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