This concrete garage floor is 3 years old. It has these quarter-inch wide sharp joints cut in every 12 feet. All settlement/curing cracks are in the joints, but I don't understand why they were cut so sharp and deep: All the depth and edge seem to accomplish is to collect dirt and provide a home for moisture and pests.

Garage floor concrete expansion joints

At this point is it OK to fill them in – at least partway to reduce the space available for dirt to accumulate? Or are they serving some other ongoing purpose?

  • you could pour in thin-set mortar
    – jsotola
    Commented Sep 21, 2020 at 18:33
  • It would be difficult to get even a sand-based mortar to flow into those narrow channels. Self-leveling compound might work.
    – isherwood
    Commented Sep 22, 2020 at 16:36

4 Answers 4


You could have caulked them a month after the slab was poured. You can caulk them now. Or vacuum/scrape them out and caulk them now.

Saw-Cut control joints (as opposed to "tool-formed while the concrete was wet" control joints) are inherently sharp. They are less expensive than formed joints. They need to be deep enough to accomplish the goal of having any cracks that form, form at/in them, not at random.

Use a caulking compound made for use on concrete. If the width precludes setting a bead in the top of the joint that will stay put, fill them with foam backer rod and then caulk on top of that.

  • 3
    Note that any latex-based caulk will shrink substantially. I'd use silicone or urethane, which shrink much less.
    – isherwood
    Commented Sep 22, 2020 at 16:37
  • Does anyone actually use latex-based caulk other than cheapskate builders and slumlords? It's so awful and silicone is not even expensive. Commented Sep 23, 2020 at 3:11

So the question is it a live joint? A live joint is one where the ground is actually moving, and you can tell by careful examination of the bottom of the control joint. If there is no crack in the bottom of the control joint it is not live and can be filled with either mortar or caulk. if there is a crack in the bottom it is easier to assume that there is movement and that the joint is live. in this case do not use mortar, and you want a bond breaker (like backer rod) under the caulk to prevent the caulk from failing.

  • There are cracks in the joint. But I thought that's to be expected when concrete cures, and that's exactly why control joints are put in: so that the inevitable cracks produced during extended curing occur in the joints where they are predictable and less visible? I.e., don't we always expect to find cracks in properly-spaced control joints in concrete that is months or years old?
    – feetwet
    Commented Sep 22, 2020 at 3:57
  • 1
    Depending on the strength to size ratio you could get curing cracks (typical example long sidewalk with too much water), but far more common in garages is differential expansion either due to moisture or ground movement. Both are very common.
    – hildred
    Commented Sep 22, 2020 at 4:38

I don't understand why they were cut so sharp and deep

For UTILITY not aesthetics.

Sharp: That's how a segmented diamond blade cuts concrete. It's just the nature of the tool being used.

Wide: The segmented diamond blade has a kerf size, apparently this one was about 1/4 inch.

Deep: They wanted to make sure the stress cracks were controlled instead of being in the middle of your slab.

If you wish to seal it up then vacuum out as much crud as you can and use a self-leveling crack sealer such as Sikaflex.

  1. Backer Rod
  2. Sikaflex Self-Leveling Sealant

Done. Perfect.

I am in no way affiliated with this company; I am just a fan of it.


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