There WAS a published booklet that based scantlings on, IIRC, steel ship scantlings. It basically provided some formula to derive one knowing the other. You needed actual data from slump tests of the intended cement for use with the formulas. I believe this was created by one of the American agencies, but it has been a while since I saw it.
The one book I've downloaded and have on this iPad might reference this conversion scheme, not sure if it does (an appendix does give what were then Lloyd's rules, but these don't seem to present a way to determine scantlings). It's "An Introduction to Design For Ferrocement Vessels", Ottowa/Hull 1972.
There was another book about building FC fishing boats that may have referenced those conversion formula but I cannot be sure if I'm remembering that correctly. As I recall, this book had extensive drawings of the shop set up to build such a boat, the requirements for same, which may help you to find the one I'm thinking of.
Chapter 10 of this:
http://www.fao.org/docrep/003/v9468e/v9468e00.htm#Contents
... gives some scantling rules "Whilst working in India, the American Bureau of Shipping was approached with a view to obtaining classification of future vessels built in India." The guidelines, from 1986, cover 10 to 20 meter boats. They may have been based on the conversion formula which may be older, I just can't remember, sorry. Still, they may give you somewhere to search.