How does road salt and the way it reacts with water break down the surfaces of roads?

This answer comes to us from the archives of The Billings Gazette:

"Salt is frequently used to melt slippery ice and snow. That same salt can damage roads.

As salt is spread on roads, it mixes with the snow and melts. If a road is not sealed, the salty water seeps underneath. This may cause steel reinforcing bars to rust. The rust expands and pushes up surrounding concrete. Steel bridges are particularly vulnerable to salt-induced rust and corrosion.

Compounding the problem is the fact that roads expand and contract in hot and cold weather. Roads that are well maintained and sealed are much less likely to be damaged by winter salting."

Thanks, coldfuse! Great answer.