Almost every concrete building on Guam will show spalling eventually. Salt in the air works its way through the concrete to the steel reinforcement inside, the steel rusts, and rust takes up more room than steel does. That expansion pushes the concrete off from the inside.
What you are looking at
The early sign is a rust-coloured stain bleeding through the surface. Later the concrete above it sounds hollow when tapped, and eventually it falls away and exposes the bar underneath.
When it is cosmetic
A shallow patch on a non-structural surface — a garden wall, a decorative column — can wait. It will keep spreading, but it is not holding anything up.
When to call someone
Exposed rebar on a beam, a column or a slab edge is structural, and the repair only gets more expensive the longer the steel is open to the air. If you can see bar, get it looked at.
How the repair actually works
Cutting back to sound concrete, cleaning or replacing the corroded steel, treating it, and rebuilding the section with a repair mortar. Skipping the steel preparation is why patch jobs fail within a year.
