Freeze-thaw cycling
Repeated freezing and thawing of water inside concrete, which expands as it freezes and progressively damages the material unless the paste contains entrained air.
What does freeze-thaw cycling mean?
Each freezing event generates internal pressure as water expands, and without microscopic air voids to relieve it, the paste is torn apart cycle by cycle. The number of cycles matters more than the minimum temperature, so climates that hover around freezing can be harsher than consistently cold ones. Damage shows up as surface scaling, internal cracking, and eventually crumbling.
In practice
Regions with wet winters that freeze and thaw repeatedly punish concrete far more than the coldest northern climates do.