March 29, 2008

Asphalt Vs. Concrete

Stupid Question ™
Jan. 10, 2005
By John Ruch
© 2005

Q: Why are streets paved with asphalt, while sidewalks tend to be concrete? Why not the same material for both?
—N.R., from the Internet

A: First, a bit of terminology. Asphalt roads are actually a form of “concrete,” that being a generic term for a substance that combines a base with crushed or powdered stones.

That black road surfacing stuff is asphalt concrete, made with a hydrocarbon asphalt base that uses bitumen as a cement. That gray sidewalk stuff is Portland cement concrete, made with a limestone base and named for a naturally occurring form of limestone.

As Prof. Stefan Romanoschi of Kansas State University’s department of civil engineering pointed out to me, you can find streets made of cement and sidewalks made of asphalt, so there is certainly a degree of interchangeability to the substances.

The choice, he said, comes down to cost, durability and configuration.
While cement is more vulnerable to anti-icing chemicals, it is more durable than asphalt under heavy traffic loads. But asphalt is cheaper to the tune of tens of thousands of dollars per mile, according to the asphalt industry. It can also be repaired less expensively with simple patches.

Therefore, Romanoschi said, asphalt is very popular for streets with relatively low traffic (especially truck traffic) volumes and speeds. It holds up well enough and repairs do not have to be extensive due to low-speed traffic, he said.

But cement can be better for highways and industrial areas, Romanoschi said, noting, “They cost more initially but you do not need to spend money on maintenance.” That also means fewer construction-area slowdowns.

This difference plays out in the asphalt industry’s own numbers, which report that 94 percent of the U.S.’s total paved streets are asphalt—but within that figure, only 65 percent of interstate highways are asphalt. The rest are cement.

As for sidewalks, Romanoschi said it comes down to the way the two materials are set down. Asphalt has to paved and compacted with a mechanical roller that by necessity is of a fixed width and shape. Cement, on the other hand, can be poured into any form and scraped level, and hardens on its own.

He noted that many sidewalks have “irregular shapes and widths”—consider street trees, sign poles, manholes and so on—and hence are easier to create with cement.

While I asked for a technical answer to this question and got one, I suspect aesthetics and habit probably play a role as well, especially on the sidewalk end of things. Cement may be considered to have a cleaner look, and it certainly doesn’t smell on a hot day.

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