15 Sept 99
l. Roofing shingles. In 1995, the yearly U.S. production of waste roofing shingles
was estimated at 12 million tons (Ali et al. 1995). Roofing shingle waste generated from
manufacturing operations is suitable for use in either hot-mix or cold-patch materials.
Roofing shingles obtained from roofing or demolition contractors are generally too
contaminated to be useful, without extensive processing, as a paving material (Paulsen
et al. 1987). In 1994, three state DOTs indicated that they had used roofing shingles in
asphalt mixtures. One application, in Illinois, involved using the shingles as aggregate in
a cold-patch material. Another study evaluating the use of shingles in asphalt paving
mixtures found the roofing shingles could be satisfactorily placed in both dense and SMA
mixtures (Newcomb et al. 1993). In 1993, a Minnesota DOT pavement containing 5 to 7
percent shingles, by weight, reported good performance after two years (FHWA 1993).
A laboratory study in Canada found that satisfactory hot-mix asphalt mixtures could be
made with up to 25 percent by weight of the mixture being roofing shingles (Ali et al.
m. Silica fume. Silica fume (condensed silica fume) or microsilica is a by-product
in the manufacture of silicon and ferrosilicon alloys. Nearly 100,000 tons of silica fume
are produced each year. Silica fume is pozzolanic because of its high silica content and
its high specific surface area. Silica fume is 10 to 20 times finer than fly ash. Silica fume
is marketed as either a powder or a slurry for partial replacement of cement in PCC
Research has shown that silica fume increases the bond between the paste and the
aggregate. This occurs when finely divided amorphous silica particles combine with
available lime to form a calcium silicate hydrate. This requires good dispersion of the
silica fume throughout the mixture. High-range water reducers or superplasticizers are
generally required. Silica fume, reduces permeability, improves resistance to freezing
and thawing, and improves resistance to chemical intrusion. Because of these properties,
silica fume is widely used for bridge decks, parking garages, and other surfaces requiring
a low permeability surface. Silica fume is usually used at a rate of 10 to 20 percent by
weight of cement (Collins and Ciesielski 1994).
Several state DOTs have used or are investigating the use of silica fume in PCC mixtures.
The CE has also investigated and recommends the use of silica fume where it would be
economically feasible. Many standard applications of concrete pavement do not require
the special properties of silica fume.
n. Sulfate waste. Sulfate wastes are mainly a by-product of the wet scrubbing of
flue gases at coal-burning power plants. The by-product generated contains various
forms of calcium sulfate, and is referred to as flue gas desulfurization (FGD) sludge. In
the early 1990s there were at least 52 web-scrubbing systems, with more planned for
construction. These plants generated approximately 18 million tons of FGD sludge
annually. To be a useful construction material, the sludge must be dewatered.
Combinations of lime-fly ash, cement fly ash, or portland cement can be used to stabilize