I know the forum is Exterior Improvement, but I couldn't find pouring Concrete House Slabs - if this is in the wrong place, kindly redirect me and I'll head over there.

I'm in SW Arizona. Had to break open my house slab to replace the 60-year-old cast iron sewer pipe. Using all ABS (and a few flex couplings where the pipes didn't quite align - easier than ripping out the complete repair job and starting over!). The hole is about 5 x 2, and tunnels under the concrete stand-up shower basing to connect to that drain. (Shower is completely tiled over and I really didn't feel like getting into all of that, too!) The hole was made using a demolition hammer, and the edges are very jagged (got the holes in my arms to prove it).

Tomorrow I complete the pipe connection to the shower and leak-check. Then I shovel all the dirt out of the shower and back into the hole. It looks like I'm going to need 6 or 7 80-lb bags of "just add water" 4000-lb Quickcrete.

Do I need to wet the dirt as I throw it in? Or can I just fill the knee-deep hole to the bottom of the slab, stomp on it a few times, and call it good? Do I need to find some rocks or brick to put under the elbow for the toilet flange (4" pipe tees off horizontal for about 18-20 inches, then elbows up to the flange)?

Ed

 

If it were me, I would drill some holes in the edges of the existing concrete and insert 3/8" rebar at least every foot, otherwise the patch over the disturbed dirt will sink. If you drill deeper on one side, you can slip a bar in to clear the span then slide it out and into a hole in the other side.

 

I would have used #57 stone under the pipe to support it before the concrete.

 

I read about doing that in some other posts, but it was almost always for holes that had been cut and had smooth edges. I did not saw this hole, and these edges are anything but smooth. I rented a Bosch demolition hammer from a hardware store and broke it out in chunks with a 2" spade bit. There's not a smooth face anywhere to drill into.

I read about having to spray the surface with water daily for a week?? And maybe taking a month to cure before I can tile?? I was thinking about using the Fast Setting (red bag) Quickrete: pour the mix into the hole and add water directly, no mixing first? Would that work better here?

Ed

 

If you don't want to insert rebar, next best thing is chip the edges so that you have a 'V' shape, wider at the top narrower at the bottom. You can cover it with plastic to help keep it wet. I don't know much about that no mix stuff, I think it is meant for setting fence posts.

 

As toolman stated the no mix concrete mix is intended for setting posts not what you were doing. You will deftly need to mix the concrete 1st in order to have it fill all the crevices and give you a good consistency to smooth the surface with. You even need to let the fast setting concrete cure for a good while before setting tiles on top of it. If you don't it will interfere with the curing of the thin set mortar you use to install the tile. Covering it with plastic for at least the 1st week will help it cure better. Take your time to do it right.

 

This is getting a bit scarier. This house was built in 1954; all the sewer plumbing should be cast iron. The shower drain, though, was ABS - so that's a remodel. I'm starting to backfill the sand into the hole, using a board to tamp it firm under the exterior wall footing and the shower. I didn't break through the concrete shower pan - I just dug back about 10"-12" to expose the pipe. But as I'm pushing dirt into the "shallow" hole under the shower, it's pushing back as deep as my arm is long!

I can't trust this sand to compact well. Not without contractor-grade tools, anyway. So I'm thinking I might have a solution. I can take some pieces of my left-over 4" ABS and create some pillars: drive them down until I hit solid firm dirt and cut them off flush with the bottom of the replacement slab. And I can put one under the lip of the shower to bolster that, too.

I'm quickly hitting a wall where I go beyond what I can do and will need to hire a contractor. And I don't think I can afford that. (Already replaced the stove, roof, and gate this year!) We don't have any groundwater here, nor any type of freeze (our low temperatures this week will get down to high 40s-low 50s).

Ed

 

Your repair would be better if you used crushed gravel instead of sand to place and compact in the voids--sand does not compact very well. I don't think your plastic-pipe-pillar idea is a good one. You'd be far better off just filling any accessible voids with solid concrete.

 

no-mix concrete sounds as if its regular conc in a different bag but w/maybe a slight price increase too

 

(Tried to upload pictures, but no go.)
I got an 8" diameter round cardboard form and cut two pillars. I sank these down to solid dirt, one at the left end and one center next to the toilet. I put a piece of the 4" ABS down to solid dirt under the shower ledge. Then I put a piece of 3/8" x 18" rebar across each of the 8" tubes (lifted a bit on rocks) and two 3/8" x 4 1/2' rebar across the length of the hole on top of the smaller pieces.

Filled the three tubes first, then the rest of the hole. Six 80-lb bags and 3 hours later, it's done! And so am I!

(Now on to phases 2 - 12 of this project!)

Ed

 

Sounds like you went through a lot of effort to accomplish exactly what? Similar to using a howitzer to kill a mosquito--it may work, but isn't really necessary.

Are you saying your bathroom floor loads are so heavy that they need reinforced concrete piling to support them? I can't imagine what you use the bathroom for to require such over-build, unless you have an extensive bowling ball collection that you store in there, floor-to-ceiling.

Could have accomplished the same end result by ramming $2 worth of gravel into the visible voids, and then pouring the floor concrete. More than strong enough for normal residential bathroom floor loads, and would have taken a small fraction of the effort you put into it.

 

Yeah, I figured it was overkill. But I didn't have any "visible voids" - just the hole I dug out to get down to the sewer pipe. I suspected a void underneath the shower, but unless I either jack-hammered that completely out or tunneled further underneath, I'll never know. And it's held for the 18 years I've been in the house so far, and all the years before (house built in 1954), so I wasn't all that concerned.

My biggest worry was that the weight of the new pour would cause it to sink a bit over time because I couldn't compact the loose sand I was using to backfill the hole. And there were a lot of suggestions to use rebar, but the edges of the hole were so jagged it would have been a lot more work to cut them smooth and drill into. So my next-best solution was the pillars and use them to support the rebar.

Anyways, it's done. Now I can move on through the next phases: sheetrock, tile, exhaust vent, window, stucco repair - oh, and remounting the toilet and sink so we can use the bathroom!

Ed