Material: Recycled
Glass
Issue: Glassphalt has been used since the early 1970’s as an alternative to conventional
hot-mix bituminous asphalt pavement. In
order to meet roadway pavement standards,
glassphalt must be properly mixed and
placed, requiring some modifications to
generally accepted asphalt procedures. For a discussion of the general issues and
economics of glassphalt, see the Recycled Glass in Asphalt
Best Practice Best Practice: Glassphalt is basically the same as conventional hot-mix
asphalt, except that some percentage of
the natural aggregate is replaced with
crushed glass.
The glass must be properly cleaned
and crushed, and combined with the natural
aggregate, preserving agents, and bitumen
in their specified proportions. The glassphalt is then placed and compacted.
This best practice briefly describes
protocols for the preparation and placement
of glassphalt meeting the requirements
for medium traffic specified by
The Asphalt Institute. The glass should contain less
than 2% by weight of metal, plastic, or
other miscellaneous debris such as paper
and food residue.
Previous studies have found that
2% of debris on a weight basis is equivalent
to about 10% to 15% debris content obtained
using a two-dimensional visual inspection
method.
This inspection method is relatively
easy to perform in the field and should
be considered for glassphalt applications.
Details of the visual inspection
method are in the Visual Inspection for Recycled Glass
Construction Aggregate Best Practice. Acceptable gradations for the
glass within the asphalt will be determined
by the purpose of the glass aggregate
in the mix.
For surface course, glass aggregate
graded to 3/8-inch and finer is recommended. Particles larger than 3/8-inch have a tendency
to align themselves parallel to the road
surface during placement and can lower
the skid resistance.
For traffic conditions above 40mph,
glassphalt with glass particles larger
than 1/2-inch can have a skid resistance
5% lower than that of conventional asphalt
(1).
Larger particles also more easily
strip out of the surface.
If the glassphalt is intended for
a base coarse, particles up to 3/4-inch
in size may be used.
Glass in larger sizes than 3/4-inch
can contain entire bottlenecks, which
can be a safety hazard. Larger glass pieces are also more difficult
to screen for contaminants. Once reduced to the desired
size, the glass aggregate is blended with
the natural aggregate to obtain a uniform
glass/natural aggregate mix prior to mixing
with the other asphalt components.
The proportion of the components
can follow the Marshall Mix Design Method
for conventional asphalt, as specified
by the Asphalt Institute.
Mixes containing 20% or less by
weight of glass are recommended in order
to meet the Marshal property requirements
for medium traffic (40mph or less).1
These properties include stability, flow,
percent air voids, and percent voids in
mineral aggregate. 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 the bond will cause the aggregate
to “strip” from the asphalt, increasing
the potential for premature failure.
The smooth surfaces of glass result
in a higher stripping potential than natural
aggregate. This is especially true when the glass particles
are larger than 3/8-inch in size.
Experience suggests that stripping
may not be a problem when particles are
smaller than 3/8-inch, or when the glass
content is less than 10% by weight.
However, an anti-stripping agent
should be used anyway, especially in surface
course glassphalt. Hydrated lime and calcium hydroxide are successful
anti-stripping agents when 1% by weight
is added to the mix. Manufacturers’ instructions should be consulted
when using specific brands. The aggregate, glass and anti-stripping
agent should be mixed so the glass and
anti-stripping agent are uniformly distributed.
This mixture is then combined with
the sand and asphalt in the same manner
as with ordinary hot mix asphalt. Glassphalt can be installed
using the same equipment and procedures
as conventional asphalt. Because glass holds heat longer than natural
aggregate, glassphalt cools slower than
conventional asphalt. Consequently, glassphalt should be allowed
to cool longer before beginning compaction.
A 24-hour waiting period is recommended
before traffic is allowed on the new glassphalt. Implementation: Before a decision is made to
use glass in asphalt, operational issues
and modifications to customary procedures
should be completely understood.
Pilot projects in glassphalt have
often yielded unsatisfying results because
inadequate provisions were made for the
proper handling of the material and incomplete
planning was done to capture the relevant
data. Benefits:
Under
the right economic circumstances, and
with proper technical expertise, specification
roads, especially those designed for medium
traffic, can be made using glass as a
portion of the fine or coarse aggregate
in asphalt. The economics of glassphalt are a local issue
and generally rely on recognition of the
value of waste diversion. Application
Sites: Glass
suppliers, asphalt plants, paving sites,
and testing laboratories. Contact: For more information about
this Best Practice, contact CWC, info@cwc.org. References: Jefferson City, MO The Asphalt Handbook, 1989, Asphalt Institute Manual
Series No. 4. 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. Malisch,
W.R., Keith, T.E., Day, D.E., and Wixson,
B.G., 1972, “Effects of contaminants in
recycled glass utilized for glassphalt,” Proceedings, Third Mineral Waste Utilization Symposium. Schneider,
J.J., 1973.
Marshall properties of asphaltic
concrete containing glass-rich facilities:
M.S. thesis, Civil Engineering, University of Missouri-Rolla. Shin,
C. J., S&EE, Inc., Bellevue, WA. Test
Results and Specifications, 1989, State
of New Jersey Department of Transportation. Issue Date / Update: November 1996
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