Why do sidewalks always have lines cut across them every few feet?
In: Engineering
There are three kinds of lines in a concrete sidewalk.
The first is an expansion joint. This is the actual end of one concrete block and the beginning of another. There is usually a little space filled with some kind of compressible material. If you have enough of these, the concrete won’t crack, but that’s kind of expensive.
The second is a control joint. These are much less expensive than expansion joints, but they don’t stop the concrete from cracking. They control where the crack appears. The best ones are saw-cut into the concrete soon after it’s set, at least 1/4 the way through, and then filled with sealant. The concrete will crack the rest of the way through at this “weakened plane”, but you will never see a crack and the slab remains waterproof. Lesser ones are grooved into the concrete with a tool while it’s still wet. They’re faster and less expensive than saw-cut joints, but more unsightly and not water-sealing. They also aren’t as deep and therefore less effective at controlling where cracks appear.
Sadly, you often see what I call “beauty marks”. They’re supposed to be control joints, but they’re just tooled marks maybe 1/8 inch deep, and are completely ineffective. You’ll often see cracks in between, or even crossing, these pathetic excuses for control joints.
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