Modulus of Rupture in Concept

Not directly. According to the manual, section 54.6.17;

• Modulus of rupture (f cr ) is 7.5 root (f ′c) times the lightweight concrete factor. The maximum f’c for the cross section is used.
• Lightweight concrete factor is assumed to be wc / 145 pcf ≤ 1.0. The maximum wc for the cross section is used.

So you can indirectly control it by changing f'c, or to a lesser extend by changing the density.

Not directly. According to the manual, section 54.6.17;

• Modulus of rupture (f cr ) is 7.5 root (f ′c) times the lightweight concrete factor. The maximum f’c for the cross section is used.
• Lightweight concrete factor is assumed to be wc / 145 pcf ≤ 1.0. The maximum wc for the cross section is used.

So you can indirectly control it by changing f'c, or to a lesser extend by changing the density.