Wang and Salmon, 3rd Edition page 809 and page 830 in 4th edition cover design of plain and unreinforced concrete.
For the bending of plain concrete, the authors use 5 root f'c for modulus of rupture with a strength reduction factor of 0.65.
Section modulus is still bh^2/6 and I = bh^3/12.
So Phi Mn = phi*f*I/(h/2) where f = 5 root f'c
Compare Mu to phi Mn like you would normally do.