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Reinforcing concrete slab with dowels



 
 
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  #1  
Old February 17th 08, 04:15 AM posted to alt.home.repair
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Posts: 665
Default Reinforcing concrete slab with dowels

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


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  #2  
Old February 17th 08, 05:09 AM posted to alt.home.repair
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Posts: 256
Default Reinforcing concrete slab with dowels

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
  #3  
Old February 17th 08, 05:38 AM posted to alt.home.repair
external usenet poster
 
Posts: 930
Default Reinforcing concrete slab with dowels

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
  #4  
Old February 17th 08, 04:08 PM posted to alt.home.repair
external usenet poster
 
Posts: 665
Default Reinforcing concrete slab with dowels


wrote in message
...
On Sat, 16 Feb 2008 23:15:08 -0500, "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



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?


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


  #5  
Old February 17th 08, 04:11 PM posted to alt.home.repair
external usenet poster
 
Posts: 665
Default Reinforcing concrete slab with dowels


"Robert Allison" wrote in message
news:fkPtj.36007$we5.12736@trnddc02...
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


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


  #6  
Old February 17th 08, 04:26 PM posted to alt.home.repair
external usenet poster
 
Posts: 665
Default Reinforcing concrete slab with dowels


"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.




  #7  
Old February 17th 08, 04:39 PM posted to alt.home.repair
external usenet poster
 
Posts: 665
Default Reinforcing concrete slab with dowels


wrote in message
...
On Sun, 17 Feb 2008 11:08:30 -0500, "MiamiCuse"
wrote:


wrote in message
. ..
What kind of surface are you installing
over this?


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


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


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.


  #8  
Old February 17th 08, 06:45 PM posted to alt.home.repair
external usenet poster
 
Posts: 191
Default Reinforcing concrete slab with dowels

You may want to conside using this or a similar product.

http://www.schluter.com/6_1_ditra.aspx
"MiamiCuse" wrote in message
...

wrote in message
...
On Sun, 17 Feb 2008 11:08:30 -0500, "MiamiCuse"
wrote:


wrote in message
...
What kind of surface are you installing
over this?

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


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


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.



  #9  
Old February 17th 08, 06:53 PM posted to alt.home.repair
external usenet poster
 
Posts: 256
Default Reinforcing concrete slab with dowels

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
  #10  
Old February 17th 08, 10:07 PM posted to alt.home.repair
external usenet poster
 
Posts: 327
Default Reinforcing concrete slab with dowels

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.
 




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