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New Concrete Over Old

 
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Old 01-04-2019, 02:58 PM   #1
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New Concrete Over Old


I am new to this forum and appreciate any advice. I am a retired architect and have been out of the trade for a while. I am helping my daughter with a project. She would like to construct a 16' x 20' concrete patio. There exists a 3' x 8' slab entirely within the new slab footprint. Can that slab remain if there is enough cover of new concrete over top. Also, this is in Hawaii - no frost concerns. Thanks.
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Old 01-04-2019, 03:23 PM   #2
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Re: New Concrete Over Old


demo the little slab.

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Old 01-04-2019, 04:02 PM   #3
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Re: New Concrete Over Old


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demo the little slab.
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Old 01-04-2019, 10:33 PM   #4
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Re: New Concrete Over Old


Quote:
Originally Posted by Parkitect View Post
I am new to this forum and appreciate any advice. I am a retired architect and have been out of the trade for a while. I am helping my daughter with a project. She would like to construct a 16' x 20' concrete patio. There exists a 3' x 8' slab entirely within the new slab footprint. Can that slab remain if there is enough cover of new concrete over top. Also, this is in Hawaii - no frost concerns. Thanks.


It will dry funny and crack due to the inconsistencies in absorption. As Griz said, demo it. My guys could have that busted up and hauled away by break.


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Old 01-04-2019, 10:47 PM   #5
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Re: New Concrete Over Old


Possibly break it into small pieces and spread it out, then pour.
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Old 01-05-2019, 05:44 PM   #6
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Re: New Concrete Over Old


Although she is in Hawaii, I would still demo the old slab. She won't have the heaving issue, we have in the colder climates, but as said above, it will be tough to finish. When I say tough, it will end up with a different finish over the slab. All of the moisture, over the slab, will have to come up through the top. Unlike the rest where a portion of the moisture will go down into the sub-grade.

It's not that big. Demo it and start fresh.
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Old 01-05-2019, 05:45 PM   #7
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Re: New Concrete Over Old


retired archie, that wasn't taught different, experienced first hand what happens when done, or recommended a fix?

i don't get it.
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Old 01-05-2019, 08:56 PM   #8
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Re: New Concrete Over Old


I think the OP's question is a good one. By placing a sheet of plastic over the area that would resolve the inconsistent finishing/curing issue mentioned. As long as all the other proper concrete steps are taken, what would be the problem(s)?
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Old 01-05-2019, 09:07 PM   #9
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Re: New Concrete Over Old


Cracking.
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Old 01-05-2019, 09:09 PM   #10
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Re: New Concrete Over Old


If it's a historically significant slab, put a plaque on it and pour around.
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Old 01-05-2019, 10:00 PM   #11
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Re: New Concrete Over Old


Quote:
Originally Posted by hdavis View Post
Cracking.
Quote:
Originally Posted by rustyjames View Post
I think the OP's question is a good one. By placing a sheet of plastic over the area that would resolve the inconsistent finishing/curing issue mentioned. As long as all the other proper concrete steps are taken, what would be the problem(s)?
Pouring over an existing slab will almost always cause the new slab to crack and essentially reflect the old slab.

I always place a vapor barrier under all our slabs so the concrete sets consistently, moister differences in the subgrade can dry out one area of a slab while another area may still be too wet to work.
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Old 01-06-2019, 10:40 AM   #12
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Re: New Concrete Over Old


break it up and spread it around.

what if they placed a new 4" pour over the existing slab and prepared the slab? I was guessing the OP was suggesting a thinner pour over the existing.
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Old 01-06-2019, 05:23 PM   #13
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Re: New Concrete Over Old


Simple decision, demo out a 3X8 or risk having to repair or demo a 16X20...
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Old 01-06-2019, 05:33 PM   #14
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Re: New Concrete Over Old


If the 3 x 8 existing SOG was at the correct elevation, how could raising the pour height ~ 4" give an optimal solution.....

Demo the old work...

Wet cut the old concrete into 3 pieces and drag(a few 3/4" pins or expansion bolt anchors, log chain, and skidsteer/ pick up out of the way for garbage can pedestals etc....

Low tech, lift one edge with a Johnson bar or two, hammer into Man and woman sized rubble, super cheap dig holes under new slab, plate tamp sand/good fill over them. If you are paying $$$ for waste disposal this might save some coin.

Barring Tsunamis and Volcano eruptions, this slab will outlast all of us, thus a good spot to invest a few extra $ for superior finish given by hiring a skilled craftsman....IMHO.

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Last edited by Fouthgeneration; 01-06-2019 at 05:47 PM.
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