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[edited to add more examples]

Along several streets in London I can see paving slabs that have become stained in the middle of the slab with white-ish marks forming different shaped patterns. The staining is permanent, all year round and not dependent on weather conditions or time-of-day.

Typically neighbouring slabs have similar patterns, in a small local area. This suggests it could be due to something underlying the slabs - possibly the Reflective Staining effect suggested by Keith McClary.

Does anyone have any other ideas what could be causing these shapes?

(These are public streets so not possible to lift the slabs to examine what is underneath.)

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  • $\begingroup$ I found "Reflective staining" but it doesn't look quite like that. $\endgroup$ – Keith McClary Jun 3 at 3:03
  • $\begingroup$ guess :some type of "pick up" system, as the stains are all at about the center of mass. Suction $\endgroup$ – anna v Jun 3 at 4:29
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Those stains look a lot like patterns that can be expected to result from the curing, shrinkage, and drying of concrete in a mold. The concrete would pull away from the mold as it cures, and dry from its edges. Any components of the mix that stay in solution will be pulled radially as the drying progresses, until the concentration reaches saturation; at which point the dissolved components will precipitate out, leaving a visible residue. The precise pattern that emerges will be a function partly of the composition and structure of the mold (for example, a wooden mold will warp differently than a plastic mold because it absorbs water and distorts as it dries).

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