600-R-93-095~
JJ.S. Department of
^.^Transportation
Federal Highway
—Administration
f
t
as specified in the
Intermodal Surface
Transportation
Efficiency Act
of 1991
Section 1038(b)
FHWA-RD-93-147
EPA/600/R-93/095
REPORT
JUNE 1993
OF THE
USE OF
RECYCLED
PAV
ING
MATERIAL
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FOREWORD
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 report 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-
ronment 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
study, 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.
NOTICE
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.
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1. Report No.
FHWA-RD-93-147
EPA/600/R-93/095
4. Title and Subtitle
Technical Report Documentation Page
2. Government Accession Mo.
A Study of the Use of Recycled Paving Material - Report to Congress
3. Recipient's Catalog No.
5. Report Date
June 1993
6. Performing Organization Code
7. Author(s)
8. Performing Organization 'Report No.
9. Performing Organization Name and Address
10. Work Unit No. (TRAIS)
Federal Highway Administration, 400 Seventh Street,SW, Washington, DC
Environmental Protection Agency, 401 M Street, SW, Washington, DC
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
13. Type of Report and Period Covered
Final Report
14. Sponsoring Agency Code
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 asphalt pavements
containing scrap tire rubber and synthesize the experience with 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 resullts 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-93-088, titled "Engineering Aspects of
Recycled Materials for Highway Construction."
7. Key Words
scrap tires, crumb rubber modifier, recycling, hot mix
asphalt, envirnmental 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)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No of Pages
50
22. Price
(PFV2.1,6/30/92)
Reproduction of completed page authorized
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TABLE OF CONTENTS
CHAPTER 1 — INTRODUCTION
CHAPTER 2 — SCRAP TIRE RUBBER.
ENGINEERING ASSESSMENT
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
HEALTH/ENVIRONMENTAL ASSESSMENT
A. Comparative Threats to Human Health and Environment,
B. Crumb Rubber Modifier
C. Recycled Crumb Rubber Modifier,
D. Conclusions
CHAPTER 3 — OTHER RECYCLED MATERIALS,
SCOPE
RECYCLED MATERIALS IN ASPHALT CONCRETE.
A. Reclaimed Asphalt Pavement
B. Recycled Glass
C. Recycled Plastic.
OTHER RECYCLED MATERIALS.
D. Blast Furnace Slag
E. Coal Fly Ash
F. Roofing Shingle Waste,
G. Mining Wastes
H. Municipal Waste Combustion Ash
I. Steel Slags
J. Reclaimed Concrete Pavement,
K. Sulfur
CURRENT DISPOSAL PRACTICE.
CHAPTER 4 — SUMMARY and CONCLUSIONS,
SCRAP TIRE RUBBER
A. Health/Environmental Assessment
B. Recycling
C. Performance
OTHER RECYCLED MATERIALS
A. Reclaimed Asphalt Pavement
B. Recycled Glass
C. Recycled Plastic
D. Other Recycled Material
CURRENT DISPOSAL PRACTICES,
CONCLUSIONS
REFERENCES
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LIST OF FIGURES
FIGURE 1. Standard CRM terminology .
LIST OF TABLES
TABLE I. 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
5
7
9
15
25
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LIST OF ACRONYMS
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 -poly vinyl 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 interlayer
SEA —sulfur extended asphalt
SHA -State highway agency
SHRP -Strategic Highway Research Program
VOC -volatile organic compounds
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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), titied .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
highways.
• 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,
Colorado.
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 ah1 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).
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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/2) 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/3) 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.
ENGINEERING ASSESSMENT
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 particle 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™ and Arm-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
corporation.
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 All 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 firms
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
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MATERIAL
PROCESS
CRM
WET
Batch
Continuous
Terminal
DRY
PRODUCT
Asphalt Rubber
Binder
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
modifier (CRM) technology applications at different
levels of development since the 1970's. Reports have
been written to document their findings and observa-
tions, but the diversity of terminology makes it diffi-
cult to determine the true benefit of a given product.
In 1.991, FHWA introduced standard terminology to
improve the ability to communicate the experience of
highway agencies who were evaluating different CRM
technologies/6) The standard terminology has been
expanded to include promising new innovations. This
report defines the standard terminology and summa-
rizes it in figure 1.
Crumb rubber is recycled rubber that has been reduced
in size by mechanical shearing or grinding. Crumb
rubber modifier is crumb rubber derived from scrap
tire rubber that has been reduced to particle sizes less
than 6.3 mm (1/4 in) and is used in asphalt paving.
The methods of producing crumb rubber impart differ-
ent shape and texture characteristics to each particle.
