What is the minimum thickness of a concrete slab that is supported directly on the ground?

What is the minimum thickness of a concrete slab that is supported directly on the ground?

• A. 4
• B. 3.5
• C. 4.5
• D. 3

Answer: (B)

The main idea is that a concrete slab on the ground must be thick enough to carry expected loads and resist cracking from shrinkage and temperature changes. For interior slabs directly on grade, 3-1/2 inches (about 90 mm) is the standard minimum. This thickness provides sufficient bending strength and room for finishing while keeping the slab economical. Slabs that are thinner would crack more easily, while thicker slabs aren’t usually required for typical interior floors. In situations with frost considerations, heavy loads, or poor subgrade, additional thickness or reinforcement may be needed, but the baseline minimum is 3.5 inches.

The main idea is that a concrete slab on the ground must be thick enough to carry expected loads and resist cracking from shrinkage and temperature changes. For interior slabs directly on grade, 3-1/2 inches (about 90 mm) is the standard minimum. This thickness provides sufficient bending strength and room for finishing while keeping the slab economical. Slabs that are thinner would crack more easily, while thicker slabs aren’t usually required for typical interior floors. In situations with frost considerations, heavy loads, or poor subgrade, additional thickness or reinforcement may be needed, but the baseline minimum is 3.5 inches.