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Utilization of Recycled Materials in Illinois Highway Construction

Bottom Ash

Bottom ash is produced in a dry-bottom coal boiler often found in coal-fired electric power plants. The coal is pulverized and blown into a burning chamber where it immediately ignites; the incombustible portion of this material not collected in the flue as fly ash is known as dry bottom ash, which drops down to a water-filled hopper at the bottom of the boiler (1, 4).
Physical Properties:
Bottom ash is a coarse, angular material of porous surface texture and size ranging from fine gravel to fine sand, predominantly sand-sized. This material is composed of silica, alumina, and iron with small amounts of calcium, magnesium, and sulfate; as a whole, the quality of the material is governed by the coal source, not by the type of furnace (1).
Engineering Value:
Bottom ash may contain pyrites or "popcorn" particles that result in low specific gravities and high losses during soundness (i.e. freeze-thaw) testing. Due to the inherent salt content and in some cases, low pH this material may exhibit corrosive properties. This material is highly susceptible to degradation under compaction and loading; as a result, bottom ash is not an acceptable aggregate for most highway construction applications (1, 4, 12).
Potential Application:
Other states have utilized bottom ash for snow and ice control, as aggregate in lightweight concrete masonry units, and as raw feed material for portland cement. This material has also been utilized as an aggregate in cold mix emulsified asphalt mixes, base or subbase courses, or in shoulder construction, where the gradation and durability requirements are not as critical. West Virginia and Texas researchers conducted a study in which some of the observations made concluded that performance depends on the amount of pyrites and sulfates present. Also, the quality of the material depends upon how the material was stockpiled before use (1, 4, 12).
Department Concern:
Besides the concerns noted above, bottom ash is considered a problematic debris, which plugs drainage structures when used for snow and ice control.
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John Bukowski
Office of Asset Management, Pavements, and Construction
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Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration