Do I need to reinforce a trench in my slab?

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 procedures?
Thanks,
MC
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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. T
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Yes it is a floor slab. Not much soil there underneath all sand.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 crack. :(
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. :(
cheers Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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?
MC
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Since the trench is pretty shallow you most likely won't have any problems.
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)
cheers Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

you can cut rebar with an angle grinder, bolt cutters, hack saw. sawzall or rebar cutter (shear)
cheers Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.