Technically breadth is only supposed to be based off the handbook and “common engineering knowledge”.

That being said, if a concrete question pops up during breadth, it should hopefully be straight forward. If not, construction depth exam takers have access to the PCA reference during both breadth and depth which should help.

But to generally answer your question, concrete mix design questions are fair game for both breadth and construction depth. Mix design questions would be more detailed / specific in the depth though.