Rebar corrosion
Rusting of embedded reinforcement, which expands to several times the volume of the original steel and forces the surrounding concrete to crack and spall.
What does rebar corrosion mean?
Fresh concrete's high alkalinity forms a passive film that protects steel indefinitely, and corrosion begins only when that protection breaks down, typically from chloride penetration or carbonation. Adequate cover, low permeability, and crack control are the primary defenses, with coated bar, corrosion inhibitors, or non-metallic reinforcement used in severe exposure. Once corrosion starts, it accelerates, because spalling exposes more steel.
In practice
Rust stains and cracks tracking in straight lines along bar locations are the visible stage of a process that started years earlier.