Shingles in Hot-mix Asphalt Pavements

Problem Statement

Each year, roofing manufacturers in the U.S. produce approximately 1,000,000 tons of new waste shingles and shingle trimmings. Moreover, residential and commercial roofing replacement generates 8 to 10 million tons of old roofing waste. Historically, about 95 percent (or 22 million cubic yards) of this valuable, non-biodegradable solid waste has ended up in landfills.

More than 500 million tons of asphalt mixture are produced annually in the U.S., about 90 percent of which is hot-mix asphalt (HMA). Using approximately two percent roofing waste in all asphalt mixtures placed would consume all manufacturing and consumer waste roofing produced each year.

Objectives

The Texas Transportation Institute (TTI) conducted study 0-1344 "Roofing Shingles and Toner in Asphalt Pavement", for TxDOT, the Texas Commission on Environmental Quality (TCEQ), and the Federal Highway Administration (FHWA) to evaluate the inclusion of roofing shingles from manufacturing and consumer waste streams in hot-mix asphalt.

Findings

Researchers examined published and unpublished information on the use of waste roofing materials in HMA. The literature search showed that several small laboratory efforts and field tests using waste shingles in HMA have been conducted.

Researchers then performed laboratory experiments on asphalt mixtures containing manufacturing waste and consumer waste. Manufacturing waste shingles are defined as new material that is obtained from a roofing shingle production plant. Manufacturing waste is a very uniform, predictable product and is the most promising source for rapid implementation of these study findings. Consumer waste shingles are those obtained during the removal of existing roofs, and make up the bulk of the scrap shingle supply. Consumer waste is inherently quite variable in quality and may be contaminated with various other building construction materials. However, consumer waste has been successfully used in hot mix asphalt (HMA) as much or more than manufacturing waste.

Two types of HMA were modified with ground roofing and tested in the laboratory. These included a dense-graded, Type D mixture and a coarse matrix-high binder (CMHB) Type C mixture. The roofing materials were added to the HMAs at 5% and 10%, and the engineering properties of the resulting HMA mixtures were compared to untreated mixtures. Laboratory tests measured the effects of the roofing wastes on Hveem stability, indirect tension, resilient modulus at several temperatures, moisture susceptibility, TxDOT static creep, air void contact, and voids in the mineral aggregate.

The findings of this study indicate:

1. Roofing waste can be used in HMA.
2. It should be possible to produce materials that will meet TxDOT's standard quality control specifications using up to 5% roofing waste into typical HMA surface mixtures and up to 10% roofing waste in base mixtures.
3. Roofing waste must be shredded in order to use it successfully in asphalt mixtures. The finer the roofing is shredded, the better it is for incorporation into asphalt mixes.
4. A CMHB mixture with higher voids in the mineral aggregate and thicker asphalt film can accommodate roofing wastes better than a dense-graded mixture.
5. Mixing and compaction temperatures of HMA may need to be increased in the laboratory and in the field to accommodate the relatively stiffer roofing-modified mixtures. Standard compaction temperatures may yield higher air voids than desirable.
6. Standard TxDOT hot mix asphalt design procedures appear satisfactory for designing mixtures containing roofing waste.
7. Standard TxDOT hot mix asphalt design procedures appear satisfactory for evaluating mixtures containing roofing waste.
8. For purposes of mixture design and construction, waste roofing can be handled using the techniques already established for reclaimed asphalt pavement (RAP).

Implementation

Guidelines and specifications have been developed for constructing asphalt concrete pavements containing scrap shingles which addresses mix design, shingle processing, plant production, placement and compaction, and shingle-use specifications.

In general, manufacturing roof waste is preferred over consumer waste because: it is more uniform in content; the asphalt is softer and more functional in a paving mixture; it is not contaminated with nails, wood, and other deleterious materials, it contains no organic roofing felt or asbestos. Deleterious materials can generally be removed from roofing materials torn off old roofs by screening the material over a 12.5 mm or smaller sieve prior to incorporation into the hot mix asphalt.

It is probable that a high percentage of the roofing waste generated in Texas cannot be used in cold-applied paving materials because of this application's relatively low volume when compared to hot mix asphalt. Nevertheless, the hard asphalt, fibers, and angular fine aggregate in ground roofing waste appear to have potential not only for use in cold-applied asphalt maintenance mixtures but also in actually improving their quality.

The contents of this summary are reported in detail in The Texas A&M University System's Texas Transportation Institute Research Report 1344-2F, "Shingle and Toner In Asphalt Pavements," Joe W. Button, Devon Williams, and James A. Scherocman, Preliminary Report Dated - November 1995. This summary does not necessarily reflect the official views of the FHWA, TCEQ, or TxDOT.