What happens on the surface while the slab is curing
Crazing sets up in the first day or two after the pour, while the top skin of the slab is curing faster than the concrete underneath it. A steel trowel or a float worked too long, or worked while there is still bleed water sitting on the surface, pulls extra fine paste up to the top. That thin, paste rich layer shrinks at a different rate than the coarser concrete below it. Hot sun, a dry wind, or low humidity on pour day speeds the same thing along, because the surface loses its moisture before the rest of the slab has caught up. The result is a shallow, connected pattern of hairline cracks across the top skin only, not a crack that runs down through the slab.
Where it shows up
Broom finished driveways and patios poured on a hot, windy afternoon are the most common place to see it, along with power troweled garage and shop floors that got worked past the point the surface could take. A slab kept damp and out of direct sun for the first day or two, the way we ask on every pour, rarely shows it this badly. Crazing also tends to read worse than it is. Low morning or evening light catches the pattern of tiny cracks at an angle and makes a slab look far more cracked than it actually is under flat daylight.
Crazing versus a crack worth a phone call
Crazing is a web, not a line. It has no depth you can catch with a fingernail, no step between one side and the other, and it does not follow a straight path across the slab. A crack that runs in a single line, that has one side sitting even a little higher than the other, or that you can feel as a step under your foot, is a different problem, and it usually means something under the slab moved rather than the surface finish. We cover how to tell a cosmetic crack from a moving one on our tear out and replacement page. Crazing on its own does not put you there.