Engineers mixed coal fly ash into the pavement base at Newark International Airport in the early to mid-1970s. The new runways, taxiways and aprons had to carry jumbo jets. Sections of that base reached 36 inches (914 millimeters), and the Federal Highway Administration (FHWA) calls it the largest such project reported in the United States.<\/p>\n\n\n\n
The same passage ends with a short verdict on this type of base, although that sentence does not name the airport. “After 20 or more years, the PSB pavements continue to perform satisfactorily,” says the FHWA guideline on fly ash in stabilized base<\/a>. PSB stands for pozzolan-stabilized base.<\/p>\n\n\n\n FHWA describes a flexible pavement system with a base of lime, Portland cement, fly ash and sand. That base had three layers, and each one used a slightly different recipe.<\/p>\n\n\n\n Fly ash made up 10 to 12 percent of the mixes. Lime and cement together came to just 3 to 4 percent.<\/p>\n\n\n\n One mix used 4 percent lime and cement, with crushed stone and sand as aggregate. Its ultimate strength reached 2,000 to 2,600 pounds per square inch.<\/p>\n\n\n\n For scale, FHWA’s lab procedure asks for more than 400 pounds per square inch after seven days. Those two numbers describe different ages, so they only hint at how much strength the material can add over time.<\/p>\n\n\n\n According to EPA, fly ash is a very fine powder, mostly silica, left by burning finely ground coal in a boiler. Most of these bases have used Class F ash. That type needs an activator such as lime or cement.<\/p>\n\n\n\n Add water and an activator, and the powder forms cementing compounds that bind stone and sand. Engineers call this a pozzolanic reaction. FHWA’s fly ash manual for highway engineers<\/a> says the method has “a long and successful history.”<\/p>\n\n\n\n Cutting cement remains a live goal today, as in work on graphene-infused concrete that needs less cement<\/a>.<\/p>\n\n\n\n The reaction between activator and ash also keeps going, so the base can mend itself to a degree. FHWA credits these mixes with an “inherent ability to heal or re-cement cracks.” Pavement age, moisture and contact between the broken faces all affect the result.<\/p>\n\n\n\n At 40\u00b0F (5\u00b0C) or lower, very little strength develops inside the material. A base that enters winter too weak may not survive repeated freezing and thawing.<\/p>\n\n\n\n Northern states responded with calendar cutoffs for placing these bases. Those dates ran from September 15 to October 15, depending on location or lab results. FHWA also recommends paving over the base with asphalt within seven days.<\/p>\n\n\n\n Lab work comes first. FHWA’s procedure tests five fly ash proportions, then five activator combinations, and adds extra activator as a safety margin in the field. Rubberized asphalt made from old tires<\/a> faces similar questions about how recycled ingredients behave on the road.<\/p>\n\n\n\n The same guideline names cracking in the base course as the major concern for highway engineers. Shrinkage causes those cracks more often than fatigue does.<\/p>\n\n\n\n Such cracks usually reflect up through the asphalt, and that raises long-term maintenance costs. Sealing them at the surface keeps water out and protects the layer, FHWA says. Even so, the agency listed the problem as unresolved, with “no universally accepted procedure” for minimizing it.<\/p>\n\n\n\n U.S. coal-fired utilities generated about 63.6 million tons of coal combustion residuals in 2024. EPA reports that number from an American Coal Ash Association survey.<\/p>\n\n\n\n Giving that material a job has clear appeal, much like sending old wind turbine blades to a cement plant<\/a>. EPA’s page on coal ash reuse<\/a> lists fewer virgin resources, lower disposal costs and stronger materials as potential benefits. It encourages reuse “in an appropriate and protective manner.”<\/p>\n\n\n\n That wording has a reason behind it. Coal ash contains mercury, cadmium and arsenic<\/a>, and EPA warns they can reach waterways, groundwater and air without proper management.<\/p>\n\n\n\n Federal rules may also shift. On April 9, 2026, EPA proposed revising its definition of beneficial use and excluding specific uses from federal coal ash regulations.<\/p>\n\n\n\n FHWA’s account of Newark gives no tonnage, no cost and no carbon estimate. Its “satisfactorily” also dates from the 1990s and describes no inspection method, so it cannot show the condition of those layers now.<\/p>\n\n\n\n In 2014, EPA judged fly ash bound into concrete an appropriate use, but that evaluation did not cover Newark’s runway base. Neither FHWA’s account nor EPA’s reuse pages include a leaching or groundwater study of the site.<\/p>\n\n\n\n Long service answers the engineering question and leaves the environmental one open. The technical summary appears in FHWA’s User Guidelines for Waste and Byproduct Materials in Pavement Construction<\/a>, publication FHWA-RD-97-148.<\/p>\n\n\n\n Photo: Jibreel23 \/ Wikimedia Commons, CC BY-SA 4.0<\/p>\n","protected":false},"excerpt":{"rendered":" Engineers mixed coal fly ash into the pavement base at Newark International Airport in the early to mid-1970s. The new … <\/p>\nThree mixes under Newark<\/h2>\n\n\n\n
Chemistry borrowed from smokestacks<\/h2>\n\n\n\n
September deadlines for northern crews<\/h2>\n\n\n\n
Cracks that climb through asphalt<\/h2>\n\n\n\n
Waste with a toxic side<\/h2>\n\n\n\n
Newark’s groundwater record stays blank<\/h2>\n\n\n\n