Reinforcing concrete slab with dowels

[MiamiCuse]

I have a trench about 4; wide and 10' long in my slab that was opened up for plumbing drain relocation and now I am ready to close it up. I will put the sand back in and compact it real well, then termicide, then moisture barrier, then I plan to add dowels to both sides of the existing slab and pour new concrete.

It is very difficult to do the dowels because the trench width it irregular. As I did the first two I started to think if I position the dowels differently would it make the job easier and actually more effective?

Instead of drilling holes on existing edges, why put the end of the dowel at the bottom of one slab, then pound it deep into the sand as far as it will go at an incline, when the dowel is shortened to be about the same width as the trench, start pounding it down until it wedges into the existing concrete on the other side. So basically the dowel will bridge the trench from below one slab to the middle of another slab, sort of pushing up the existing slab.

If I continue this pattern, alternating the dowels say every 16 inches, I will have an even number of dowels on each side, angled up. If I pour concrete in and embed it into the new concrete, would it not be like pretensioning it? Seems to me logically would hold the new concrete up better...or am I nutz?

MC

[gfretwell]

I think you are working too hard on this. Compact the dirt and pour in the patch using an acrylic patch mix that will bond to the old slab and be done with it. Use a low slump on the concrete and it won't shrink as much (less water) What kind of surface are you installing over this?

[MiamiCuse]

Ceramic tiles, a preslope for shower pan, more mortar on top of it and more tiles. Thanks.

[gfretwell]

Typically they will lay tile over a membrane down here. Concrete will crack and transfer the crack through the tile otherwise.

[MiamiCuse]

I have not seen membrane under tiles in the places I lived and visited. Seems they just apply thinset and tile on. Wood floors or laminated floor a moisture barrier is common. There is a layer of moisture barrier on the underside of the slab which I torn up when digging up the trench. I will need to fix that back in. Thanks.

[BobK207]

MC-

I have nothing to add to Robert's post...do everything he suggests & you'll be fine. Esp the straigtht cut & min thickess. filling with a "floable" mix is much better than packing sand.

cheers Bob

[MiamiCuse]

Robert:

Just one more question regarding using concrete mix under the overhanging portions of the existing slab - since there are copper supply lines and PVC drain lines tucked under the slab (the plumber deliberately excavated there to route the pipes through...by packing concrete there wouldn't I be embedding the drains and supply in concrete? Something about this makes me uneasy. I can wrap the copper with sleeves (I plan to anyways) to avoid contact with concrete, but seems there should be a way to provide support without encasing the lines into hard concrete.

What is I take the big rocky chunks from the demolition of the original slab, and wedge as many pieces as I can underneath, and pound them in with rubber mallet? Will this work at all?

Thanks,

MC

[Robert Allison]

Snipped

No, don't do that. Sleave the copper and then fill with sand up to the top of the pipes, so that they are just visible. Wet this down to compact it and then add more sand if needed to almost cover the pipes and repeat. As you wet it down, it should fill all the voids in and around the pipes. After doing this, do the slurry mix. (You should wet down the sand under all of the patch just before you place your concrete anyway.)

If you think it is too tight, its not. Just make your slurry wet enough to flow easily into all the gaps and crevices. As long as you do this, your pipes may be touching the concrete, but it won't be embedded in it. And even if it was, that is not a bad thing in and of itself. It is bad if this causes your slab to be less than 3 inches thick, because it would cause the concrete over that pipe to crack along that pipe. Kind of like a control joint in reverse.

Don't worry, be happy!

[Mike Dobony]

I don't know how good of a contractor you are, but I know whenever my b-i-l bid a job he got it based on his reputation as a quality worker. He learned from many experts in the field.

You also said you tied steel together, but you didn't mention running the dowels the full length of the slab either.

[MiamiCuse]

Thanks Robert. I will try it.