Air entrainment
Deliberately creating billions of microscopic air bubbles in concrete so that freezing water has somewhere to expand. It is the primary defense against freeze-thaw damage.
What does air entrainment mean?
Entrained bubbles are far smaller and far more evenly spaced than the accidental voids left by poor consolidation, and it is the spacing that provides protection. Air improves workability and reduces bleeding, but it costs compressive strength roughly in proportion to the amount added, so targets are ranges rather than maximums. Air content can drop during pumping, long hauls, and over-finishing, so the number at the truck is not necessarily the number in the slab.
In practice
Any exterior slab in a freezing climate should be air-entrained, and it should be broom finished rather than hard troweled.