I would suspect that the interior portions of the dam run lower strength
(less % of portland cement) mixes. Major sections of the dam are
mostly filler.

I did some reading years back, the "gravel" used for the dam was graded
with rocks up to 6" or so. Why not, the best concrete mixes use a
variety of aggregate sizes to produce the best 'fit' of small pieces
next to larger pieces.

Jim Stewart wrote:
I just finished a book describing the building
of Boulder Dam, and there's something I don't
understand. The book mentioned that two concrete
batch plants were made, one to product "low-mix"
concrete for the lower portions of the dam and
another to produce "high-mix" concrete for the
top.

The two plants were not run at the same time.
Any ideas as to why they needed two separate
plants and what the difference in the concrete
was?