The size, shape, and texture of the CRM have a signif-
icant effect on the performance of the asphalt pave-
ment.
CRM is incorporated with asphalt paving materials by
one of two construction processes: a wet process or a
dry process. The wet process blends the CRM into
the asphalt cement to modify the properties of the
binder. The method of blending can generally be
divided into three categories: batch blending, continu-
ous blending, and terminal blending. Batch blending
defines those wet process technologies that mix batch-
es of CRM and asphalt in production. Continuous
blending describes those wet process technologies
that have a continuous production system. Terminal
blending is associated with wet process technologies
that have products with extended storage (shelf life)
characteristics and are produced at an asphalt cement
supply terminal. The terminal blending technologies
may use either a batch blending or continuous blend-
ing system to actually produce the product at the ter-
minal.
The dry process adds the CRM to the heated aggre-
gate or hot mix asphalt (HMA) mixture during the
production of the mix. The basic concept of the dry
process limits its use to the production of HMA mix-
tures. The flexibility of the dry process is reflected in
the type and degree of modification the CRM imparts
to the paving mixture. There are different types of hot
mix production facilities, batch and numerous drum
configurations. The type of plant may play a role in
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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 asphalt 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-
ently.
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 RUMAC 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 hi
RUMAC mixtures are characterized by the gradation
of the stone aggregate. These mixtures are classified
as dense-graded, gap-graded, and open-graded.
There are 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, thin 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 pattern(s) that are being addressed by the project
design.
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 preconstruction 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
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TABLE 1
TECHNOLOGY
McDonald (1)
pressure
continuous blending
terminal blending
Ecoflex™
Flexochape™
• PlusRide™
generic dry (RUMAC)
chunk rubber
generic dry (AR)
CRUMB RUBBER MODIFIER TECHNOLOGIES
DEVELOPMENT DATE
AND LOCATION
PROCESS/PRODUCT
1960's - Arizona
wet/batch/AR
1990 - Missouri
wet/batch/AR
1989 - Florida
wet/continuous(terminal)/AR
1992 - Arizona
- Washington
wet/terminal/AR
1992 - Canada
wet/terminal/AR
1986 - France
wet/terminal/AR
1960's - Sweden
dry/RUMAC-gap
1989 - New York
dry/RUMAC-gap, dense
1990 - SHRP
dry/RUMAC-gap
1992 - Kansas
dry/AR-open,gap,dense
PATENTED ?
MARKETING FIRM
FIELD EVALUATION
patented (2)
(3)
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
Neste
U.S. Oil
limited evaluations since 1992
patented
Bitumar
limited evaluations since 1992
patented
BAS Recycling (Beugnet)
has not been field-evaluated in U.S.
patented
extensive evalua
not patented
EnvirOtire
ions since 1978
TAK (4)
limited evaluations since 1989
not patented
CRREL
has not been field-evaluated
not patented
(4)
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.
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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
same.
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-
formance.
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/11)
Thin Surface Treatments - The performance of AR
binder in thin surface treatments has been extensively
evaluated/10) 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 (SAMI) 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 <7X9> - 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 CRM 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.
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TABLE 2. SUMMARY OF EXPERIENCE
TECHNOLOGY
McDonald
pressure react.
cont. blending
terminal blend
Ecoflex
Flexochape
PlusRide™
generic dry-RUMAC
chunk rubber
generic dry-AR
EXTENSIVE
AZ,CA
AK
LIMITED
AL,AR,CO,CT, DE,FL,
GA.ID, IA,KS,ME,MD,
MA,MI,MS,MO, NC.NE,
OH,OK, OR,PA,TN,TX,
VA,WA,WI,WY
FL,IA,KS,MS, NJ,
PA,VA,WA
AZ,FL,OR,WA
NC
AZ,CA,IA,MN,MT,
NJ,NM,NY,NV,OK,
OR,SC,UT,WA
CA,IA,IN,IL,NY,OR
FL,KS
COMMENT
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
specifications.
Very limited experience.
Has not been field-evaluated in U.S.
Projects constructed prior to 1985
do not represent existing PlusRide™
design guidelines.
Projects represent early
technology development.
Has not been field-evaluated.
Very limited experience.
This table does not reflect the use of crack/joint sealant and does not distinguish between various types of applications
for each technology.
