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

Construction & Demolition Recycling, March-April, 2004 by Ronald Collins, Kevin Vaughan

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;