On Mon, 18 Feb 2008 23:57:46 GMT, Robert Allison wrote:

Mike Dobony wrote:
On Mon, 18 Feb 2008 00:32:10 GMT, Robert Allison wrote:


Mike Dobony wrote:


On Sun, 17 Feb 2008 11:26:06 -0500, MiamiCuse wrote:



"BobK207" wrote in message
...


On Feb 16, 9:09 pm, Robert Allison wrote:


MiamiCuse wrote:


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

No. Bad idea. The rebar is supposed to be protected from
exposure to earth or even air, because the most common problem
for rebar is failure due to rust. Embedding the rebar in the
dirt will drastically accelerate its deterioration due to
oxidation (rust). Your rebar should be no closer than 2" to the
dirt (or at the center of the slab, vertically).

In addition, the rebar in this instance is not to strengthen the
concrete, but to tie the old concrete to the new. This is to
keep the new and the old from settling differently. You want
both to act as a unit, therefore the dowels.

To make the job easier, instead of trying to drill straight into
the concrete, drill at a 45 degree or less angle (on the
horizontal axis) . Then install your rebar and bend it straight.
This allows you to get your hammer drill into tighter spaces.

--
Robert Allison
Rimshot, Inc.
Georgetown, TX

MC-

Robert hit all the important points....follow his advice & your slab
will easily outlast you.

Don't & the slab will fail in a few years.

A variable speed rotary hammer will make drilling easier.

cheers
Bob

Thanks Bob! Again!

well part of the reason I was having that wild idea (bad idea) is because as
I sat in the trench drilling I realized the "existing slab" I am drilling
into on the opposing end is along an exterior wall. Here is a picture of
the trench:

http://i173.photobucket.com/albums/w...g?t=1203265107

The PVC drain is 4" in diameter so the trench is about 3' wide and a bit
more in some areas. To the right there you can see the supply line to the
toilet and that is where the exterior wall is. See where the PVC pipe from
"north" of the toilet drain goes near the wall and underneath? Well the
plumber had that section of the pipe tucked along the length of the wall
until it gets to the lav, which means the underside of the slab along half
the trench has been disturbed, moved, dig around...the slab is just
"hanging" out there, I am not sure how strong that is. It's only 10" side
until it hits the 8" concrete wall. Can that 10" of hanging slab with
nothing underneath (I will pack sand back in sideways) be strong enough to
"tie into" and hold the 4' wide new concrete I will be pouring?

I don't know. I am just not comfortable what I am looking at, may be I am
overanalyzing, but in retrospect I should have limited what the plumbers
torn up, but that's too late now.



Do NOT run the rebar all the way from one side to the other. The rebar
needed to tie into the existing slab should be no longer than about 8-12"
total, half in the old and half in the new. You can run the rebar the rest
of the new work, but don't tie it into the stubs from the existing slab.

Why not?



It has something to do with expansion. My brother in law did commercial
construction for a number of years and concrete work was central in a lot
of it. When he was helping me with repairs he taught me this one.

I am a general contractor (38 years in construction) and worked
for five years as a general foreman for a concrete placement
company. I have never heard that before and I would need a much
better reason not to do it than you have given me.

I always tie the steel together. For what its worth, the times
that a concrete repair was spec'd by an engineer, they required
the dowels with epoxy and all the steel tied together. Since
that is the way that slabs with rebar are done, I think I will
trust the engineers and my own experience.

But I am always willing to learn.

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.