For decades coal fly ash has been used as mineral filler in thousands of products and processes. From low-tech soil stabilization in highway construction to high-tech plastics for spacecraft, fly ash is used to increase quality and technical performance, usually while reducing cost. So it is not surprising that fly ash is now being used in the highest quality asphalt pavements ever produced.

Stone Matrix Asphalt (SMA), an European mix design philosophy imported to the United States in the early 1990’s, is the preferred asphalt pavement for the heaviest traffic/ tire-weight interstate highways in Georgia. Now other states are also redesigning their asphalt pavements to incorporate this highly rut-resistant, durable mix design. SMA is a gap-graded hot-mix asphalt pavement with high asphalt cement and high mineral filler contents. SMA often employs fibers and polymers as stabilizing agents.

The key to SMA’s rut-resistance is the “stone-on-stone” contact that provides a skeleton to support the pavement under constant loading from continuous heavy truck traffic. The key to SMA’s long durability is the binder of mineral filler, asphalt cement, and usually fibers, which provides a dense, stiff mortar that binds the mix together.

In order to maximize the life of SMA pavement, clean, well-graded mineral filler must be used. Fly ash, produced and collected according to SEFA’s quality control program, provides the high-quality mineral filler required for SMA.

SEFA has marketed Department of Transportation approved fly ash for mineral filler in SMA since the early 1990’s. SEFA continues to lead in the research and implementation of coal fly ash as mineral filler in SMA around the country through the American Coal Ash Association, National Center for Asphalt Research, and various state Departments of Transportation.