Concrete backfill of slab after plumbing repair


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Old 07-20-17, 06:10 PM
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Concrete backfill of slab after plumbing repair

I have a slab foundation house and there were both a supply line leak and a cast iron drain leak. The pipes run along the kitchen wall so I made a cut in the slab, excavated the soil and performed the repairs. Now that the plumbing repairs has been completed I backfilled the sandy soil into the trench.

The slab cut is shaped like a letter "T", as shown below. To get a sense of scale the trench from point A to B is around 11" wide, and the tiles are 20"X20" tiles.





The next few days I will be compacting the trench best I could with a 4X4 post, and hosing the soil down. Then I will be adding termicide to the soil.

Question #1 is do I need a vapor barrier? That was a plastic sheet under the concrete when I made the cut, but now the barrier is cut open, and if I put a new barrier down it's still going to have a seam all the way around. Is it even going to make a difference?

Question #2 is for a small and narrow trench like this, do I need rebars? I have done bigger slab repairs before, drilling holes in the old concrete and epoxy in rebars, but for this trench I am debating whether rebars are needed. Thoughts? By the way I noticed Home Depot sells some newer blue colored "Epoxy rebars", anyone tried them?

Question #3 is the concrete product to use...I am going to manually mix with a drill in a 5 gallon bucket. Figured I need about 12 bags. I am thinking to use Sakrete High Strenth Concrete Mix. Is there a better or easier to use product? Should I brush on some blue bonding agent on the old slab cut surface prior to backfilling concrete.

The area is a kitchen so there will be cabinets, range, refrigerator, dish washer sitting on the floor.
 
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Old 07-20-17, 07:36 PM
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You didn't tell us the depth. My guess is 4". Anyway, you don't need rebar but I would use mesh. Float the mesh about an inch off the sand. If you are going to use a vapor barrier, why wet the soil? Tamp it either by hand or by machine. The Sakcrete mix has a lot of rocks in it. A type S mortar mix maybe easier & just as strong.
 
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Old 07-21-17, 03:49 AM
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I don't think you need rebar in the slab, but if it were mine, I'd drill some dowels into the edge of the existing slab to be sure there's absolutely no chance of that slab moving up or down as it cures and shrinks. It doesn't have to be much, just some 1/2" holes about 4" deep into the edge of the slab, pound in some #4 bars and let them stick out about 4". Do that every 18" or so and you'll be good, with some mesh in the slab. With a tile floor, even a very small amount of movement can crack tile.
 
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Old 07-21-17, 04:53 AM
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I agree with Bruce, I also would tie the slabs together using 1/2" rebar.
 
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Old 07-22-17, 07:45 AM
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The depth of the slab is 4".

I wet the soil with a garden hose because it does a better job of compacting the soil since it's VERY sandy.

I must wet the soil thoroughly anyway to apply termicide as I backfill. I dug down about 24" and sideways under the slab about another 18". So I backfilled three times applied termicide each times and thoroughly soaked it.

Isn't Type S mortar closer to it being more like a stucco mix?
 
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Old 07-22-17, 07:49 AM
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Anyone saw or used the green epoxy coated rebars? I saw them at the Home Depot, and it's priced cheaper than same size non-coated ones.

 
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Old 07-22-17, 07:54 AM
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Another thought occured to me.

Since my trench is a T shape, and the width is bigger than that of a flat head shovel, and I have a vapor barrier underneath it. What if I put the bags of concrete mix directly into the trench, end to end, then break them open and toss the bags away, hose down the mix, mix with a shovel right int the trench, work my way from one end to the other...would that work? The only thing I see is if I am not careful I might rip up the vapor barrier. Thoughts?
 
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Old 07-22-17, 03:05 PM
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Mortar Stucco Mix Type S | Sakrete

All the uses for Type S mortar mix are listed there.
 
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Old 07-22-17, 05:32 PM
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Nothing wrong with using epoxy coated rebar. Years ago, epoxy coated was more expensive. I guess things have changed. I used epoxy bars in a structural slab of a 2 level garage because it more resistant to road salt that MN uses in the winter.

You want to use a concrete mix (not a mortar mix) and mix it in something like a wheel barrow. If you try mixing it in the trench, chances are very good (like you say) that your shovel or hoe will punch holes in the vapor barrier. While it sounds good, I really don't think that would work.
 
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Old 07-23-17, 08:49 AM
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I can get a wheel barrow to that spot.

What I do worry is I might end up breaking more tiles. I have limited replacement tiles (and it's been discontinued). During the entire process of cut slab, remove debris, excavate dirt, repaired plumbing etc...I broke additional tiles. Shovel accidental hits, removing debris and a piece fell off...

I worry the wheel barrow mixing, pouring, will again break more tiles. Yes I put tarps down, I even put a blanket down and didn't matter much.

So I am still pondering if I can do this in tench mixing because not only it would be easier, but it would reduce damages.

What if I cut a piece of 1/4" plywood, cut it to the size of the trench, in two skinny rectangular pieces. Lay them on top of the vapor barrier. Drill holes through them, put a rope through them. Then do the mixing in the trench, the shovel would just hit the plywood. Once the concrete mixing is done, I can pull on the ropes and tease out the pieces of plywood...although if I use rebars that can't be done.
 
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Old 07-23-17, 09:10 AM
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You can sure try mixing it in the trench. If it doesn't work, poly is cheap and you've only wasted 1 or 2 bags of concrete mix.

If it were me, I'd lay tarps/blankets on the floor, then lay 7/16" OSB on top of that, right up to the edge of the trench. The tarps keep the OSB from scratching your tile. Then you can dump from the wheel barrow; or you can shovel it directly from the wheel barrow into the trench, just a little slower.

Is some of the trench under kitchen cabinets? If you run a little short of replacement tile, put mis-matched tile under the cabinets where it won't be seen.
 
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Old 07-25-17, 04:58 AM
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Arghhh...I just realized (can't believe I didn't before) that due to the narrow trench width, I couldn't lay my SDS rotary hammer down enough to drill horizontally into the holes. Even my 1/2" bit is 8"-10" long. It has to be drilled at an angle. So I can't put a 20" rebar crossing both sides of the existing slab, unless I bent the rebars which is too much hassle. Furthermore I can't even drill into the side of the trench facing the wall not even at an angle. I think I will put in one long piece from one end to the other along the wall, then a few short pieces coming out of the side of the slab I can drill into, and tie them together.
 
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Old 07-25-17, 06:15 AM
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Is some of the trench under kitchen cabinets? If you run a little short of replacement tile, put mis-matched tile under the cabinets where it won't be seen.
Not sure yet what type of cabinets. There is a possibility the base cabinets will not have kick plates to conceal the tiles underneath, but instead cabinets are on legs.
 
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Old 07-25-17, 06:32 AM
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Is there a window where you can place a chute to pour the mix? Rent a one bag mixer.
 
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Old 07-26-17, 01:24 PM
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Well, that's an unforeseen bummer about the rebar into the edges. I would say that anything is better than nothing. I would suggest some Tapcons screwed into the edge and projecting maybe 3/4", every 12". I have to believe you can get some Tapcons in there; drilled at an angle doesn't matter, anything for the concrete to grab onto.
 
 

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