I had some plumbing work done to relocate some drains, broke the 4" concrete
slab and the trench was 2' x 8', but the main drain was located in a
different location and in order to reconfigure properly, the plumber
eventually made a trench 4' x 12' in size. This is a pretty big trench and
I don't think I can just put back the sand, compact it, throw in a layer of
moisture barrier and pour new concrete.
Do I need some sort of rebar or wire mesh? If so any recommendation on
If you are talking about a floor slab, I think you are correct.
Even if the existing slab was not reinforced, I would assume the new
soil is not fully compacted.
I would do my best to compact the soil that is placed in the trench.
The slab you put over it will not be 'structural' and will break up if
the soil subsides.
Dowel into the edges of the existing slab and add some 'pencil rods'
to help the new slab span the disturbed earth.
The performance of a SOG (slab on grade) is very dependent on the
underlying soil conditions.
Often it is difficult for a DIY'r to be able to compact well unless
the soil is very amenable to compaction. In your case you have
sand...should be pretty much self-compacting.
How deep is the trench (width are not AS important)?
How upset will you be if the repair slab cracks or breaks up?
I'd go thicker, as a 4" slab is really too thin for rebar to help
much, gotta go to at least 6".
Keep the water / cement ratio of your mix low to minimize shrinkage.
Edge doweling & rebar is something I would do (because anything bigger
than hairline cracks would bother me) but I'm not really sure it would
be worth the extra effort. Plus if you've got chronically wet soil
conditions the added dowels in the 4" slab might rust & cause slab to
But if you want to do the "belt & suspenders" approach; dowel with #3
bars 12" o/c using an epoxy setting compound like Sika AnchorFIx #1 or
#3, span the trenched area with #3's at 12" (both ways) and know you
made every reasonable (or unreasonable) effort to prevent
cracking....but concrete (unless PT'd) cracks. :(
Thanks Bob. I would say the "disturbed" depth of the trench is about 8" to
12" below the bottom of the concrete slab. It's about 8 inches now but the
pipes are half buried at this point I think the plumbers shoveled some sand
back in after the did the work.
It's Miami, Florida so yes it has very high water table the cast iron pipe
we replaced were corroded on the external surface after 35 years.
If its hair line cracks it probably wouldn't bother me, but I don't think I
would know unless the tiles pop or there is an ant trail or subterrainean
termites. I plan to spray some termicides after the sand is compacted. I
was not going to do anything special other than spraying some water and jump
up and down repeatedly to compact the sand.
Originally the slab had lots of wire mesh in it.
I would be upset if the new section of the slab settles.
If I pour 6" thick slab instead of the original 4" will it offset the need
to do dowels?
Since the trench is pretty shallow you most likely won't have any
Think about the conditons that existed...more or less uniform soil,
thin slab with wire. Now you've cut through the slab & the wire.
The bonded dowels are there to re-establish the slab continuity that
was lost by cutting the slab & the wire
If the wire in the removed section was un-rusted...then I would go
thicker and add rebar or wire. Thicker to handle any compaction
issues & to make the rebar work.
Thicker w/o wire or rebar can work but it depends on the compaction.
Sand is pretty self compacting so you'll probably be ok.
BTW sand is a type of soil (at least to the CE types....but they used
to ding me when I called soil "dirt", now its all soil to me)
The wesh wire in the concrete were rusted, but they still work, I mean I had
to work hard to cut them they were not rusted through like some I have seen
that you can twist left and right once or twice to break them off.
I also have to compact the sand a bit sideways. The plumbers cut the floor
open but some of the lines they ran they ran it very close to the edge of
the trench so now there are some discturbed soil around the edge creating an
"overhang condition" that is may be 6" inward.
Oh one last question how do one cut rebars? with an angle grinder?
Thanks Bob. I recheck and using a thicker slab may not be easy. The
copper supply lines have joints and elbows that may be half way inside
the slab if I deepen the slab, and that may not be a good idea
especially if the slab settles and stress the joints. I will try some
dowels and hope that works.
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