The modulus of rupture and the subgrade modulus combined determine the required slab thickness.

• A. True
• B. False
• C. Not enough information
• D. Both

Answer: (A)

The main idea is that a slab’s thickness is governed by how much bending it must resist and how well the ground supports that load. The modulus of rupture measures concrete’s ability to resist flexural cracking and failure; the higher this value, the more bending stress the slab can carry before cracking, allowing a thinner section for the same loads. The subgrade modulus describes how stiff the soil is under the slab; a stiffer subgrade distributes loads more effectively and limits deflection, which also supports a thinner slab. In design, you choose a thickness so that the bending stresses under the expected loads stay within what the concrete can safely carry (based on the modulus of rupture) and the deflection and bearing behavior are acceptable given the soil support (the subgrade modulus). If either property is less favorable—low MOR or soft subgrade—the slab needs to be thicker to meet those criteria. So, together, MOR and subgrade modulus largely determine the required slab thickness.