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Please see below for picture. I would like to repair the cracks, but am unsure as to the ideal product. For reference, the picture represents an area that is approximately 3 feet by 2 feet, with a slight depression of a couple inches. Leading contenders seem to be Quikrete Patching Compound and Quikrete Polymer-modified Structural Repair. It is a shared driveway that sees regular vehicle traffic and would be difficult to shut down for any period of time. Ideally, I would just patch the cracks and resurface/fill the depression at a later time. Primary objective is keep from getting worse. Thanks.

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You typically don't repair something like that. It's likely very unstable, and any rigid repairs will quickly disintegrate. If you are limited by logistics or budget, consider a flexible skimcoat overlay to mask the damage and bring it to a rough level. Specific product recommendations are off-topic (and I don't have any anyway).

Really, the corners (or entire sections) should be cut out, and new concrete poured.

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    It also looks like the underlying ground needs to be augmented and stabilized. – BillDOe Sep 12 '17 at 18:15
  • Possibly, but the slab appears old enough that most of the settling has probably already occurred. A qualified mason should make that call. – isherwood Sep 12 '17 at 18:23
  • Hmm – Fizz Sep 12 '17 at 18:47
  • @Fizz, I'm not sure how relevant that is. The original repair was 3" thick, and the new repair is to a fairly stable-looking base. – isherwood Sep 12 '17 at 18:52
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A simple repair would be to Hire a concrete cutting saw and cut and smash out out a nice section beyond the cracks.

Deal with any subsidence below by filling and pack the hole with suitable material. It might just be that it wasnt compacted properly, or the concrete was laid too thin.

Drill holes into the sides of the concrete in the the cut out section and insert rebar. This will prevent the section of new concrete from slumping against the surrounding concrete. It's optional but I'd do this if this is a driveway. If it's not a driveway it might be ok with just friction. But hopefully you will have dealt with the subsidence in that section at least.

Pour new concrete and skim over it with a trowel to match the height of the surrounding concrete.

At least that's the way I'd do it if I didn't care too much about the look as any concrete your pour next to old concrete isn't going to match in colour or texture exactly.

If you want to go further, you could remove a much larger section and do much the same.

Just skimming it with concrete isn't going to last. There is obviously movement there and the skim coat will crack very quickly. If you don't care at all about look, you could also sledge hammer those chunks out and pour something thicker. It won't look as clean as a nicely cut edge with a saw.

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