James wrote:
I am building a split-rail fence out of locust rails and posts. The posts
will be set about 8 feet apart. I plan to dig postholes about 18 inches
deep, and set the posts into concrete.

snip

Thanks for any advice on this !!

James

Your 18" post hole seems way too shallow for a fence. Better do some
more research on local conditions and methods and then worry about
concrete mix. Typically, some metal fence posts are 7' tall and 3'
into the ground. For a relatively short fence 2' would be a good
choice. YMMV

Joe