From roof to road: techniques are advancing to formulate a cold mix asphalt product using both RAP and shingles.

The benefits of using scrap and recycled shingles as a paving material have been well documented in recent years. Because the base components of shingles, including asphalt, mineral granules and fiber, are so similar to asphalt paving materials, ground shingles can be quite easily incorporated into hot-mix asphalt and other parching materials.

Many contractors who use shingles typically grind the shingles and screen them over a 1/2-inch or 3/8-inch screen. The shingles retained on the top size screen are normally ground again or disposed of. Dykes Paving, an Atlanta-based contractor that uses ground shingles regularly in its hot mix asphalt, has been experimenting with using these oversized ground shingles (plus-3/8 inch) in combination with course (plus-3/8-inch) recycled asphalt pavement (RAP) to make an inexpensive cold-mix patching/paving mixture.

In addition to being an inexpensive product for paving and patching, using this product could also reduce the stockpiles of excess oversize RAP and oversize shingles that the contractor currently deals with.

The purpose of our current study is to develop a low-cost cold mix for paving and patching containing 100 percent RAP and shingles that uses a significant portion of oversize RAP and oversize shingles.

TESTING AND RESULTS

The products available for blending were an oversize RAP, an oversize shingle and fine RAP. The 5- to 19-millimeter nominal maximum size shingle/RAP blends were developed based on recommendations from the contractor's previous experimentation. The as-received gradation of the RAP and shingles was used because it was thought that when incorporated into a mixture, the individual aggregate particles would not disperse, but remain in their conglomerate form.

Each blend was mixed at room temperature with 2 percent diesel fuel by weight of total mix (including diesel fuel). Two samples of each blend were mixed and then compacted in the gyratory compactor to 50 gyrations, which is the standard compaction level for lightly traveled roads and streets. The samples were immediately transferred into plastic molds used for rut testing in the Asphalt Pavement Analyzer (APA). All samples were then placed into a 60C (140 F) oven for 24 hours of curing. Upon completion of the curing cycle, the samples were tested in the APA for resistance to rutting.

The APA has primarily been used for testing rutting resistance of hot-mix asphalt. However, some users have developed test methods for testing cold-mix asphalt as well. One method developed for rut testing of cold-mix asphalt uses the following variables:

* Test temperature: 49C (120 F)

* Wheel load: 100 lbs.

* Hose pressure: 100 psi

* Number of cycles: 8,000

Using these test variables, a cold mix is considered acceptable if it demonstrates less than 7 millimeter rutting in the APA. Between 7 millimeter and 12 millimeter is considered marginal and greater than 12 millimeter is considered poor.

The single/RAP mixes were tested in the APA using the above described procedure, but it was found that most of the mixes developed deep ruts very rapidly. In most cases, testing of the sample was ceased before 8,000 cycles of the loaded wheel were applied in order not to damage the wheel and hose. Because of this, the results reported are based on the number of cycles required to develop a 12-millimeter rut depth. Using this form of analysis, the more cycles required to develop a 12-millimeter rut depth, the better the rut resistance of the mixture.

These results indicate that, in general, as the gradation becomes finer, the rutting resistance decreases, with the exception of one of the blends.

There may be several potential reasons for the relatively poor resistance to rutting test results found for these mixtures:

* Typically, for laboratory compaction of cold mixes, holes are drilled in the molds to allow any excess fluids (solvent or waste) to escape during mix consolidation. This was not done for this compaction and could have resulted in decreased density of the mix as well as in an excess of diesel fuel being left in the mixture;

* Although the samples were cured for 24 hours at 60C (140 F), diesel fuel could still be smelled in the samples during APA testing. This would indicate that the samples were not completely cured and may require a longer cure time or curing in molds that had holes drilled in them for more ventilation.

The appearance of the mixture is also an important factor. The finer the mixture, the smoother the surface texture. However, all five blends had a relatively smooth surface texture that may be considered acceptable.

CONCLUSIONS

Based on the above testing techniques, several conclusions could be reached:

* The combination of the blends described here and 2 percent diesel fuel resulted in a mixture that mixed relatively easily and appeared to consolidate well. This was demonstrated by the fact that the samples were easily transferred from a compaction mold to the APA mold with no degradation or decompaction of the samples;

* All of the blends resulted in a relatively smooth surface texture, with the finer blends being the smoothest;

* In general, as the gradation of the mixes became finer, the rutting resistance decreased;

* The rutting resistance of all of the mixes was generally below acceptable standards. However, the development of better compaction and curing methods for this type of mix may result in better resistance to rutting test results;

* The results from this testing indicate that one particular blend may be the best in terms of performance. Furthermore, this blend will use a greater quantity of excess oversize RAP and oversize shingles;

* Further testing of this same blend with better compaction and curing methods may result in more acceptable rutting performance. It may also be advisable to perform more testing to determine an optimum amount of diesel fuel to be used in this blend, which may also affect rutting results.

ALL IN ONE PLACE

The Construction Materials Recycling Association (CMRA) has been among the contributors to the www.shinglerecycling.org Web site, a central site offering case study summaries, research reports and links to other pertinent roofing shingle recycling Web sites.

The Web site offers information on potential end markets, processing techniques and environmental and permitting issues.

In addition to the CMRA, the U.S. EPA Region 5, the University of Florida and the National Roofing Contractors Association (NRCA) have also contributed to the site.

CLEARING THE AIR

The 2003 Asphalt Shingle Recycling Forum, which was held last spring in Bloomington, Minn., offered researchers, recyclers and other interested parties a chance to share information on advances in recycling asphalt roofing shingles.

One of the event's sessions featured a summary of shingle recycling research presented by Dan Krivit of Dan Krivit and Associates (DKA), St. Paul, Minn.

As part of his presentation, Krivit reviewed part of a larger shingles recycling research and development project being conducted for the State of Minnesota and funded in part by the Recycled Materials Resource Center (RMRC). Researchers tried to answer whether asbestos levels in batches of asphalt shingles will trigger regulatory oversight.

Applied Environmental Sciences Inc. (AES) conducted a series of air sampling as part of the larger project coordinated by Dan Krivit and DKA. Importantly, the air samples indicated "total fibers" were well within OSHA-regulated asbestos limits for workers.

According to Krivit, "AES provided expert field inspections, sampling, lab analyses and advice relating to the issue of asbestos management in shingles recycling."

In his presentation, Krivit noted that "Environmental and health/safety issues are a major component to the development of any new or amended specifications that would address tear-off roofing shingles" as a recyclable material.

A draft environmental white paper report was prepared by DKA and contains a summary of findings of fact, conclusions and recommendations for mitigating any asbestos or other health, safety, or environmental risks associated with the proposed tear-off shingles recycling demonstration. "A major component of the review of feasibility is this environmental assessment of risks and benefits, especially the review of alternative management schemes for the asbestos issue," said Krivit.

On the Web: MAKING THE MARKET

Read more about Dykes Paving and its use of asphalt shingles in last issue's cover story, found in the archives section of www.CDRecycler.com.

Mr. Collins is president of Pavement Technology Inc., and Mr. Vaughan is employed by Moreland Altobelli Associates, a consulting firm. Both are based in the Atlanta area.