Newark airport used coal waste in runways built for jumbo jets: What FHWA found 20 years later

Building runways capable of handling repeated loads from large aircraft requires pavement structures that can withstand heavy stress without excessive failure or deformation. A major example emerged at Newark International Airport in New Jersey during the 1970s, when engineers used coal fly ash as part of a stabilised pavement base beneath runways, taxiways and aircraft aprons.

Rather than being used simply as a disposal material, the fly ash was incorporated into a cementitious mixture containing lime, Portland cement and sand to form a structural base for flexible pavement. More than two decades after construction, a later Federal Highway Administration (FHWA) evaluation found that the stabilised pavement sections were still performing satisfactorily.

The project became an important early example of how a byproduct of coal combustion could be incorporated into long-life infrastructure. It also showed that the successful reuse of fly ash depended on engineering the material into a carefully designed pavement system rather than treating it as a straightforward replacement for conventional construction materials.

How Newark used coal fly ash in its pavement

Construction at Newark in the early to mid-1970s included work on runways, taxiways and aircraft aprons intended to accommodate the era of jumbo jets. FHWA research on stabilised fly-ash bases describes the pavement as a flexible system with a stabilised base beneath the asphalt surface.

The base was constructed in three levels, with slightly different mix compositions used across the sections. Fly ash accounted for about 10% to 12% of the mixture, while lime and Portland cement were used at roughly 3% to 4%. The completed stabilised base was between 24 and 36 inches deep, making it a significant structural layer within the runway pavement system.

The engineering principle relied on a chemical reaction between the fly ash and the activating materials. That reaction created a cementitious matrix capable of binding the aggregate together. Instead of behaving like loose fill, the resulting layer had properties closer to a low-strength concrete material.

FHWA classifies this type of construction as a pozzolan-stabilised base, designed to provide load-bearing support beneath flexible pavement. The material was therefore being incorporated as an engineered structural component intended to withstand the demands associated with heavy aircraft.

One of the mixtures containing 4% lime and Portland cement achieved ultimate strengths of approximately 2,000 to 2,600 pounds per square inch, according to the FHWA record. The figures indicate the extent to which the stabilised fly-ash mixture was being designed and used as a structural pavement material rather than simply as a method of disposing of coal ash.

More than 20 years of performance

The significance of the Newark project was not limited to laboratory testing. The stabilised pavement had to withstand years of actual service under demanding operating conditions.

When the FHWA later examined the pavement, the stabilised-base sections were reported to be functioning satisfactorily after more than 20 years of operation. The finding added to the longer US history of using fly ash in pavement bases. FHWA notes that fly-ash stabilised bases had been used in the United States since the 1950s, with the material combined with lime, Portland cement or other activators to create a binding matrix.

The long-term record, however, did not mean the material was without drawbacks. FHWA has noted that stabilised bases can develop shrinkage cracking, which may eventually reflect through the asphalt surface and increase maintenance needs. As a result, the performance of such pavement depends on factors beyond compressive strength, including mix design, curing and placement.

Why the mix design mattered

Fly ash is not a uniform material. Its characteristics can vary depending on its source and chemical composition, while the performance of the final pavement also depends on the aggregate, activating agent and moisture conditions.

FHWA guidance therefore treats fly ash as one part of a broader stabilised-base system. The proportions of the materials, the conditions during construction, curing and the development of strength all influence how the finished pavement performs.

The Newark project was particularly notable because the pavement was designed for the repeated loads associated with large aircraft. Its later performance demonstrated that a carefully formulated fly-ash mixture could serve as a structural pavement layer under demanding conditions, although it did not establish that all coal ash could be used in the same way.

From waste material to engineering resource

Coal fly ash was historically regarded primarily as a waste product generated by burning coal and therefore requiring disposal. Its use in stabilised pavement bases offered another option by placing part of that material into a regulated engineering application where its chemical properties could contribute to strength.

There was also an economic consideration. FHWA research found that stabilised fly-ash bases had generally been more economical than some alternative base materials in many locations. Actual savings, however, would depend on the availability of local materials, the mix design and construction conditions.

The Newark experience ultimately underscored a broader principle of material reuse: successful substitution depends on engineering rather than simply replacing one material with another. In this case, the fly ash worked as part of a planned system involving specific proportions, activating materials, curing and strength development. The fact that FHWA found the stabilised pavement sections performing satisfactorily more than 20 years later made Newark an early and notable example of coal ash being put to structural use in major airport infrastructure.