We don't make concrete like that anymore. That old stuff is indestructible. It's full of local creek gravel, which around here, is hard chert and quartz. I once watched a guy try to break a well cover made from that old concrete with an 8,000 lb excavator. Pounded on it for ten minutes, and barely chipped the surface. I think I'd trust that old silo more than an identical new one made from modern concrete.