Lay down some hot black stuff, and you’ve got a road, right? Not quite. Much more is hidden underneath.
Terminology explains a lot about road construction. Scottish engineer John McAdam is generally credited with designing in the early 19th century the first modern roads made by compressing thick deposits of crushed, angular stones. Builders later poured hot tar to bind the top layer, producing a “tarmacadam” pathway, or tarmac. Although this term lingers, the method has not been used for decades (not even at airports). By the later 1800s asphalt had become the binder of choice. And blacktop? A synonym for asphalt.
Today asphalt roads dominate the landscape, followed by concrete and “unbound aggregate”—gravel. All three formats are built up in layers comprising increasingly finer, denser and harder rock particles. Top levels of asphalt or concrete reduce wear and seal out water that can cause cracking. Most fissures form from the bottom up as underbeds shift or erode. To repair them, road crews typically add a new layer. But if damage is extensive, they may mill (scrape off) several inches and resurface.
Recent innovations include “perpetual pavement,” built on a costly but tough base of asphalt that is supposed to resist damage for twice as long as conventional structures. Stone-matrix designs, in which the size and shape of stones within surface layers is carefully controlled, also promise longer life; they are popular in Europe and are becoming widespread in the U.S. Porous pavement, which allows water to percolate through instead of run off, is being tried in parking lots.
Asphalt and concrete proponents present various arguments about which compositions are better for a given application, but ultimately the choice comes down to economics. “The decision should be made on life-cycle costs,” says David E. Newcomb, vice president of research and technology at the National Asphalt Pavement Association in Lanham, Md. “It’s a balancing act” of materials and labor, time needed to lay the road (and therefore traffic inconvenience), years of durability, and maintenance and repair. The final analysis, Newcomb allows, “can be a nebulous thing.”
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