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Old 08-16-2011, 07:47 AM   #1
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Concrete form over uneven surface


I am remodeling a raised box fireplace with thinstone, and I have demolished the badly made raised hearth. I need to repour a new raised hearth and intend to make concrete forms. Problem is that the existing remains of the old hearth are bumpy.

I am looking for any tips on building my concrete form over this uneven surface.



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Old 08-16-2011, 11:35 AM   #2
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Have you tried putting a level down to see how much difference there is between areas? If there is seepage after you pour, you would just grind or chisel for clean up. Otherwise you can chisel before you do the pour. Any chance of a picture of the area that you are dealing with, showing what you are talking about.


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Old 08-16-2011, 01:00 PM   #3
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Depends on how bumpy. Slight irregularities will not be an issue. Just keep the mix away from the soupy side. A standard consistancy will leak a bit, but not much. If it's more then 1/2", use backer rod for the gaps and hold it down with a second piece of wood on the outside.
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Old 08-16-2011, 10:25 PM   #4
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Run duct tape along the uneven concrete with the edge even with the inside of where the form board will be (shown above in black. Run a second strip of duct tape as shown in blue. Set the form and fill the space between form and tape with spray foam. After the foam sets, use a sharp knife to trim the foam even with the inside of the form. Remove the excess foam and tape from the inside of the forming.
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Old 08-16-2011, 10:47 PM   #5
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Not sure I fully understand what you are trying to accomplish. But why be mindset on concrete for the repair? Would an epoxy compound work? I use this stuff all the time for concrete repairs. It is lighter and stronger when cured. You can sculpt it as you go with some practice.

Not sure about standing up to heat in a hearth though. You would have to ask the company I guess.

Last edited by user1007; 08-16-2011 at 10:49 PM.
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Old 08-21-2011, 06:55 AM   #6
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thanks everyone. I got the form made, and there only seem to be moderate gaps between the form and the existing concrete pad. I am going to try tape on the inside and spray foam or caulk on the outside.
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Old 08-21-2011, 07:32 AM   #7
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Pack the gap with plumbers putty.


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