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What is “Asphalt Milling”? And How Does It Work?

It may be a little jarring to drive on a stretch of road that’s been scraped. But milling is the cost-cutting, environmentally friendly way to improve pavement.

An environmentally meaningful innovation came out of the oil shocks of the 1970s that is often overlooked. It is asphalt milling, also referred to as cold planing, pavement milling and profiling. It’s a pavement improvement technique used by asphalt paving contractors when the surface of aging asphalt pavement is scraped off, which later is overlaid with new asphalt that is partially composed of what was scraped.

So what does this have to do with the 1970s? A 1973-74 steep uptick in oil prices happened coincidentally with an eco-consciousness of that era (asphalt milling technology goes back to the earlier parts of the 20th century, but went largely unused for decades). When the cost of asphalt binder, which is almost entirely based in petroleum and its byproducts, increased by nearly 400% from a supply-limiting decision of the Organization of Arab Petroleum Exporting Countries (OAPEC), the asphalt pavement industry began to seriously consider recycling existing pavement to counter the cost burden. Note this was just a few years after the first Earth Day in 1970, a symbol the modern environmental movement.

It was wildly successful and widely adopted by contractors who specialize in asphalt repair as well as new asphalt paving. According to the US Environmental Protection Agency and the Federal Highway Administration, about 80 million tons of asphalt material is reclaimed through pavement milling every year, and of that almost 100 percent is recycled for reuse again on roads, streets and highways. Compare that to recycling rates for aluminum cans (60%), newsprint (56%), soft plastic drinking bottles (37%), glass beverage bottles (31%) and magazines (23%).

By reducing demand for oil-based binder and aggregate, American taxpayers save about $2.5 billion dollars on public highways while none of that material is hauled to and deposited into landfills. 

The process of milling involves, with some variation, carbide cutters and large rotating drums that remove and grind the road surface. The removed material is then channeled onto a conveyor belt that is part of the milling vehicle. Heat generated by the process and the resultant dust are managed with a constant spray of water. Lasers or string-lines are used to achieve the milled depth of the existing pavement.

The intermediate result is the scraped street with which all motorists and bicyclists are familiar. The end result is the addition of a new “lift” (top layer) of smooth asphalt, much of it composed of recycled material that has been reprocessed at an asphalt  plant. According to a 2011 study conducted for the US Department of Transportation/Federal Highway Administration (“Reclaimed Asphalt Pavement in Asphalt Mixtures State of the Practice”), mixtures containing up to 30% RAP in newly laid asphalt perform as well as pavement made entirely of virgin material.

So next time you find yourself enduring the mild vibrations of a milled street, take heart. It not only means a new, smooth road will be installed soon, it also means less money was spent and a lower carbon footprint was made in the process.