expe-
riencing early failure, others performing comparably
to conventional asphalt pavements, and some CRM
pavements have performed better than conventional
mixes.  Due to limited documentation, the exact cause
of the premature distress in CRM pavements has not
been established. However, when properly designed
and constructed, there is no reliable evidence to show
that pavements containing recycled rubber will  not
perform adequately as a paving material.

We will continue national research on CRM technolo-
gies to develop reliable engineering and economic cri-
teria for the CRM pavements.  Additionally, many
States are conducting coordinated research to evaluate
the effects of local conditions and materials. The
results of these studies will be  included in long-term
performance evaluations.


In the last 30 years, the generation of solid waste in
the United States has increased twofold.  This increase
coupled with the concern of society regarding environ-
mentally safe and efficient disposal of these materials
dictates the need to find alternative uses. Economic
and engineering alternatives for reuse of waste prod-
ucts in highway applications should continue to be
identified, evaluated, and developed.
The highway community pioneered the use of waste
materials beginning with asphalt, a waste product of
the crude oil refining industry.  A long history of
incorporating by-products and wa.->te materials exists
today. Recycling of asphalt pavements has received
extensive use in the United States since the mid-
1970's. Current recycling practice today is deter-
mined by the availability of suitable materials, eco-
nomic costs, and performance.

Studies were conducted on the use and application of
waste products within the highway environment. A
wide array of ideas, concepts, and applications for
waste products exist. Documentation on environmen-
tal and human health risks, engineering criteria, costs.
economic savings, and performance varies from limit-
ed to extensive, depending on the material and appli-
cation. Only limited information on the environmental
benefits of using these materials in highway applica-
tions exists today.

A. Reclaimed Asphalt Pavement

Most State highway specifications permit the contrac-
tor to incorporate a percentage of RAP into asphalt
pavements to the extent the recycled HMA meets
existing specifications for new materials.  In the
United States, over 80 percent of the asphalt pavement
removed is reused in highway applications.

Current asphalt pavement recycling practices utilize
10 to 22 percent RAP in recycled HMA production
using conventional hot mix plant technology.  State-
of-the-practice conventional technology has demon-
strated the capability to recycle asphalt pavements at a
maximum of 50 to 70 percent RAP for properly engi-
neered hot mix materials without adverse engineering
or environmental problems. The exact percentage of
RAP that can be successfully incorporated into a given
recycled mix is dependent on the in-service pavement
materials properties and field conditions. Recycling.
as a pavement rehabilitation technique, generally will
not enhance the basic materials properties of the pre-
existing pavement. To meet materials engineering cri-
teria for many recycled mixes, RAP is often included
at a lower percentage than the maximum percentage at
which a conventional HMA plant can operate effi-
ciently and continue to meet environmental standards.
Hot in-place recycling has been developing since the
mid-1970's. Hot in-place recycling has been per-
formed on asphalt pavements using in excess of 80

 percent RAP. but the results have been aging of the
 asphalt cement and excessive emissions. New tech-
 nology is under development to address this problem.
 Cold recycling has been used successfully on medium-
 10 low-volume roads to recycle 100 percent RAP.
 Microwave technology is now available that has
 demonstrated the capability of hot recycling of asphalt
 pavement within current emissions standards at RAP
 percentages of 80 percent and greater. This technolo-
 gy has had only limited utilization to date and is pro-

 HMA pavements utilizing 80 percent RAP produced
 with conventional hot mix technology result in early
 aging and oxidation  of the asphalt cement and unac-
 ceptable air quality emissions. Cold-mix recycling has
 been performed successfully for in-place and central
 plant production. Comprehensive information on trie
 performance of cold in-place recycling is not available
 and life-cycle costs have not been determined.
 Mixture design and analysis procedures are limited
 .and require further development.  Paving projects con-
 structed utilizing microwave technology are perform-
 ing satisfactorily to date.

 State highway agencies report a cost savings when
 using RAP.  Recycling of asphalt pavements using
 various percentages of RAP is a proven technology
 and with proper engineering and mixture design, recy-
 cled HMA can be considered an appropriate substitute
 material as provided  for under subsection 1038(d)(2)

 Additional information on the use of RAP at the 80
 percent or greater level for the various recycled
 asphalt mix  production technologies is needed for
 long-term performance, engineering design, econom-
 ics, and environmental and human health impacts.
 FHWA will continue to develop and advance this
 technology as. a viable alternative reuse resource.