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 parts 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 part of a planned research pro-
gram, the Ontario Ministry of Transportation recycled
aRUMAC [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/12)
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.^)
HEALTH/ENVIRONMENTAL ASSESSMENT
A. Comparative Threats to Human Health and
Environment
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 vary 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, 82-85% com-
bined carbon, 12-15% hydrogen, 2-8% sulphur, 0-3%
nitrogen and 0-2% oxygen."<14> 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."<15) 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
-------
TABLE 3. CRUMB RUBBER MODIFIER RECYCLING VARIABLES
Type of CRM RAP
Percent RAP
Recycled HMA
(plant mixed)
Conventional mix
CRM mix
Hot in-place recycling
Cold in-place recycling
Asphalt Rubber
low
N/A
N/A
high
Rubber Modified
low
Canada
New Jersey
N/A
N/A
high
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 particles 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/14) 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 mutagens
and have been reported to cause skin cancer in treated
animals and have been associated with skin and lung
cancers in exposed workers/16) <17X18) Many of the
known carcinogenic PAH's, in particular,
benzo(a)pyrene, have been reported in asphalt cement
itself and in its emissions/14) 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/19)
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/20) 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.<2°)
In June 1992, the Occupational Safety and Health
Administration (OSHA) 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.(21>
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
HMA production plants, recycling of asphalt pave-
ments, or as 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-
tions.
• 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
comparisons.
• Varied plant configurations and emissions con-
trols.
• 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).<22)
Their 21/2-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
appended.^)
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
10
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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 isobutyl ketone (MIBK) in
only the CRM asphalt paving mixes.
Contained in one of the 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-
tures.
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."^24)
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/25) The CRM was added
to the asphalt binder using the wet process, resulting 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
particulate 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 yOC'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 riot 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,
particulate emissions than either CRM mix. For the
most part, 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
,11
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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
mixtures.
The National Asphalt Paving Association (NAPA) 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.(27) 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."(28> "An analysis of
air quality testing performed for this project shows
that PlusRide™ can be recycled within current air
quality standards.'^28) The air emissions study ana-
lyzed particulate, carbon monoxide, total hydrocarbon
(as methane), oxygen, stack opacity, and odor.<29) The
New Jersey Department of Transportation study was
the only one of this type identified and is limited to the
study of the recyclability of one dry 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 vary 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 similar
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.
12
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CHAPTER 3 - OTHER RECYCLED MATERIALS
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.(3°) 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).(3D
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.<31) 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-
ble.(32)(33)(34)(35) xhis report 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.
SCOPE
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-
ed.(30)(31)(36)(37) ' . - .
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. All 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,
1993.
Many of the materials appearing in table 4 have had
some use in the construction of highways. These uses
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 1038(d)(2) of ISTEA.
13
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RECYCLED MATERIALS IN
ASPHALT CONCRETE
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, hot in-place recycling, and cold in-place recy-
cling.
(2) Recycled Hot 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
HMA.
a. Recycled Hot Mix Asphalt (Conventional HMA
Plants)
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/41) This range of initial cost savings
is consistent with those reported and predicted by
FHWA/39) 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 dram, 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/42) 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
14
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TABLE 4. SUMMARY OF KNOWN WASTE APPLICATIONS
ANNUAL RATES IN
MILLIONS OF METRIC TONS
WASTE CATEGORY
AGRICULTURAL WASTES
Animal Manure
Crop Wastes
Logging and Wood Wastes
Miscellaneous Organics
DOMESTIC WASTES
Paper and Paperboard
Yard Waste
Plastic*
Glass
Municipal Waste Ash*
Sewage SludgeXAsh
Scrap Tires*
INDUSTRIAL WASTE
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
MINING AND MINERAL WASTE
Waste Rock*
Mine Tailings*
Coal Refuse
Phosphogypsum*
Washery Rejects
Kev to Abbreviations
PRODUCED
1,910
1,460
360
64
27
180
66.7
31.9
14.7
12.0
7.3
7.3
2.3
273
91
45
23
21
16
16
14.5
9
9
6.4
7.3
3
2
1,640
930
473
109
96
32
RECYCLED
16.4
3.8
0.3
2.4
0.4
30
73
11
13
5
CURRENT AND PAST HIGHWAY USES
ASPHALT
PAVEMENT
NA
ER
NA
UN
NA
NA
LR
LA
LR
LR
AR
AA
AA
ER
ER
ER
LR
AA
LR
ER
AR
AR
AR
ER
AA
AR
ER
UN
NA
CONCRETE
PAVEMENT
NA
ER
NA
NA
NA
NA
ER
NA
ER
LR
ER
LR
AA
UN
UN
ER
UR
AA
ER
UN
UN
NA
AR
UN
UN
UN
UN
UN
NA
BASE
COURSE
NA
UN
NA
NA
NA
NA
UN
LA
LA
ER
ER
AA
AA
ER
ER
ER
LR
LR
LR
UN.
