Durability
Concrete's ability to resist its service environment — weather, chemicals, abrasion, and corrosion — over its intended life. It is distinct from strength.
What does durability mean?
A mix can meet its strength requirement and still fail in service if it was not designed for the exposure, and durability failures are far more common than strength failures. The main levers are low permeability, appropriate air entrainment, adequate cover, and real curing. Because durability problems take years to appear, they are systematically undervalued during construction.
In practice
Almost every concrete complaint a homeowner has after year five is a durability issue, not a strength issue.