Just about everybody knows when it starts raining in your living room, it’s probably time for a new roof. However, not everybody knows what can be done with those roof shingles once they come off your house. Turns out, the roof over your house and the road in front of it have a lot in common. Both are held together with asphalt, a petroleum based binder. This does not mean that it is acceptable for you to drive on your roof, or anyone else’s for that matter. However, the presence of asphalt in roofing shingles does mean that recycled shingles can be used as a component in an asphalt pavement. This practice has quickly been gaining acceptance throughout the U.S. and has a number of environmental and economic benefits. In this edition of the RoadReady newsletter, we will explore recycled asphalt shingles, how these materials are incorporated into asphalt pavements, the benefits of doing so, and necessary considerations.
Types of Shingle Waste
Two types of shingle waste streams exist that can be recycled in pavements. The first, manufacturer waste shingles, come directly from manufacturers of asphalt shingles, and are desirable because the composition of the material is known to a high degree.
The second type of waste shingle is known as “tear-off” and represents those shingles removed during re-roofing or roof removal projects. While this is a much larger source of shingle waste, issues can arise due to the presence of deleterious or harmful material such as wood, asbestos, and nails. This generally means that regulations are stricter regarding the use of tear-off shingles. Recycling these shingles generally requires sorting of material from other construction wastes and proper inspection for the presence of asbestos. In addition to their greater abundance, these shingles are also distributed more evenly across the U.S. than manufacturer waste, which is concentrated where shingle plants are located.
Properties and Components of Asphalt Shingles
Asphalt shingles make up the majority of roofing in the United States. These shingles can contain between 20% and 36% asphalt, which is used to bind a number of other materials such as fine aggregate, granular aggregate, fiberglass, or organic felt and hold the shingles together. This is quite analogous to a roadway surface, where asphalt is used to bind together blends of aggregate to form a cohesive structure. Some older generations of shingles contain asbestos, a group of compounds which can be extremely harmful if inhaled. While the production of shingles using asbestos was discontinued in the late 70’s and early 80’s, asbestos-containing shingles are still present in tear off waste. If these shingles were to be used in asphalt mixes, there would be a significant health hazard to roadway workers. As a result, an important part of shingle recycling is screening for shingles that may contain asbestos through regular testing. That said, because of the 12 to 25 year lifespan of shingles and their discontinued use, shingles with asbestos are rapidly disappearing.
Availability of Shingle Waste
Millions of tons of asphalt shingles are disposed of in the U.S. every year. Most of these are roof tear-offs, which account for about 7-10 million tons annually. Manufacturers produce about a tenth of this amount in factory waste. While tear-offs are produced just about everywhere in the U.S. Manufacturer waste is only produced at shingle manufacturing facilities. About 30 states are home to such facilities, making manufacturer wasted shingles more available to these regions. A study of municipal waste in Wisconsin showed that shingles were the third highest source of landfill material behind wood and food scraps. So until we can start recycling our coffee grounds and banana peels into our pavements, shingles seem like a pretty good option.
Use of RAS in Asphalt Pavements
A number of DOTs throughout the United States have some level of experience using recycled asphalt shingles in asphalt pavements. Before use in pavement, shingles must be processed by screening for deleterious materials before undergoing grinding into smaller particles. This makes the asphalt in the shingles more available to other materials in the hot-mix. The maximum size specified for the ground shingles is usually on the order of one quarter inch to a half inch. However, the smaller the particle size, the better the RAS will typically perform in a roadway. In general, state DOTs that allow the use of RAS limit the shingle content of pavement to 5%. Of these states, many only allow manufacturers waste and prohibit the inclusion of tear-offs.
The addition of asphalt shingles to asphalt mix also allows reduction of virgin asphalt use. This is one of the primary benefits of shingle recycling. Experimental studies have shown extreme values of replacement of up to 50% of the asphalt binder in a mix, but values like 20-30% are more realistic for most cases. In addition, the smaller particles that make up the shingles can reduce the amount of virgin fine aggregate needed.
Benefits of Using RAS
There are several benefits associated with the use of RAS. These include environmental and economic benefits, as well as positive effects on the actual roadway surface. Many of the economic and environmental benefits are associated with the reduced amounts of virgin asphalt required when RAS is added to the asphalt pavement.
- Cost savings from reduced virgin aggregate needs
- Cost savings from reduced virgin asphalt needs
- Avoidance of landfill tipping fees
- Increased rutting resistance
- Reduced cracking
- Stiffer mixes
- Higher density with less compaction effort
Considerations in Using RAS
When deciding whether or not to use shingles on a project, there are a few things to watch out for. The added stiffness that mixes exhibit when asphalt shingle waste is incorporated can increase the risk of thermal cracking in colder climates. In addition, in some cases mix working time can be reduced, and moisture retention by the pavement can increase. The processing of shingles requires facilities for screening of deleterious materials, grinding of shingles and appropriate space for storage. These capital investments require consideration of the extent of which shingles can be used in the long term. Finally, shingles tend to wear down equipment, including both processing equipment and standard paving equipment.