Why do random slabs stay dry in the rain?
Answers
The only thing I can think of is that the porosity between different sidewalk squres must be different. Perhaps a more porous square sort of wicks the water down into the concrete, leaving less water visible on the surface. But what's got me confused is how two different squares can have different porosities because I would imagine that two bordering squares would have come from the same batch and dried under the same conditions.
Depending on how the sidewalk is built, one square of sidewalk can sometimes take the majority of cement that a single mixer can hold. They use different brands for different batches of concrete, or varied how they mixed/poured it enough, it could make the concrete of different squares appear differently. My guess is that the water hitting it just accentuates the differences and makes them easier for you to notice.