Most parking garages in NYC are valet parking.
In another lifetime I worked for an AE (architecture and engineering) firm that specialized in repairs to existing buildings. Many of our projects were parking garages, and some were rather terrifying. Parking structures are almost all reinforced concrete. The floor slabs are designed to be the minimum possible thickness, both to save weight (which saves money) and the minimize the height of a multi-level structure.
The problem is that in northeast, where they use salt on the roads in winter, cars pick up wet salt slush as they drive, and then drip the salty solution onto the concrete while they're parked. The salt soaks into the concrete and corrodes the reinforcing steel. It's a recipe for disaster, but many developers prefer to roll the dice and skip putting a protective coating on the top of the slabs, probably on the theory that they'll sell the structure before it becomes enough of a problem to require expensive repairs -- or just give up and collapse.