UN .
LL
AA
ER
LR
AR
UN
ER
NA
EMBANK-
MENT
NA
UN
AA
UN
NA
NA
UN
LA
NA
LR
AR
AA
LR
ER
ER
ER
ER
LR
UN
ER
AR
AA
AA
ER
LR
AL
ER
ER
ER
OTHER
AA
UN
AA
UN
LA
AA
AR
LR
UN
LR
AR
UN
UN
• UN
UN
LR
UN
AA
LR
ER
UN
LA
AA
ER
UN
UN
UN
UN
UN
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 - Limited use; Design and performance research suggested * There are environmental concerns with this
ER - Experimental; Design and performance research suggested material that may require further research.
15
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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.^4) 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 $ll/metric ton ($10/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 $32/ton
and the combined cost of the Cyclean™ recycled
HMA was $21.29/ton. Bid tabulations from the Texas
1-35 project showed that the Cyclean™ recycled HMA
provided an initial cost savings of $6.60/metric ton
(S6.00/ton) compared with bid items for conventional
HMA on that project.^) 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 [113,547 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
applications/46)
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)/4?) 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.(48X49X5°)
However, research has shown that CIPR does not
structurally improve the existing pavement/48)
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/51)
(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
16
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reports show that cost savings of less than 10 percent
have been realized.(38)(52)(53)
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 for cullet in glass manufacturing/30) 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.(31X36)(55) 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-/3 D(36)(56)(57)
• 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)
sieve.
• 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 $1 to
$4/metric ton ($1 to $4/ton).C7) These costs include
crushing and transportation to the construction site.
However, in some metropolitan areas, fine aggregates
can be as much as $13/metric ton ($12/ton).<54>
Glass disposal costs vary depending upon location. •
Disposal costs range from $22 to $55/metric ton ($20
to $50/ton).(54) The purchase price for sorted
uncrushed glass varies by region, but is generally $44
to $55/metric ton ($40 to $50/ton) for clear glass, $28
to $55/metric ton ($25 to $50/ton) for brown glass,
and $0 to $55/metric ton ($0 to $50/ton) for green
glass.(58> In major metropolitan centers in the
Northeast, unsorted uncrushed glass can sometimes be
obtained at no cost.(7)(59) 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
transporting.^)
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.<7X3°) Approximately 14.7
million metric tons (16.2 million tons) of plastics are
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:<31X6°)
• Low-density polyethylene (LDPE) - film and
trash bags.
• High-density polyethylene (HOPE) - 1-gal milk
jugs.
• Polypropylene (PP) - luggage and battery «asings.
• Polystyrene (PS) - egg cartons, plates, and cups.
• Poly vinyl chloride (PVC) - siding, flooring, and
pipes.
• Polyethylene terephthalate (PET) - 2-L soda bot-
tles. :
17
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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
landfills.
(2) Experience
The use of polyethylene as an additive to asphalt pave-
ments is not 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™ .0X31X60) 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).c?X60)
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.CWeo) FHWA, as part of the
$150 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)(3i)(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 $8/metric
ton($7/ton)ofmix/6°)
(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-
essary to substantiate their findings.
OTHER RECYCLED MATERIALS
D. Blast Furnace Slag
(1) Material Availability
Blast furnace slag is an 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/36) Approximately 14 million
metric tons (16 million tons) of blast furnace slag is
produced annually/31) 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/31) Air-
cooled blast furnace slag has a number of desirable
aggregate properties, including hardness, angularity,
high durability, wear resistance, and low specific grav-
ity/61)
Research studies indicate that at least 10 States have
experience using air-cooled blast furnace slag as an
aggregate in asphalt pavements/31X36X55X61) 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
18
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limited use of air-cooled blast furnace slag in asphalt
pavement due to higher than normal asphalt cement
content requirements/36)
(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 or equal to conventional aggre-
gates/36) 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/31) 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
reclaimed/63)
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/65)
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/32) 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 m
(No. 200) sieve and is typically between 3 percent to 6
percent of the mix by weight/66) Mineral fillers are
readily available by-products of aggregate production
and the operation of baghouses in hot mix asphalt
plants.
States that have used fly ash as the dust portion of a
mineral filler generally have been successful/31)
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 ($70/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/ton). 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
19
-------
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/36)
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/68) 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
films/6?)
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/68) Roofing shingles, as
a component of construction and demolition debris,
are generally landfilled in either MS W 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-
sions^69)
• "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 30% 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
waste."
