Freeze and thaw
Why does asphalt crack over the winter?
Water gets into the asphalt, freezes, expands about nine percent, and levers the sides of every opening a little further apart, dozens of times a winter.
The numbers
- Water expands roughly nine percent when it turns to ice.
- One freeze does almost nothing. A winter of them is what opens a crack.
- A crack that takes water in October is a wider crack in April.
- In northwestern Wisconsin freeze and thaw does more damage to asphalt than sun does.
The cycle, step by step
A hairline crack takes rain. The temperature drops overnight and the water in it turns to ice, which takes up more room than the water did, so it pushes the sides of the crack apart. In the afternoon it thaws, the crack is now very slightly wider, and it takes a little more water than it did yesterday.
Run that dozens of times between November and April and a hairline becomes a crack you can put a finger in. Nothing dramatic happens on any one night, which is exactly why it goes unnoticed until spring.
Why it is worse here than further south
The damage is done by the number of times it crosses freezing, not by how cold it gets. A place that goes cold in December and stays cold does less of this than northwestern Wisconsin does, where a sunny February afternoon thaws the surface and the night freezes it again.
Why it turns into a base problem
Once a crack goes all the way through the asphalt, the water reaches the compacted stone underneath, which is the part carrying the load. Wet base loses strength and freezes and heaves, and the asphalt over it starts flexing. That is the point at which filling a crack stops being the fix.
It is also why crack filling is the cheapest work on any surface and the work that pays for itself the most times over.