By John Davis, Contributing Editor
Asphalt is a truly versatile material. As an example, look at asphalt shingles. Asphalt is used to make the shingles for roofs. Then, at the end of their service life, the shingles can become part of an asphalt pavement.
Composed of liquid asphalt, hard rock granules and mineral filler, recycled asphalt shingles (RAS) have a number of uses. They can be used in hot mix asphalt (HMA) pavement mixes or as cold patch mixes on roads and streets. They can be used as a subbase in road construction, for temporary roads, driveways and parking lots, or to minimize dust on rural and unpaved roads.
Recycled shingles can be either manufacturer’s scrap or recovered material—“tear-offs” from old roofs. The scrap pieces generated by the shingle manufacturer consist primarily of tab punch-outs, off-colors, damaged shingles, ends and beginnings of rolls, and flat-roof polymer modified membranes. Recovered scrap material is usually tear-offs from residential roofing jobs.
Asphalt shingles provide a significant source of liquid asphalt for asphalt mixes. Currently more than 11 million tons of asphalt shingles are wasted each year. To give a sense of the potential value of RAS, consider the following: If you multiply 11 million tons times the current cost of asphalt per ton, you have a potential value of more than $1 billion.
Hot mix asphalt is currently the largest market for RAS. HMA uses both waste from newly manufactured shingles and old tear-off shingles from residential roofs. There is usually about 20 percent asphalt in new shingles, but old tear-off shingles contain more asphalt. The old shingles have lost as much as 50 percent of their granules, leaving a higher percentage of asphalt.
Waste from shingle factories can be cut or ground up and added to the HMA process or regenerated with rejuvenating chemicals prior to the HMA process. Recovered RAS is typically ground to approximately ¼-inch and passed under a magnetic separator in order to remove the nails.
Asphalt pavement properties usually improve with the addition of RAS. Research shows that RAS improves rutting and cracking resistance and the organic fibers reinforce the pavement.
RAS in asphalt cold patch has been used in several states for a number of years. New Jersey, Washington and California use it as well as the city of Chicago. The purported advantages include a longer life compared to other patch materials due to the fibers from the felts and the fiberglass in the shingles. The RAS cold patch can be stored longer because it does not “clump” as quickly as other materials.
Recycled asphalt shingles in an asphalt mix can be used to cover rural, unpaved roads. The dust is minimized, loss of gravel into side ditches is reduced and vehicle noise is reduced. The country roads have a longer life and require less maintenance.
RAS has been used in temporary roads, driveways and parking lots. The tear-off shingles are ground and passed through a magnetic separator, then spread and compacted to make a functional temporary road or parking lot.
RAS has also been used as a subbase in road construction. The RAS can be blended with RAP to provide a compactable subbase. RAS oftentimes improves the compaction of the subbase.
Several states have sponsored laboratory and field studies on recycled asphalt shingles. Ohio, North Carolina, Minnesota and Texas have performed laboratory studies; Pennsylvania, North Carolina and Minnesota have conducted field studies. Missouri began using RAS in 2005 and plans to use it in 36 projects in 2008. The studies have shown that recycled asphalt shingles provide:
- Increased asphalt stiffness
- Decreased cracking
- No effect on moisture sensitivity
- Decreased rutting susceptibility
- Decreased optimum content of virgin asphalt cement.
Laboratory studies of scrap asphalt shingles show that they improve rutting resistance and result in less temperature susceptible asphalt mixes. Also, in an Ontario study, HMA mixes using up to 5 percent shingles experienced lower tensile strength at 18ºC, which improves the resistance to low temperature cracking. But the use of tear-off shingles may cause the HMA to become brittle, which may contribute to low temperature cracking.
Asbestos in Shingles
The presence of asbestos in recycled tear-off shingles is still a concern among government agencies. The Chicago Environmental Protection Agency took 27,000 samples of asphalt shingles and found that 1.5 percent contained trace amounts of asbestos. The roofing caulk or sealant contained the fibers, not the shingles.
5 Percent RAS in HMA
Currently, state agencies such as Minnesota, New Jersey and North Carolina allow up to 5 percent processed shingles in HMA. Experience in blending shingle asphalt binder with virgin asphalt binder showed that asphalt shingles can cause increased stiffness in the recycled binder. Use of 5 percent singles in the HMA can increase the Performance Grade high temperature of the blended asphalt binder by one grade.
Typically, states are allowing up to 5 percent RAS in HMA mixes. Florida, Georgia, Indiana, Maryland, Michigan, North Carolina, New Jersey and Pennsylvania all allow up to 5 percent manufacturer’s scrap shingles.
RAS in SMA
Research work with processed asphalt shingles in Stone Mastic Asphalt (SMA) shows that the use of shingles may successfully replace the use of cellulose fibers. A field performance evaluation in Waterloo, Ontario, Canada, compares asphalt control sections with SMA asphalt test sections containing processed manufactured scrap shingles.
The control section was paved with a mix using 85/100 penetration grade asphalt binder and the test sections were paved with 3 percent manufactured shingle (modifier) using 150/200 penetration graded asphalt binder. The traffic in 2002 on Highway 86 was 19,400 ADT with 9 percent commercial trucks. The truck traffic has remained constant since 1999. The estimated high and low temperatures on the pavement surface was 54ºC and -25ºC. PG 58-28 was the standard binder.
Pavement Condition Survey
The visual condition survey indicated that there was extensive, moderate to severe centerline cracking in the control section. There was also slight wheel path fatigue cracking and moderate to extensive pavement edge cracking in the control section.
The RAS test sections showed minimal mid-lane and pavement edge cracking. However, a significant portion of the longitudinal construction joint was exhibiting medium to severe cracking in the test section at the interface with the acceleration ramp. The test sections showed an excellent centerline joint and excellent pavement appearance. The PCI rating for both the control and test sections was 88.
Although the various uses of recycled asphalt shingles for roads are still relatively small, their use in HMA and for other road-related purposes continues to grow. The recent Missouri DOT showcase, “Utilization of Recycled Asphalt Shingles in Hot Mix Asphalt,” drew nearly 150 participants from 22 states and Canada. Interest in using asphalt shingles will increase as the cost of highway construction rises and the awareness of shingles as a significant source of asphalt increases.
|A useful resource for additional information on the use of recycled asphalt shingles is the website www.shinglerecycling.org.