MiamiCuse wrote:

"MiamiCuse" wrote in message
...

"Robert Allison" wrote in message
newso%tj.4388$kD3.611@trnddc08...

MiamiCuse wrote:


Thanks Robert did not think about that...ok

although I think they will rust anyways. The original slab I broke has
wire meshes in the concrete and they were all rusted, the cast iron
drain below that I replaced has exterior corrosion. The water table in
Miami is very high.

Install the rebars eccentrically and bent it straight? What size rebar?

MC

OK, this is a response to both your reply to Bob above, and to this
message. I have looked at the photo that you provided and here are my
recommendations;

You have enough room to drill rebar dowels straight into the existing
concrete, so do that. It is best if they are straight and in the middle
of the slab vertically. (I was picturing a much narrower trench, where
it would be difficult to get the drill straight.)

Use #4 (1/2") rebar. Drill 1/2" holes and drive the dowels about 4-6"
into the existing concrete. You could do this on 2' centers along both
sides of the trench, probably just two in the ends (the short sides of
your trench). Tie these together to form the standard grid pattern. I
usually install a short piece of rebar in one side, and a long one in the
other, then just tie the two together with tie wire. I try to lap the
two pieces by about 12 inches.

Don't shove sand up under the overhanging edges. It will compact
sideways, but not vertically (which is what you want). Instead, mix up
some slurry and pour it under there. You can make slurry out of a very
wet mix of standard premix, but I usually spend the extra money and use a
wet mix of non shrink grout. The deleterious effects of the added water
in the mix is minimized by the extra strength of the grout. Pour this in
so that it runs under the existing concrete and fill it up to the BOTTOM
of the existing concrete. Let this set up, then pour your concrete over
this. You can use the sand to dam up the edge so you don't have to use
much of this slurry, just along the overhanging edges.

In addition, you could use a higher strength concrete, mix it wet, and
vibrate it under the existing slab. I don't like adding water to
concrete, so I would recommend the previous method. Makes for a stronger
concrete end product.

Last, but not least, use your concrete blade in your grinder to get a
straight vertical edge along the sides of the trench. Some of the edges
are straight enough, but you do NOT want concrete feathered like it would
be if you poured it like it is right now.

The best way to achieve straight edges is to cut the concrete to about
half its thickness with a masonry blade in either a circular saw, or a
grinder, then break off the remainder. You want a fairly straight
vertical edge on the existing concrete, but it does not need to be
perfect. You do not want your new concrete to be less than 2" thick,
ANYWHERE. I am not talking about the sum of the new and the old, I am
talking about just the new portion that you are placing. If it feathers
to nothing over the existing concrete, it will not have the strength it
needs to maintain its integrity.

Seems like a lot here, but it really isn't.

--
Robert Allison
Rimshot, Inc.
Georgetown, TX

Thanks Robert for all the advise. I have just one more questions.

This is another bathroom I need to pour concrete on. This trench is
smaller, where the widest part is about 3 feet and the narrow part 18".

The problem is the trench ends at the exterior wall B.

So in this case if I add dowels do I drill it into the wall?

Thanks again,

MC


oops forgot to attach an image link:

http://i173.photobucket.com/albums/w...8annotated.jpg

I can't tell from that photo, but if the slab is tied into the
wall, then I would also dowel the new patch as well. If they are
separate entities, leave them that way.

--
Robert Allison
Rimshot, Inc.
Georgetown, TX