Ramping Armor Armor

So I was thinking of ways to help with the whole armor pen and critical hits issue.

What about an armor that has "ok" stats (decent amount of hp but low armor), however; the passive on the armor would increase the armor of the champion the more the champion was hit by AA and Physical damage spells. Also, the shorter the duration between hits would cause the armor to increase more each time, allowing it to scale into late game with attack speed.