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I had a driveway poured three years ago. After a crack in the walkway appeared and two more developed in the driveway itself, my husband began to get suspicious that I wasn't given a 4" slab. If we dig around the side of the drive and measure the concrete, should it be 4"? or could it settle to a little over 3".

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    4" of concrete doesn't really 'settle'. So what was poured is the thickness you'd be left with. As for the edges, that may not necessarily be the overall thickness depending on how well the base was prepared. As for cracking, that happens. There should have been expansion joints in the driveway that would control the cracking. If there weren't, that's a different issue than the thickness. It could also be that there was a poorly prepared based, settling out of their control, bad rebar, or all sorts of other issues. As for checking the actual depth, probably best to drill a hole in the center. – DA01 Apr 22 '15 at 0:17
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    Good list, but there's always the "Reinforcing? What's that?" option for extensive cracking - either missing altogether or improperly placed. And no, concrete itself does not shrink or settle, so 4" should be 4." – Ecnerwal Apr 22 '15 at 0:59
  • Can you describe the approximate length and width of the cracks? What is the drainage like in the driveway area? Are there any expansion joints in the driveway? – Hank Apr 22 '15 at 1:28
  • Concrete settling, LOL. – Tyler Durden Apr 24 '15 at 19:53
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If you are only interested in thickness of your concrete slab there is a method to measure it. Sometimes when someone doesn't know the strength of some old concrete element which was poured by someone else people hire specialist for this; they use special tool which is cylinder shaped and which drills the concrete and extracts concrete cylinder, which than you can measure. Don’t be afraid that you will damage the slab because it is only inch or two wide. On the other hand if you are wondering what is wrong and how to fix it you need to provide us with more details. People who wrote comments have asked the right questions, especially Ecnerwal.

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Cracking concrete rarely has anything to do with its thickness.

If the concrete has no footing below the frost line, cracking is virtually certain in an area with hard freezes.

If a no-load paving like a concrete sidewalk cracks, it is either because of a root, or because it is improperly drained. If concrete sits in water, it will absorb the water and then crack when the water freezes. Sometimes a sidewalk will crack due to adiabatic expansion of an improper subsurface (like dirt), but 95% of the time it is a water-related problem.

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  • What are you talking about? There is tons of good concrete with footings. I have a 150 foot driveway poured in the late 60s that is in perfect condition. It has no footings and is about 6 inches thick. Thickness is a huge factor for cracks, as is rebar use, as is concrete formula, as is a proper bed, and a few other factors. But thickness matters. It might not have been the main issue but what you are saying is just wrong. – DMoore Apr 24 '15 at 21:06
  • @DMoore Where do you live? The OP lives in a winter area with hard freezes. As for using rebar, it is true it will prevent cracking, at least until the rebar rusts, but I don't think the OP has rebar in her driveway. – Tyler Durden Apr 24 '15 at 21:11
  • Midwest. We have 3 months of freeze and terrible heat in summer. Terrible for concrete. I have been dealing with concrete and mixing plants half my life. Thickness matters. There might be a point where it doesn't but it is no where near 2-3 inches. – DMoore Apr 24 '15 at 21:14
  • @DMoore Midwest where? What state? – Tyler Durden Apr 24 '15 at 21:16
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    What is adiabatic expansion? – Joel Keene Apr 25 '15 at 4:04

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