The chemical reaction of cement in a concrete mix ends after about 116 years. From then on the the concrete mix becomes inert and degrades.
It was figured the dam would become a huge sand pit after silting in after a hundred years and before the concrete could fail.
So from 2050 on, people should be wary of being below the Dam, just damn man!
On another note, there are still die-hards that insist upon "Boulder Dam" as the name - something to do with a depression and FDR not getting the naming rights.