Why do they put lines in sidewalks?

Dear Cecil:

Jill and I walk every day on our lunch hour, so we see a lot of street and sidewalk repairing going on. What we'd like to know is, why do they put lines in cement sidewalks? They pour and smooth out a perfectly good sidewalk, then they draw lines in it. The lines only go a quarter-inch deep, so what good do they do? We remember the saying when we were kids, "step on a crack, break your mother's back." Do we have sadistic city employees?

Cecil replies:

I was sure this was going to be another one of those “I dunno, we’ve always done it that way” questions, which have been turning up with dismaying regularity lately. But it turns out there’s a good reason for the lines. Not that cement contractors necessarily know what it is, of course. Many of them have the idea that the lines allow the concrete to expand and contract with changes in temperature — not true, strictly speaking.

Concrete does expand and contract, and for that reason expansion joints, typically some sort of compressible fiber board, are put in every 40 feet or so. But the lines you’re talking about, which are called “contraction joints,” serve another, admittedly related, purpose. Concrete normally shrinks a bit as it dries, resulting in unsightly cracks. Cement finishers put in contraction joints so that when the concrete does crack, it’ll do so at the joints, where the slab is thinnest, rather than just any old place. That way it won’t look so bad. Of course, a lot of times it cracks any old place regardless of what the cement finishers do, but hey, they tried.

Send questions to Cecil via cecil@straightdope.com.

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