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Mortared bricks have been removed from a 50-year-old 12' x 12' concrete slab patio, leaving a rough surface and exposing a curved crack in the concrete which is about six feet long.

If small-diameter pea-gravel is spread onto the rough surface to make it smooth and then some 25 PSI 3/4" thick XPS foam insulation panels are placed on top of the gravel, and then new concrete is poured onto that surface, would there be sufficient decoupling to prevent the crack from propagating up through the new pour?

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  • $\begingroup$ I feel like the problem would be not having a stable base. no contractor would warrant their work pouring on top of such a base. $\endgroup$
    – Tiger Guy
    Jul 7, 2022 at 18:53
  • $\begingroup$ I don't have the hands-on experience. I'm curious why a concrete slab with deep haunch footers, which has been in place for 50 years, would be less stable than freshly compacted soil, and less stable in a way that a decoupling or crack-isolation membrane couldn't address? What is the nature of the instability? $\endgroup$
    – Tim
    Jul 7, 2022 at 19:28
  • $\begingroup$ Also, warranties tend to be for one year in my town, so there's not much to them, and that's the case whether you spend \$1,000 or \$10,000. $\endgroup$
    – Tim
    Jul 7, 2022 at 19:38

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If it is so important not to have cracks, going along with your solution I would just add this.

If the concrete below is slab on grade, cut some joints on it using a diamond blade saw to release the strain built up in it after years of creep and shrinking.

Then place the top layers and possibly use vinyl underlayment that is designed to resist moisture. And new pour has to be placed with adequate #3 rebars and cured properly.

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  • $\begingroup$ +1 Thanks for the good suggestions. The concrete contractors want to demolish the whole thing and start fresh, which ups the cost quite a bit, so I'm hoping to convince them to do it with the decoupling membrane. $\endgroup$
    – Tim
    Jul 7, 2022 at 13:01

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