Representative image of workers laying a road base made with recycled incinerator ash. Image Credits: ChatGPT.
In 1974, Houston, Texas, tried an unusual material for road construction: ash left behind after municipal waste was burned. The mix contained 89% incinerator ash, 9% asphalt and 2% lime by weight, meaning that most of the material in the experimental road base came from waste rather than newly quarried aggregate. After three years of traffic, a 1977 study compared the littercrete section with a conventional hot-mixed asphaltic concrete base, known as blackbase. The report’s reference to an 89 per cent waste content described the composition of the littercrete mixture by weight. In Houston’s 1974 project, the aggregate used in a six-inch road base consisted entirely of incinerator ash. By total mixture weight, it contained 89 per cent incinerator ash, 9 per cent asphalt and 2 per cent lime. Another Houston project in 1977 used a road-base mixture containing 70 per cent ash. In 1974, Houston pioneered the innovative use of incinerator ash in road construction instead of disposing of it in landfills.
Because asphalt mixtures already required large amounts of aggregate, road construction seemed like a practical option. Three years later, the experimental section was reported to be performing almost like a conventional asphalt pavement. The littercrete section was reported to be performing well and was described as almost identical in appearance and performance to the conventional control section.
The project was an early example of turning a difficult waste stream into a construction material, according to the US Department of Transportation’s ROSA P. collection . Instead of sending the residue to landfill, engineers used it in an asphalt concrete base course on a city street. A road base made largely from waste In the Houston experiment, engineers used littercrete, a stabilised asphalt material in which incinerator waste replaced much of the conventional aggregate. The experimental and control sections were laid on a municipal street, with an asphaltic layer placed over the base. Researchers assessed the road through field observations and tests on core samples taken from the pavement. The results were encouraging. What happened after three years? This project was part of a broader effort to find a use for the ash left behind when municipal waste was burned. Incineration reduced the amount of waste going to landfill, but it also created a new problem: what to do with the ash. A later Federal Highway Administration review recorded several projects that tested municipal waste combustor ash in roads. The littercrete mixture itself was different. These projects showed that incinerator ash could be used in road construction rather than simply being sent for disposal. However, they were still demonstration projects, so they did not establish that ash could replace conventional road materials on a large scale. They were early attempts to turn a waste product from incineration into a useful construction material. Image Credits: Wikimedia Commons. An early lesson in waste-based construction
Thus, the test Houston carried out in 1974 is located in an interesting position within the chronology of circular construction. However, what made the Houston road project noteworthy was that it showed an asphalt base containing a large proportion of incinerator ash could remain serviceable for several years under traffic. Environmental concerns were also part of the discussion. Municipal waste incinerator ash can contain soluble salts and trace metals, making proper processing and testing important before it is considered for construction use. Historical FHWA reports on the use of incinerator ash in construction identified advantages such as high stability and low unit weight, but those findings related to the projects and assessments of that period rather than a current recommendation for the unrestricted use of municipal waste incinerator ash. A study in the Journal of Cleaner Production found that the performance of asphalt mixtures containing municipal solid waste incinerator bottom ash depended on factors such as ash porosity and replacement rate. Higher ash contents could reduce high-temperature rutting resistance, while certain ash characteristics could improve moisture and low-temperature performance. The test suggests that waste material, which had hitherto been thought of as a waste product fit only to be disposed of in the incinerator, can actually be used to construct functional roads. The test does not present a method for building such roads, but it demonstrates their feasibility. You use AI every day. Now get your AI Quotient. Take the AIQ test.