Which of the following is a primary design consideration for slab-on-grade thickness?

• A. Cranes capability
• B. Fine vs coarse aggregate
• C. Modulus of rupture and subgrade modulus
• D. Location of lifting anchors

Answer: (C)

The main factor in slab-on-grade thickness is balancing the concrete’s bending strength with the supporting soil’s stiffness. Modulus of rupture tells you how much bending stress the concrete can safely carry before cracking, while the subgrade modulus describes how the soil beneath will react and deform under load. Together, they determine how thick the slab must be to distribute loads, limit bending, and keep deflection within allowable limits. If the soil is soft or weak (low subgrade modulus), more thickness helps spread the load and control cracking and settlement; a stronger concrete (higher modulus of rupture) can permit a thinner slab, but overall thickness is still governed by the combination of these two factors and the project’s performance criteria. Other considerations, like cranes’ lifting capacity, aggregate type, or where lifting anchors are placed, influence construction practicality or concrete mix details but are not the primary drivers of slab thickness.