Filling in concrete cavity below grade

[MiamiCuse]

In order to pass a 3/4" PVC electrical conduit from inside of the house to outside of the house below grade, I had to chip out part of the concrete block wall, and since it's below grade on the outside and then travels vertically up on the inside wall (to get to a junction box), I had to put two 45 degree elbows into the hollowed out section and that makes the hole more than the original drilled out 1" hole. Now I want to fill it back up with something. I don't want to use those expansion foam I would like to back fill with concrete. However, the cavity inside the 8" thick concrete block wall is bigger than the hole. I don't think I can "pour" concrete in.

Is it ok to add more water to get the concrete "soupier" so I can pour it in? or is there another way? It's not a big cavity I think just about the size of a milk carton.

Thanks,

MC

[David Nebenzahl]

On 2/27/2009 11:48 PM MiamiCuse spake thus:

The fill isn't going to be structural, right? So even though it's poor practice to soup-ify concrete, in this case it should be OK; you just need to get some stuff in there, not develop any strength.

At least that's the way this non-concrete expert sees it.

[dadiOH]

Sure, add some water. That's what they do when doing concrete construction and they want to fill the top, "U" shaped lintel by pumping "grout" into it.

You might want to enlarge the hole some too in an upwards direction...the soupy grout will pretty much self level so you won't be able to fill above the bottom of the hole; once the soupy stuff sets, mix some stiffer mortar to stuff into the upper part.

[aemeijers]

I'd use foam and concrete- foam to make a plug in the cavity in the block, and then the stiff pack-it-in-by-hand concrete patch mix to plug the holes in both faces. A lot less messy, and a whole lot less weight to lug around. The problem is, the cavity you can see probably leads to the same cavity in the row of block below it, unless you made the hole in the bottom row. If you don't want to mess with foam, another trick is to wad up some window screen or other similar material, and stuff it in as best you can. It doesn't matter if there are some voids inside the wall- they were there to begin with. You just need something to take up space, that the concrete can lock into place around, but not flow past. Main objective is to get the patch on both faces to bind well to the surrounding concrete, so you don't get water leaks. You want it to be pretty on the inside wall, and extra solid and leakproof on the outside. Don't forget to heavily wirebrush the outside, so the patch has fresh concrete edge to bind to, and after the concrete sets, lots of the black stuff at the transition with the pipe. The black stuff should completely hide the patch mix on the outside, including on the bottom of the pipe, and extend down the pipe a couple of inches. Penetrations in the envelope like this are a common leak point in heavy rains or the spring thaw.

-- aem sends...

[Joe]

Mason mix is more tolerant of water additions, so that may help the problem in a small cavity.

Joe