Material: Recycled Glass Issue: Asphalt containing
glass cullet as an aggregate is called
“glassphalt,” and has been widely tried
as a means to dispose of surplus waste
glass since the 1960’s.
Glassphalt is basically the same
as conventional hot-mix asphalt, except
that 5% to 40% of the rock and/or sand
aggregate is replaced by crushed glass.
The cost-effectiveness of substituting
glass for conventional aggregate is highly
dependent on the location, the quality
and cost of local aggregates, and any
credits available for using recycled materials
in beneficial re-use applications. Best Practice: This Best Practice discusses the benefits and applicability
of glassphalt.
For details on the preparation
and installation of glassphalt, refer
to the Preparation and Placement of Glassphalt Best Practice.
For a complete discussion of the
history of glassphalt also refer to the
Glasphalt
Paving Handbook. Glassphalt was originally developed
as an alternative to landfill disposal
of mixed color waste glass.
Mixed color glass, which is unsuitable
for recycling into new containers, is
generated by most recycling programs.
If there are no alternative local
markets for mixed color glass and the
only other option is disposal with landfill
tip fees, using processed glass as a substitute
for natural aggregate in asphalt may be
an option to be considered. A great number of glassphalt
demonstration projects have been performed
in cities around the country. Most of these projects have not progressed
past the pilot stage because of economics.
It is not economical in most parts
of the United States to collect glass,
process it to a specification aggregate,
blend the glass with natural aggregate,
add the batch modifiers needed to meet
specifications, and deal with the operational
changes required for glassphalt. The best possibility for sustained production of glassphalt is in
communities with municipal asphalt plants,
because the community can make a direct
correlation between the extra costs incurred
in glassphalt installation and the savings
from diverted solid waste tip fees. The best possibility for sustained use of glassphalt by private
sector asphalt manufacturers and contractors
is through the creation of ongoing financial
incentives to use the glass. Once in place, glassphalt is
difficult to recognize by the ordinary
person unless large glass particles are
present in the surface layer.
When properly installed, glassphalt
presents no danger to humans, nor does
it damage vehicle tires.
Due to its glass content, glassphalt
will hold heat longer than conventional
asphalt. This characteristic could prove useful in situations
where roadwork is conducted in cold weather,
or when long periods of post-mix transportation
are necessary.
In addition, glassphalt surfaces
appear to dry faster than traditional
paving after rain because the glass particles
do not absorb water. Glassphalt surfaces are also more reflective
than conventional asphalt, and may improve
nighttime road visibility. Use Limitations: Most installations of glassphalt have been designed to meet the standards
of The Asphalt Institute for medium
traffic asphalt, which specify a maximum
speed limit of 40 mph. These standards include requirements for stability,
flow, voids in mineral aggregate, percentage
of air voids in the mix, and unit weight.
The most common applications are
as surface pavement (surface coarse) for
residential streets, secondary roads,
parking lots, sidewalks, and curbing. When glassphalt is placed and
compacted, larger glass particles will
align themselves parallel to the road
surface.
This can cause the skid resistance
of glassphalt to be slightly lower than
that of conventional asphalt.
Consequently, glassphalt is not
recommended for surface pavement on highways. However, the skid resistance of glassphalt containing less than
10% glass by weight, with cullet particles
smaller than ¼-inch in size, shows no
appreciable difference from asphalt containing
100% natural aggregate. If the glass aggregate is used as part of the
base, or lower, asphalt course, then the
size of the glass particles and the skid
resistance of the material are not a concern. All types of asphalt paving
face the possibility that the bonding
of the asphalt to the aggregate will deteriorate
under adverse conditions, particularly
water exposure. Weakening this bond will cause the asphalt
to “strip” from the aggregate, increasing
the potential for premature failure. The smooth surfaces of glass result in a higher
stripping potential than that of rougher
natural aggregate. Consequently, an anti-stripping agent such
as hydrated lime or calcium hydroxide
should be used as an additive in glassphalt,
especially in surface coarse glassphalt. Implementation: Glassphalt has probably received
more national publicity than any other
alternative use for recycled glass.
The technical basis for substituting
processed glass for a portion of the natural
aggregate in asphalt has been well established.
However, before committing resources
to pilot projects or demonstration sites,
recyclers and communities should consider
this Best Practice and the Preparation and Installation of Glassphalt
Best Practice, and study the references
below to determine whether the application
is sustainable under local conditions.
It may be economically more practical
to use glass as an unbound construction
aggregate. See the Developing Specifications for Recycled Glass Aggregate
Best Practice. Benefits: For over twenty years, recycled glass has been used
as an aggregate supplement in hot-mix
bituminous asphalt pavement. The material
has both benefits and limitations, which
should be considered before use. Understanding these factors will help producers
and end users to decide whether glassphalt
is a viable alternative to conventional
asphalt. Application Sites: Glass suppliers, asphalt plants, construction
sites, and testing laboratories. Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org. References: The Asphalt Handbook, Asphalt Institute Manual Series
No. 4, 1989; Day,
D.E., and Schaffer, R., Glasphalt
Paving Handbook, University of Missouri-Rolla; Malisch, W.R., Day, D.E., and
Wixson B.G., 1975, Use
of Domestic Waste Glass for Urban Paving,
Summary Report, National Environmental
Research Center, Office of Research and
Development, U.S. Environmental Protection
Agency, Report EPA-670/2-75-053. Shin, C. J., S&EE, Inc.,
Bellevue, WA Shin, C.J., and Sonntag, V., Using recycled glass as construction aggregate,
Transportation Research Board, National
Research Council, Record No. 1437, Washington,
D.C., 1994; Issue Date / Update: November 1996
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