- Pavement Terminology
- California Specifications
- Local Government Testing
- Private Markets and Testing
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.
The waste shingles are first ground and screened to produce the appropriate size pieces, then mixed with aggregate in the required percentage.
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 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.
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.
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.
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 CalRecycle staff's knowledge.
CalRecycle 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.
In October 1996, CalRecycle 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.
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.
Local Government Testing
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 and Testing
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.
CalRecycle has published a series of fact sheets, case studies, and resource lists on construction and demolition recycling. The are available on line via CalRecycle's online publications catalog, which includes a C&D section. From the catalog you may also order hard copies by e-mail or phone.