In 1987, Kentucky put 40% power-plant bottom ash into 1 mile of State Route 3; later testing found excellent performance and strong skid resistance on the road

In October 1987, a one-mile stretch of State Route 3 in Lawrence County, Kentucky, became a test of whether waste from a coal-fired power plant could serve a useful purpose in road construction. Engineers incorporated bottom ash from the Big Sandy Power Plant near Louisa into an asphalt surface mixture, replacing part of the conventional coarse aggregate. The experiment used a mixture containing 40 per cent bottom ash. Centreweight, turning an industrial by-product into a component of a public road.

According to a report published by the University of Kentucky Transportation Center in August 1992, the experimental pavement performed well, with skid-resistance tests indicating that the surface offered strong friction. The project was noteworthy because road surfaces must do more than carry traffic: they also need to provide reliable grip, particularly when vehicles brake or turn. The results suggested that bottom ash could be a useful source of high-friction aggregate, although the findings applied to the tested mixture and road section rather than every type of ash or asphalt pavement.

An industrial by-product finds a place in road construction


Bottom ash is formed due to the settlement of minerals from coal at the bottom of the boiler during the combustion process. Unlike the fly ash, which is finer and carried by the flue gases, bottom ash is normally coarse with angular particles that have porous surfaces. As such, the uses of bottom ash in construction are different from those of sand and crushed stones. The bottom ash used in the Kentucky test was obtained from the Big Sandy Power Plant run by Kentucky Power Company. The University of Kentucky report indicated that about 539 tons of the experimental bituminous surface material were laid on the one-mile stretch. Bottom ash comprised 40 per cent of the mixture in dry form.

The angular nature of the material had some potential advantages since the aggregate pieces play a role in creating texture in asphalt pavements. This is because their effect on the tyre of a moving vehicle results in friction, but the non-polishing quality is critical because of the possibility of traffic causing the aggregates to become smooth over time. But the porous nature of bottom ash means that mix design becomes an important factor in the process, which is influenced by the interaction with the binder, water absorption, and other components. This study has therefore moved past the goal of getting rid of an industrial by-product and into whether this particular one can be used in practice.

Tests found strong skid resistance

However, the experimental road section proved to be successful in subsequent evaluations. The pavement was rated highly in terms of overall performance, while skid resistance tests indicated that the road had high friction resistance. This was significant because sufficient friction resistance ensures the tyre grips the road, especially when cars brake or turn.

These findings also revealed the possible use of bottom ash as a material that can be used as a high-friction aggregate that is resistant to polishing due to traffic and can serve to substitute some part of the coarse aggregates in the asphalt layer. This made it possible to use an industrial waste in road construction rather than discarding it. However, the above findings only apply to the mixture and the road section examined in Kentucky. Not all bottom ash is fit for asphalt production, nor will all mixtures including bottom ash prove equally successful.

A wider opportunity for recycled road materials

The experiment in Kentucky is an example of the wider interest that there has been in reusing industrial waste products in civil engineering. There is a huge amount of residue from the burning of coal, and reuse can conserve virgin materials. One area of application could be in road-building due to the need for mineral aggregate in asphalt mixes.

A review published in the journal Materials in 2020 examined the use of ash from waste combustion in construction, including historical road-paving trials in the United States. It illustrated the Kentucky case as an example of the application where ash was added to bituminous surface and proved to work well in terms of having high friction potential. This sort of implementation requires careful consideration of the material rather than the belief that the material will be suitable just because it is a waste product. It requires engineers to take into account the size of particles, absorptive nature, durability, environmental aspects, and compatibility with asphalt.

Still, the State Route 3 case has shown that there is a possibility to find an appropriate use of bottom ash. A waste produced at the power plant was included in a mile of public roadway, and its later assessment showed that it resulted in pavement that worked extremely well and provided great skid resistance. This case did not answer all the questions concerning the application of ashes in the asphalt but proved that it is possible to find an appropriate use for an industrial waste.