Concrete scaling
The surface flakes off in thin patches, typically a sixteenth to a quarter inch deep, exposing the aggregate underneath. It usually starts at edges and joints and spreads across the slab over successive winters.
VerdictCosmetic — does not compromise the slab
What causes it
Scaling is freeze-thaw damage on a surface that could not resist it. Water in the near-surface pores expands about nine percent when it freezes, and with nowhere to go it breaks the paste apart. The two conditions that make a slab vulnerable are missing air entrainment and a weak surface layer — usually from finishing while bleed water was still present, which works excess water back into the top and leaves it porous. Deicing salts accelerate everything by driving more freeze-thaw cycles and pulling water into the slab osmotically.
How it is repaired
Scaling does not repair invisibly. Shallow scaling can be resurfaced with a polymer-modified overlay after the loose material is removed and the surface is profiled, but the overlay is only as sound as what it bonds to. Extensive scaling on a slab that was never air-entrained will keep scaling, and replacement is often the honest answer. Do not skim-coat over loose material.
How to prevent it
Specify air entrainment for any exterior flatwork in a freezing climate — ACI 318 requires it for exposure classes F1, F2 and F3. Do not finish while bleed water is on the surface. Cure properly, seal with a penetrating silane or siloxane, and keep deicing salts off the slab for at least the first winter.