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Is there concrete all around or is it in my head?

July 22, 2010 at 7:41 AM by Dr. Drang

With In Our Time on hiatus until September and The Bugle in reruns for a month, I’ve been casting about for other podcasts to listen to. I decided to give Grammar Girl another try—I was a subscriber a couple of years ago—and downloaded several episodes.

One of the first I listened to was “Concrete versus Cement,” a usage issue near and dear to my heart. Unfortunately, the episode stabbed that heart with error after error. The show was written1 by Sal Glynn, a book editor who probably should have consulted a civil engineer before releasing the final copy.2

In fairness, the show gets its main point right: concrete and cement are not the same thing. Cement is a constituent of concrete and is almost never used as a building material by itself. From pools to patios, sidewalks to stoops; everything that’s commonly called cement is really made of concrete.

OK, fine. But things go south after that. Here’s the most egregious passage:

In 1824 Joseph Aspdin patented “Portland cement,” a powder made of limestone and clay. He called it Portland cement because when it was mixed with sand, gravel, pebbles, bits of rock, and water, the resulting dried concrete resembled the limestone from the English Isle of Portland.

This is the division: cement is a powder that is mixed with other materials and water to create the solid mass known as concrete. There is no such thing as a cement overpass, a cement porch, or a cement pond. These are all concrete. If you stub your toe on concrete you’ll yell like a Cretin. Get it? Concrete. Cretin.

And in case you were wondering, pavement is also not concrete. Pavement is a solid material made of sand, gravel, or crushed stone much like concrete, but the cement binder is replaced with asphalt or tar.

The history lesson is basically right, although I wouldn’t say Portland cement concrete “resembled” Portland stone (no matter what the Wikipedia article says). The Portland name was mostly a marketing gimmick. Aspdin was a successful salesman, not above pretending his cement had secret ingredients that made it superior.

Also, I don’t understand the distinction Glynn is trying to make between gravel, pebbles, and bits of rock. Gravel is made up of pebbles, each of which is a bit of rock. In concrete, they’re all categorized as coarse aggregate.

But the real problem in that first paragraph is the word dried.

Concrete doesn’t harden by drying.

This is a common but mistaken belief (as common and as mistaken as thinking that concrete and cement are the same). The cement in concrete undergoes a chemical reaction with the water added to it. That reaction, called hydration, produces a new material that becomes harder as the reaction progresses. The new material binds the sand and gravel together and fills the spaces between them.

Portland cement is a hydraulic cement—it can harden under water. Underwater concrete placement is fairly common, and even though that concrete never dries, it hardens just the same.

So never, ever look at newly-placed concrete and say it’s drying. Say it’s setting or setting up or hardening or curing. You can even say it’s hydrating if you want to get funny looks. Just don’t say it’s drying.

The real problem with drying is that it’s a word tha has a very specific meaning in reference to concrete and that meaning has nothing to do with hardening—it has to do with shrinking and cracking. Here’s why:

The hydration reaction requires a certain proportion of water to cement. When concrete is mixed, it usually has more water than is necessary to hydrate the cement. The extra water is added to make the fresh (i.e., unhardened) concrete a bit mushier and easy to work with. After the concrete sets, that extra water slowly migrates out of the hardened concrete and evaporates, and the concrete shrinks as a result. This is called drying shrinkage.

Usually, the concrete is prevented from shrinking as much as it wants to because it’s being held in place by something. Sidewalks, for example, can’t shrink the way they’d like to because the friction between their underside and the ground below prevents it. When that happens, tensile stresses develop in the concrete and—being weak in tension—it cracks.

These shrinkage cracks are so common, and so difficult to prevent, that in many structures prevention isn’t even attempted. You can’t, for example, eliminate shrinkage cracks from a sidewalk, so instead you try to control them. The joints that run across sidewalks—mistakenly called expansion joints by many people—are control joints. They’re put there to make the sidewalk weaker along those lines and cause the shrinkage cracks to form out of sight down at the bottom of the grooves rather than haphazardly over the walking surface.

So that’s the first paragraph. I dislike the second paragraph, too, because it presents an overly narrow definition of cement. The word cement is used not just for the gray powder, but also for the paste that’s formed after we mix that powder with water and for the hardened material that results from the hydration reaction. If the meaning isn’t clear from the context, we may be more explicit and refer to these latter two materials as cement paste and hardened cement paste, respectively, but they’re cement nonetheless. Saying that cement refers only to the powder, which Glynn does elsewhere in the episode, too, is simply wrong.

The third paragraph is just ridiculous. Pavement can’t be made of concrete? That would be news to

Of course pavements can be made of concrete. Back in Pavement Design class, we learned that those were called rigid pavements, while the ones made of asphalt are called flexible pavements. It’s right there on Page 5 of Yoder & Witczak.

Honestly, where would anyone, even a book editor, get the idea that pavement is exclusively asphalt?

I’m sure Mr. Glynn is a very nice person and quite knowledgeable in his field. But concrete is not his field. You don’t see me writing about gerunds.

  1. Grammar Girl herself, Mignon Fogarty, still narrates the episodes, but she seems to be outsourcing more and more of the scripts. ↩

  2. You might well ask why I’m bothering to correct a podcast that’s over a year and a half old. First, I just heard it a few days ago—I’d have corrected it earlier if I’d known about it. Second, it’s still sitting there on the Grammar Girl website, misinforming people who come across it. And third, I’m kind of pissed that a book editor thinks he can look up a few things on Wikipedia and present himself as an expert on a topic that I and thousands of others spent years learning. ↩

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