Concrete crazing
A network of fine, shallow cracks across the surface, often described as looking like a dried lake bed or cracked glaze. Individual cracks are hairline and rarely more than an eighth of an inch deep. Most visible when the surface is drying after being wet.
VerdictCosmetic — does not compromise the slab
What causes it
Crazing is surface shrinkage. The top few millimetres dry and shrink faster than the concrete beneath, and the restraint from below tears the surface into a fine network. Anything that dries the surface quickly causes it: sun, wind, low humidity, or finishing operations that bring too much water and fines to the top. Intermittent wetting during curing — letting the surface dry between soakings — is a classic cause.
How it is repaired
Crazing is cosmetic and does not need repair for durability. It cannot be removed without resurfacing — grinding and polishing, or an overlay — and both are worth doing only if the appearance matters enough. A sealer will make the pattern more visible, not less, so test on a small area first.
How to prevent it
Cure continuously rather than intermittently. Protect fresh concrete from sun and wind, use an evaporation retarder in hot, dry or windy conditions, and avoid overworking the surface or sprinkling water on it during finishing.