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Hi, I'm Tim Carter from AskTheBuilder.com. It's a cold winter day, and I want to show you something
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that's really pretty neat. If you remember, a couple of weeks ago we poured this great concrete
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slab. Look at this. Yeah, here's the slab. And it's nice and hard. It's curing out really well
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But I want to show you something. These were added later. These are concrete control joints
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and what these are, the contractor came back and looked down here. See this line right here
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This was not in here when the slab was poured. These were saw cut in later with a special
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diamond saw that cuts a groove in the concrete. And you'll notice they were put here for a
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reason. See that? That's an inside corner on the concrete foundation. That's exactly
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where the slab wants to crack, right at this corner, and if you did not put in these two
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saw joints, you'd probably get a very random squiggly crack that just went all the way
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across the slab. And understand this about concrete. When you pour concrete and as it
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cures and hardens, it shrinks. The water in the concrete leaves, some of it leaves the
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concrete some of it goes into the chemical reaction hydration and this concrete shrinks It shrinks 1 16th of an inch for every 10 linear feet that you pour So that shrinkage creates tension where it tries to literally
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rip the concrete apart. So, you need to saw cut in these concrete control joints after
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the fact on these nice smooth slabs. Sometimes on sidewalks and driveways, they actually
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have a nice tool that will cut that control joint in as the concrete is still in its plastic
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or wet state. Understand this though, that joint needs to be at least, the depth of the
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joint needs to be one fourth minimum the thickness of the slab, so if this is a four inch thick
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slab, it means that saw cut line should be an inch deep, and many, many times you'll
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notice that contractors actually don't cut the crack or the line that deep. So concrete
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control joints. They're called control joints because they control where the slab is supposed
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to crack. It doesn't always work that way, but that's what they're supposed to do. I'm Tim Carter
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for AskTheBuilder.com. If you want to discover more home improvement tips, go to AskTheBuilder.com
