I did observe what appeared to be corrosion on some of the prestressing strands at the failure point. You can make your own judgement from that. The test and all will confirm or deny whether that was the real mechanism of failure. The question is how rapid you might see it. I have not seen the plans is was designed for 100 pounds per square foot.
Moisture getting on the steel is a typical reason.
I know this type of bridge is used in parking decks fairly extensively, used some in highway pedestrian bridges, but exact numbers I do not know. Visual inspection might not catch it all. You can get some ideas, but to 100-percent sure, there's probably no foolproof technique. The best way that I can describe the effects of a prestressed girder is if you took a stack of books, put your hands on both ends and squeezed them together, you can put a lot of load right in the middle of that. That's what strands do on a concrete block.