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The previous owners of my house had the laundry machine drain running into the storm sewer, through a rough hole cut into the concrete floor (pictured).

Photo of the hole to be sealed

The pipe at the bottom of the hole runs out to the storm sewer, and I plan to seal it permanently. I also plan to fill the hole, but I am not sure which concrete/mortar product to use. The hardware stores near me mostly carry quikrete and most of the repair mortar compounds I can find say they're for "holes up to 4 inches deep", etc.

This hole is approximately 9 inches deep and 3-4 inches in diameter, making it narrow but significantly deeper than what most of the mortar repair compounds I can find are advertised for. My desired end state is a hard concrete surface that I can paint. Please advise.

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  • Once the pipe is fixed, might need a bigger hole to do a good job. See if you can use concrete or mortar. The 9 inch hole can be filled up with 4 or 5 inches of sand/fine gravel before using concrete/mortar patch.
    – crip659
    Commented Dec 28, 2023 at 20:45

3 Answers 3

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Concrete, sand mix concrete and type S masonry cement are are fine for doing what you need. So use whatever is the cheapest of the lot. You will not need a lot, and by all means do not fill the whole depth of the pipe. No telling what else may be connected to the pipe that you may not see, or the concrete may run to anyway, blocking a pipe you may not want blocked in any amount.

As mentioned in another answer, 2" will work, it is a non structural fill, I would go as thick as practical. I like the idea of using that "shelf" I see down in the pipe a bit. If you can cut something similar to 1/2" plywood... OSB...thick plastic... something that can handle a little "punching action" with a stick or other small diameter rod or stick to insure the cement is settled all the way into the walls of the pipe.

Make the top flush with the surface, wait a couple of hours, maybe more and give it another trowel, wait another hour or 2 more and give it a little "touch" with a damp sponge. After the concrete has set up enough, the light wipe with a damp sponge should mimic the surface of your concrete floor, or as close as it may get....

The damp sponge needs to be squeezed and hard as possible to get it barely damp, before using

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  • Won't "regular" concrete shrink too much when it dries, and/or bond improperly with the edges of the hole?
    – nathanvy
    Commented Dec 29, 2023 at 4:39
  • The amount of shrinkage that any of these cementitious products have is negligible. If you still draw concern, consider the "type S" masonry cement recommendation. That is used for laying brick and block, made to cure out in thin applications and is quite strong. Readily available too. There is also "non shrink grout" but it handling is different than the others I suggest, but is very strong too. Different in handling, as in it is more "plasticy". At least that is the term used for that type of fluidity after it is mixed.
    – Jack
    Commented Dec 29, 2023 at 4:47
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    Also that is what the rodding does, it forces the concrete to make hard contact with the sides of the pipe. Also, only use enough water to get the mix to combine. More water, weaker the mix, more shrinkage.
    – Jack
    Commented Dec 29, 2023 at 4:49
  • Thanks very much.
    – nathanvy
    Commented Dec 29, 2023 at 21:06
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My suggestion is to fill the whole with a piece of polystyrene roughly cut to shape (or even thick cardboard), about 2inches down the hole or on the inner ledge/lip. Then fill it up with concrete and level off.

The plug stops the concrete from disappearing while you work on it.

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  • Storm drain, can't block the pipe, +1. Plastic bag full of newspaper shoved into the hole to keep it out of the pipe. Then more newspaper until it's only 6" deep. Then wet the sides of the hole and pour any cement (!) that says portland (otherwise it isn't) in. Then get fancy with a hand grind stone and/or give it a shot of tile grout before you paint.
    – Mazura
    Commented Dec 29, 2023 at 5:26
  • Hydraulic cement actually expands a bit as it cures, if I'm remembering correctly. That's why it is the standard choice for patching holes and achieving s watertight seal. Seems the obvious choice here... after, as noted, putting in a barrier so it doesn't go down the drain.
    – keshlam
    Commented Dec 29, 2023 at 14:45
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Use concrete. Assuming it runs down the horizontal line an extra 5 inches beyond the 9 inches of vertical line, I compute a volume of 0.1 ft³. That's about 1/4 of a 60 pound bag.

If you don't have any concrete finishing tools, a drywall putty knife will work for that tiny surface. Mix the concrete pretty dry so that it doesn't pour down the drain line. What does "pretty dry" mean? You should be able to build a 12" tall by 8" diameter tapering "sand castle" out of the mix with it sagging about 2" under its own weight. Quikrete defines its concrete as 2" to 3" slump at minimum water, so use the minimum water specified (if not a tiny bit less, but don't ignore common sense if it's too dry). Slap the surface a few times with your gloved hand after placing the concrete to bring some cream to the surface, and strike it off flat with the putty knife.

A Note on Shrinkage

You seem concerned about concrete shrinkage. You shouldn't be in this case. According to the ACI, normal shrink concrete has shrinkage strains ranging from 0.052% to 0.078%. For the 9" concrete dimension, this implies shrinkage between 0.005" and 0.007". 0.006" is about 10% of 1/16". For the 3" to 4" concrete dimension, the shrinkage should be less than half that, 5% of 1/16". There are all sorts of dimensional irregularities to your existing concrete's cylindrical surface, with plenty of these much larger than 5% of 1/16", so concrete bond isn't necessary. The fresh concrete will mechanically lock into the wider spots in your existing concrete, and this mechanical lock mechanism provides plenty of strength with or without cement connecting the old concrete to the new concrete.

Paint will easily fill a 2.5% of 1/16" gap around the perimeter of the patch, so that shouldn't be a concern. (A human hair is about 3% of 1/16".)

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  • My understanding is that "regular" concrete won't bond with the edges of the hole and will shrink too much, which is why a repair mortar compound is needed.
    – nathanvy
    Commented Dec 29, 2023 at 4:38
  • @nathanvy, concrete shrinkage is a function of its water/cement ratio. The advocated dry mix will minimize shrinkage. To get a shower's dry pack mortar to bond well with an underlying concrete slab, shower installers will put down a layer of modified thinset between the old slab and their mortar (you place the concrete when the thinset is still wet). This level of complexity for your project would be silly and expensive. I suppose you might install the concrete to an inch or two below top of slab and use hydraulic or anchoring cement for the last bit (they swell as they cure). Again, silly.
    – popham
    Commented Dec 29, 2023 at 5:05
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    @nathanvy, 0.0008 is considered a lot of shrinkage strain. For the 9" dimension that's 0.0008(9") = 0.007" or about 10% of 1/16". For the 4" dimension, it's about half that.
    – popham
    Commented Dec 29, 2023 at 5:11
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    @nathanvy, as for the bond, there's plenty of roughness there for mechanical interlock. The concrete will be pinched between the bottom of the hole and that concrete at the top that sticks out further than the concrete below it. If you consider that insufficient (it's not), then you could chisel the sides a little.
    – popham
    Commented Dec 29, 2023 at 5:15
  • @nathanvy - if you're worried about bond you spray the whole thing clean with a garden hose, and then you should still wet the adjacent stone/masonry right before you pour. Excessive shrinkage is only if you mixed it with too much water, or don't let any standing water absorb first after the garden hose. And if you don't float it correctly (again) which brings out excess moisture; push hard, at about an hour later. Ideally about 30 seconds before you can't anymore. But that's for a new slab. You might get half an hour +/- 15m, so watch out.
    – Mazura
    Commented Dec 29, 2023 at 5:39

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