I need to anchor a shed to cement or concrete with ⅜" x 3" anchor bolts.  I have a Bosch "long life in concrete" ⅜" bit.  The drill motor is medium-size but was able to get three holes deep enough (three inches) without great difficulty, and one is almost there.  But the fifth one is only two inches, and it seems that it just won't go any further. (More than a minute with much of my weight pushing down on the drill.)  The bit does not appear to be worn out.  Putting a punch in the hole and beating on it with a sixteen-ounce hammer doesn't seem to make a difference.

Does this mean I've gone through the cement and encountered a particularly hard rock?  Or what might it mean? A solution?

UPDATE: Whatever was interfering suddenly surrendered and it took less then fifteen seconds to do the rest of the hole.  So it was probably a rock that moved out of the way.  Rebar is embedded in the cement and would not have moved.

  • Wonder if you are hitting some rebar? Assume the foundation would be reinforced when laid
    – Chris
    Mar 26, 2022 at 20:18
  • Could be that the bit actually worn, even though it appears to be OK. Mar 26, 2022 at 20:31
  • I certainly hope that a piece of aggregate in your concrete slab didn't suddenly "move out of the way". That would mean that there was a void for it to have moved into, and that's not good... I'd say you finally cracked it and the bit was able to get a purchase in it and drive through.
    – FreeMan
    Mar 26, 2022 at 22:44

1 Answer 1


Probably right on a stone in the cement. Move over a few inches and try again.

Stones usually only about 3/4 to an inch big.

  • 1
    Could be hitting rebar too. Always good to move over a bit. Mar 26, 2022 at 20:18
  • Not "always"—the pre-fab floor has a depression designated for the anchor. Anyway, whatever was interfering suddenly surrendered and It too less then fifteen seconds to go the rest of the way.
    – WGroleau
    Mar 26, 2022 at 22:03
  • The stone poured underneath, best I remember, is ¾" and less. I'm thinking it was one of those that moved out of the way after sufficient harassment.
    – WGroleau
    Mar 26, 2022 at 22:09
  • 1
    Concrete is a mix of sand, cement, and stone(about 3/4"). Sometimes you just hit the right spot and hit stone.
    – crip659
    Mar 26, 2022 at 22:45
  • I am not sure whether it is concrete or cement. That's why I mentioned both in the question. When they were pouring it, I didn't see any stones. But there were definitely ¾" underneath.
    – WGroleau
    Mar 27, 2022 at 6:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.