According to the National Asphalt Pavement Association (NAPA), the earliest HMA production units consisted of shallow iron trays heated over open coal fires. The operator dried the aggregate on the tray, poured hot asphalt on top, and stirred the mixture by hand. The quality of the mix usually depended on the skill and experience of the operator.
The Cummer Company opened the first central asphalt pavement production facilities in the U.S. in 1870. By the end of the 19th century, builders on both sides of the Atlantic were producing mixers and dryers in a variety of forms.
The first asphalt facilities
The first asphalt facility to contain virtually all the basic components we have today was built in 1901 by Warren Brothers in East Cambridge, Mass. (It lacked only a cold feed and pollution control equipment.) The first drum mixers and drum dryer-mixers, which came into use around 1910, were Portland cement concrete mixers that were adapted for use with HMA. Mechanization took another step forward in the 1920s with the improvement of cold feed systems for portable and semi-portable systems. Vibrating screens and pressure injection systems were added in the 1930s.
The asphalt plants of the early '50s might have included a dryer, a tower with a screed, and a mixer. They were dirty, dusty operations, says NAPA. But by the mid-'60s, with air pollution a serious concern across the country, many had added wet scrubbers and a few had baghouses.
The other major change in the mid to late '60s was the addition of surge bins and storage bins. Prior to that, everything was loaded right from the plant into the truck.
Today's asphalt production facilities have come a long way from the dirty, dusty operations of the '50s. What can we expect from asphalt plants in the next 20 years?
Keeping it dry
Dr. J Don Brock, chairman of Astec Industries, and Dennis Hunt, senior vice president of Gencor, agree that asphalt plants of tomorrow will most likely be enclosed in a building. "There are some plants both in the U.S. and internationally that are already doing this," says Hunt.
Besides keeping the plant "out of sight" from neighbors, the covered buildings will help keep aggregates drier. The drier the material is going in, the less fuel you will use drying it, which translates into energy savings.
"Covering aggregates will become more popular," says Hunt. "You'll see more of aggregates in containers that help leech moisture out."
The use of recycled materials – reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) will increase significantly in the coming years. These materials will also be stored in a building as well as crushed inside a building.
"Seeing 60 to 70 percent recycle will not be uncommon, and an indirect tube type dryer will be used to preheat the recycle prior to its entering a plant," says Brock.
"Over the next 20 years, I see a substantial increase in both fuel for drying and the cost of the liquid asphalt," he continues. "That will lead to the need to store the aggregate in a building to keep it as dry as possible; paving the inside of the building with slopes to drain away as much of the moisture contained in the material as possible, prior to picking it up with a loader or a loading facility. Most likely plants will have top loading capabilities, such as using a drag chain for pulling the driest material off of the top of the pile."