ASPHALT ANSWERS

By: Prithvi "Ken" Kandhal, P.E.

Roofing shingles reduce cost of HMA It is estimated that 11 million tons of waste roofing material are generated annually in the U.S. This is more than four times the 2.5 million tons of scrap tires generated each year. However, during the last five years, considerable time and resources have been devoted to researching the use of scrap-tire rubber in hot mix asphalt (HMA) pavements, whereas almost negligible research effort has been directed at the use of waste-roofing material (shingles) in HMA.

Unlike the use of crumb rubber from scrap tires, the use of roofing shingles is both technically feasible and has the potential of providing significant economic benefits by reducing overall cost of HMA mixtures . Such a use also eliminates the environmental concerns and costs associated with the disposal of roofing shingles in landfills.

Of the estimated 11 million tons of waste-roofing material, about 10 million tons come from roofing replacements or "tear-offs". The remaining million tons is generated as a waste (seconds or factory scrap) from new roofing-shingle manufacturing. Most of the waste-roofing material is currently disposed of in landfills at a cost ranging from $18­p;$45 per ton.

Roofing shingles contain a substantial amount of asphalt cement binder (about 30% in organic shingles and 20% in fiberglass shingles). Therefore, the use of roofing shingles in HMA will reduce the amount of required virgin asphalt binder. Moreover, asphalt roofing shingles contain high grade mineral filler and cellulose fiber, which are very likely to enhance the engineering properties of the HMA.

Field trails in Florida, Indiana and Minnesota, states which generally use 5% roofing shingles in HMA, have shown encouraging pavement-performance results. The commercially available, shingle-shredding equipment, has been developed and modified based on the experience gained in the field trails. It has been estimated that by introducing 5% organic shingles, the HMA cost can be reduced by $2.79 per ton, which is economically very attractive.

There is a need for a concerted research effort (similar to the efforts devoted to scrap-tire rubber) by the highway agencies and the industry for utilizing waste-roofing shingles in HMA. Mix design procedures should be developed for HMA containing both "tear-offs" and scrap waste from the manufacturing process. Construction of field demonstration projects also should be encouraged by state highway agencies. The use of roofing shingles in HMA is very likely to provide an excellent opportunity for reducing the overall costs of HMA and enhancing its engineering properties, while eliminating a major environmental problem.

About The Author: Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.