• "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 $21/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
20
-------
cost up to $66/metric ton ($60/ton) for disposal.
Landfills are charging between $20 to $50/metric ton
($18 to $45/ton) to accept old roofing shingles/6*)
Based on these figures, an asphalt cement cost of
$130/metric ton ($120/ton), and an aggregate cost of
SB/metric ton ($7/ton), using 5 percent roofing shin-
gles by weight in an asphalt mix can save up to
$3.08/metric ton ($2.79/ton) over conventional HMA.
(4) Health and Environmental Effects
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.(31) 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/61^71)
• 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 hi remote geographical areas/61)
(2) Experience
There has been a wide range of experience with the
use of the various mining wastes in highway construc-
tion/31) Their use as an aggregate in asphalt concrete
mixes depends upon the type of mineral waste used.
Research indicates that four States have used mine
tailings in asphalt pavements, primarily to improve
skid resistance, with good to excellent results/31) 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/61)
(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
available.
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
21
-------
asphalt concrete base course/72) 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 anti-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/73) Other
test sections have also been placed with satisfactory
performance results.C31)
(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 MWC 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/31) 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/61) 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/61)
(2) Experience
Steel slags are highly variable materials that have been
shown to have a potentially expansive nature/36)^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 m (No. 200) sieve; however, for
use as an aggregate in asphalt pavements, it will need
to be regraded or blended with natural aggregates/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/61)
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/36)
(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)
22
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J. Reclaimed Concrete Pavement
(4) Health and Environmental Concerns
(1) Material Availability
It is estimated that approximately 3 million metric tons
(3 million tons) of concrete pavement is being recy-
cled annually/75) 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/31)
(2) Experience
Reclaimed concrete can be crushed or rubblized 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/31) Collin's survey indicated that its most
common usage was as an aggregate subbase or base
course.
(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
$3.30/metric ton ($3.00/ton), while hauling costs an
average of approximately $0.10/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/76) Recycled concrete aggre-
gate normally requires 0.5 to 1.0 percent more asphalt
cement than most conventional aggregates.
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/77) 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/78) 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/77)
Due to the high variability in the cost of sulfur, it is
not typically substituted for asphalt cement. ',--
23
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(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. P9) 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 SO2
were reported.
Results from stack emission testing results indicated
that the sulfur gaseous emissions were far hi excess of
those for conventional asphalt (78 to 84 ppm vs. 469
ppm).^9) 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-
wind.
CURRENT DISPOSAL PRACTICE
State legislatures throughout the Nation have
expressed concern over the increasing amounts of
waste materials that are being produced. This concern
has resulted hi several types of legislation aimed at
reducing the generation of waste and promoting recy-
cling.
By 1992,39 States had some form of statewide law
encouraging or mandating recycling/80) Every State
has passed legislation promoting the procurement of
certain 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/7) 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
projects.
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.
24
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TABLE 5. SUMMARY OF DISPOSAL PRACTICES
Material/Appurtenance Type
Asphalt Concrete: Surface Course
Base Course
Stabilized Base
Crashed Stone
Crashed 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 Percentage of Material
Disposed (1)
16
16
27
16
19
22
26
22
74
87
100
66
48
54
47
56
54
56
63
83
89
Reused/Recycled (2)
82
82
65
67
77
73
50
74
22
13
0
26
52
42
53
44
44
44
37
12
11
(1) These materials 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.
25
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CHAPTER 4-SUMMARY AND CONCLUSIONS
Section 1038(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 (C) the performance of asphalt pavement con-
taining CRM. The study also directs the examination
of the use of other waste materials in highways.
Section 1038(d) requires the minimum utilization of
tire rubber in asphalt paving materials beginning in
1994.
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.
SCRAP TIRE RUBBER
A. Health/Environmental Assessment
The weight-of-evidence from the currently available
information shows that the emissions from any asphalt
plant, either producing conventional HMA 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-
duction.
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 tfie 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
(1993).
26
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C. Performance
While pavements containing CRM have been con-
structed and have been in service 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 CPJVI 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.
OTHER RECYCLED MATERIALS
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 waste 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
27
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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-
to 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-
prietary.
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 the
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)
oflSTEA.
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 recycled 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
1038(d)(2)ofISTEA.
C. Recycled Plastic
Like glass, plastic is also a significant part 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.
28
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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
ISTEA.
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.
CURRENT DISPOSAL PRACTICES
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
tableS.
CONCLUSIONS
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
available:
• 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
pavements.
• 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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32
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33
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34
*U.S. G.P.O.:1993-343-273:80067
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