Is making asphalt one of the black arts? Better believe it.
For sure, the end product is black. And there's art in precisely controlling the content and quality of the final mix.
At the Shelly facility in Coshocton, Ohio, the artistry begins as sand and gravel is dredged from a pit on site, screened and sorted by size. (Plant operator Shelly Company is a division of Oldcastle Materials, North America's largest manufacturer of building products and materials and the Number 1 asphalt producer in the United States.) When the big mixing drum, powered by four 20-hp electric motors, starts turning at six to seven RPM, that aggregate is transferred by conveyor belt into the upper end of the drum. The exact amount is metered, using calibrated conveyor belt scales and asphalt meters.
There a powerful 98 million BTU burner, fueled by waste oil at this plant, dries the tumbling sand/aggregate mix—the proportions and sizes of particles precisely tailored for the final use—and heats it to around 400o F (205° C). The mix also includes fine particles recycled from the bag house, a giant air filter nearby powered by two 100-hp, 480 V motors.
Next bitumen, a dense petroleum byproduct, flows through heated supply pipes into the drum, where it coats the spinning particles. Operators carefully control both the temperature of the bitumen and its hardness when it cools. A harder mix might be needed as the final layer in a highway paving project, where it must resist years or decades of heavy traffic, while softer bitumen might be used for lower layers.
Then ground, recycled asphalt is added to the mix. Precision is imperative here too, as the percentage of recycled material allowed is closely regulated. This material, ground off old highways before they are resurfaced, is added at ambient temperature and cools the asphalt mix to a final temperature of around 290o F (143° C).
Now the mix, produced at up to 250 tons per hour, is ready to be trucked to the jobsite. But the wizardry isn't over. For drivers zooming over a smooth and seamless new highway, the magic rolls on and on.