 B. Recycled Glass

 Glass is a significant  component in the solid waste
 stream. It is highly suitable for solid waste recycling.
 Its use as a substitute paving material has been demon-
 strated. The economics of using waste glass are high-
 ly dependent upon availability. In general, large quan-
tities of waste glass are found primarily in major met-
ropolitan areas. The analysis indicates limited poten-
tial for risks to human health and the environment.
 Significant literature and experimental project data are
 available to support the use of rec\cled glass in prop-
 erly engineered asphalt pavement mixtures up to 15
 percent. Thus, the addition of recycled glass into
 HMA mixtures can be considered as an appropriate
 substitute material as provided for under subsection

 C. Recycled Plastic

 Like glass, plastic is also a significant pan of the solid
 waste stream.  However, only limited reuse of waste
 plastics exists today.  Plastics in the waste stream vary
 significantly in chemical composition.  To date, we
 have extremely limited experience with the use of
 recycled plastics in highway applications.  The use of
 plastics as a polymer modifier in asphalt pavements
 exists today. While there are several technologies
 available to blend  virgin  plastics with asphalt cements
 to produce a polymer modified binder, the chemical
 variability in recycled plastics has been a significant
 deterrent to the use of waste plastics in pavements.
 Two known HMA paving products that utilize waste
 plastics exist.  Only limited performance, economic,
 and environmental data are currently available.
 Therefore, the use  of recycled plastics in asphalt pave-
 ments is not considered an appropriate substitute
 material under  subsection 1038(d)(2) of ISTEA at this
 time. We will continue to work with the States and
 industry to evaluate the emerging asphalt paving prod-
 ucts and applications.

 Based on the review, we have identified other poten-
 tial highway applications for reuse of recycled plas-
 tics. We will continue to develop and promote the use
 of these technologies as appropriate.

 D. Other Recycled Materials

 Our research revealed many potential applications for
 reuse of waste and  by-product materials within the
 highway setting. Only limited information is available
 for many of these waste products. A waste materials
 symposium, "Recovery and Effective Reuse of
 Discarded Materials and By-Products for the
 Construction of Highway Facilities," is scheduled for
October 1993.  The objective of this symposium is to
 identify and disseminate current state-of-the-art infor-
mation on new and innovative methods for effective
recycling and reuse of waste by-products within the
highway system.

 Other waste materials identified as currently applica-
 ble for use in asphalt pavements include coal fly ash.
 blast furnace slags, reclaimed concrete pavement, and
 waste rock.  With proper materials mixture design.
 these materials would be an acceptable substitute
 material as provided for in subsection 1038(d)(2) of

 Several other materials were identified as having
 potential asphalt pavement applications,  but we have
 inadequate information or performance experience
 with these materials at this time. These include coal
 bottom ash. non-ferrous slags, steel slags, roofing
 shingles, and mine tailings.

 A majority of the States have some form of statewide
 law encouraging or mandating recycling or reuse of
 waste materials.  Current practices by the State high-
 way agencies regarding the reuse and disposal of
 materials in federally assisted highway projects vary.
 All States responding to our survey practice reuse of
 waste materials where technically and economically
 feasible. The results of our survey are summarized in
 table  5.


 Highway agencies across the United States have rec-
ognized the importance that the highway system plays
in providing for an improved environment.
 Significant contributions are being made in current
 recycling practices. Additional development is under-
 way to identify and develop opportunities to reduce
 highway waste generation and increase recycling and
 reuse where technically and economically feasible.
 Major investments in developing environmental.
 health, engineering, and  economic performance
 criteria and guidance are underway.

 Based on the studies to date and limited project data

    •  There is no reliable evidence indicating that the
        manufacture, application, or use of asphalt
        pavement containing recycled rubber substan-
        tially increases the threat to human health or
        the environment as compared to the threats
       associated with conventional hot mix asphalt
   •  There is no reliable  evidence that asphalt pave-
       ments containing recycled rubber cannot be
       recycled to substantially the same degree as
       conventional pavement.
   •  There is no reliable evidence that asphalt pave-
       ment containing recycled rubber does not per-
       form adequately as a material for the construc-
       tion or surfacing of highways and roads.

Additional research is underway to continue to
develop our understanding of factors influencing the
reuse of waste products within the highway system
and to develop sound environmental, economic, and
engineering criteria.


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