Concrete cover
The thickness of concrete between reinforcement and the nearest surface. Cover protects steel from corrosion and fire and lets the bar develop its strength.
What does concrete cover mean?
Required cover varies with exposure, since steel near a surface exposed to weather and deicing salt needs more protection than steel in dry interior concrete. Too little cover leads to rust staining, cracking along bar lines, and eventual spalling as the corroding steel expands. Too much cover reduces the member's effective depth and its capacity.
In practice
Rust stains running in straight lines on a concrete face are cover failing, and they signal a repair that gets more expensive the longer it waits.