Concrete spalling
Chunks break away from the surface, deeper than scaling — often a half inch or more — sometimes exposing reinforcing steel. Rust staining around the damage is a strong signal about the cause.
VerdictMay be structural — get an engineer
What causes it
Spalling with rust staining is corroding reinforcement. Sound concrete is alkaline enough to keep embedded steel passive, but chlorides from deicing salt or seawater, or carbonation over time, destroy that protection. Corroding steel expands to several times its original volume and breaks the concrete off from inside. Insufficient cover is the usual enabler — ACI 318 requires three inches where concrete is cast against earth. Spalling without rust is usually freeze-thaw, impact, or fire damage.
How it is repaired
Spalling over reinforcement is a structural repair, not a cosmetic one. The concrete must be removed back past the corroded steel — including behind the bar, not just in front of it — the steel cleaned or replaced, treated, and the section rebuilt with a repair mortar specified for the job. Patching over rusting steel traps the corrosion and the repair fails within a few years. On anything load-bearing, this is engineer territory.
How to prevent it
Adequate cover, specified concrete strength, and air entrainment where it freezes. Keep chlorides off reinforced slabs, and seal exposed surfaces in salt-exposed environments. Corrosion inhibitors and epoxy-coated or GFRP reinforcement are options where exposure is severe.