What's the limiting factor on parking ramp life? Is it exposure, or are they just more cheaply constructed due to their function? Since concrete continues curing for along time I'd think a ramp would get stronger with age, at least for a while. Though, why was the Division and Fulton ramp torn down? I thought it was because of structural issues.
Winter salt (sodium chloride) / pellet deicers (calcium chloride) are really hard on the ramps. The newer ones seem to be more corrosion resistant but it's still hard on them. Security wise, I question the placing of the ramps under occupied structures or connected to occupied structures. I would think terrorism awareness should dictate separate parking structures.