More than half of the concrete section is under tension usually.  If the beam have zero axial load their strength is not very highly dependent on the f'c of the concrete.  For instance doubling f'c does not half the required steel.  If axial load is involved then the f'c has a much greater influence on the strength capacity.  Try sizing some beams with a spreadsheet and change only the f'c value and see what happens.  It isn't that difficult to make a spreadsheet for nominal moment strength.