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This fact sheet is an overview of the use of ground asphalt roofing shingles in
aggregate base and subbase, including potential markets and a discussion on
specifications.
A typical roadway section is built in several layers. The pavement is the
surface layer, and is made of concrete or asphalt. The base supports the pavement,
and is made of a layer of aggregate base (AB) and sometimes a layer of aggregate
subbase (ASB). The subbase layer allows more sand, silt, and clay. This layer has less
strength, but is used because it is more economical when bringing the road up to grade.
In addition to saving landfill space, the benefits to recycling asphalt shingles in AB
may possibly include improved compactability, though this has not yet been tested or
proved. An economic benefit has not yet been demonstrated, though this is dependent on
many factors that change over time, such as tipping fees, urban quarry reserves, and costs
for processing and transportation.
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The waste shingles are first ground and screened to produce the appropriate size
pieces, then mixed with aggregate in the required percentage.
Size Considerations
Considerations of different size pieces include:
- Cost. The larger pieces (1" or 1-1/2") cost less to produce than the smaller
(3/4" or 1/2").
- Identification. The larger pieces can be identified as shingles in the AB, while the
smaller pieces (1/2" and less) are difficult to identify as shingles.
- Compactability. The smaller pieces may contribute more to compactability.
The key to opening large markets for asphalt shingles in AB is to allow the shingles in
construction specifications. The specifications most widely used in California are
Caltrans specifications and the Greenbook.
Caltrans Specifications
Caltrans specifications are used by Caltrans projects, most local government public works
departments in Northern California, and many private projects. Caltrans testing of new
road products adds a high level of confidence to a product or method. Caltrans
specifications for shingle-content AB would significantly improve marketing prospects for
recycled shingles. Caltrans
specifications currently do not allow asphalt shingles in AB.
Caltrans Procedures
If Caltrans were to write specifications for shingles in AB, the process might include the
following steps:
- Perform further testing in the laboratory.
- Write draft specifications, called Special Provisions (SP).
- Test in the field, perhaps on a heavily trafficked road such as a truck weigh station.
- Monitor for several years.
- Write specifications called Standard Special Provisions (SSP). These SSPs would then be
available for routine use in Caltrans and local government projects.
Currently, SSPs for base and subbase allow "reclaimed asphalt concrete, portland
cement concrete, lean concrete base, cement treated base," or "glass," but
not roofing shingles.
Greenbook Specifications
The Greenbook is officially titled the Standard Specifications for Public Works
Construction. It is used by the City and County of Los Angeles and 200 other local
governments and agencies in the Los Angeles area. The Greenbook specifies crushed
aggregate base (CAB) for virgin base, and two categories of recycled base, crushed
miscellaneous base (CMB) and processed miscellaneous base (PMB). The Greenbook does
not mention roofing shingles in its base specifications.
Greenbook Procedures
If the Greenbook were to include specifications for shingles in AB, the process
might include the following steps:
- A local government tests the method in the field.
- The local government submits test results to the Greenbook Committee.
- The Greenbook Committee assigns it to a specialized Greenbook committee,
perhaps the Surface Committee, which looks at test results, and recommends approval or
rejection of the specification.
- The Greenbook Committee votes to approve or reject the specification.
Caltrans Previous
Testing
As documented in a memo from Caltrans dated July 24, 1996, Caltrans tested
"crushed" asphalt roofing shingles in spring 1996 as 10 percent of AB for
durability, and the results were favorable. Caltrans Office of Materials Engineering and
Testing Services recommends in the memo "that Caltrans respond favorably to any
contractor requests to substitute asphalt roofing shingles for virgin aggregate so long as
the resulting blend complies with Caltrans AB specifications." The memo also
emphasizes that the AB "be free from organic matter and other deleterious
substances," as the shingles can sometimes contain nails and/or wood. However,
Caltrans projects have not yet included shingles in AB, to CIWMB staff's knowledge.
CIWMB Proposal to
Caltrans
Though the above mentioned memo is a positive step, an SSP is still important to ensure
Caltrans backing and encourage market confidence.
Proposal
In October 1996, CIWMB submitted a proposal to Caltrans Pavement Design and Rehabilitation
Committee (PDRC) to allow ground shingles in AB. The proposal suggested that Caltrans:
(1) add an allowance of 10 percent ground tear-off shingles into an existing SSP that
currently allows crushed concrete and asphalt and various other reclaimed materials in
Class 1, 2, and 3 AB, and (2) conduct any tests needed to accomplish this.
Response
Caltrans responded with a letter dated October 25, 1996 stating that the 10 percent
shingle AB seems feasible; however, further tests are needed to resolve several technical
issues such as the possibility of deleterious materials, product uniformity, and material
handling characteristics. Financial support for the additional testing must also be
resolved. Further response from Caltrans regarding field testing is pending.
Mailing
To keep the industry informed on CIWMB/ Caltrans discussions, in November 1996 CIWMB
mailed a packet of information to 111 California recycled AB producers. The packet
included several letters from Caltrans and a memo outlining preliminary test procedures
and results.
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The most promising market at this time may be local government public works. Although
local governments usually use Caltrans or Greenbook specifications, they are free
to use any specifications of their choosing, or develop their own, for projects using
local funds. Local governments could start with specifications allowing 10 percent ground
shingles in AB as tested by Caltrans lab. Test projects could begin with parking lots and
low impact roads in urban areas where most of the tear-off shingle waste stream is
generated.
Private markets include private roads, parking lots, driveways, and farm roads.
Although many private owners also depend on Caltrans, Greenbook, or local public
works specifications, some may be willing to accept shingles in AB, especially if the AB
is tested for certain quality standards including the usual tests for gradation, R-value,
sand equivalent, and durability index. There are approximately 14 private laboratories in
California that test aggregate.
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CIWMB has published a series of fact sheets, case studies, and resource lists on
construction and demolition recycling. The are available on line via the Board's online publications
catalog, which includes a C&D
section. From the catalog you may also order hard copies by e-mail or phone.
Asphalt Shingle Recycling Home
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