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Re: Misc Comments on Concrete Discoloration

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I think in most cases what you are seeing is moisture.  The concrete at the edges of a crack is more pourous than a finished surface.  It will retain more moisture and will thus stay dark longer.  One way to see if you have cracking in a slab or wall is wet it down and let the surface dry out.  The cracks will still be dark.  That is just a temporary discoloration though.
If you are seeing permanent discoloration along cracks,  I am guessing it is dirt or grease.  As far as I know, you don't get any discoloration from carbonation, at least until you put the phenolthalein on.
Gail Keley