Answer by Isaac Gaetz:
In the USA, the most common practice is to use a single layer of reinforcing, placed in the upper half of a slab on grade. The reinforcing is commonly welded wire fabric, not rebar. Typically the reinforcing will be placed at 2″ below the top of slab, at the first 1/3 point, or mid depth. The specific choice of the reinforcement depth will depend on the slab thickness (2″ and the first 1/3 are the same for a 6″ slab, for example).
For most soil conditions, uplift loading is not a significant concern, nor is flexure. Instead the reinforcing’s primary role is to limit crack width and reduce shrinkage. If we only place reinforcing below the neutral axis, we’ll create large cracking at the top surface of the slab. This happens for two reasons:
- First, the bottom concrete, enclosed on all sides by soil, moisture barriers, and the concrete above, naturally has less access to air and will generally cure more slowly than the concrete on top of it. Because the curing process involves the concrete losing moisture and shrinking, the top concrete with tend to shrink more and faster than the concrete below it.
- Second, the reinforcing, if placed in the bottom of the slab, with further restrict the bottom concrete from shrinking. This will amplify the problem identified in the first point above.
Instead, we use reinforcing in the top of the slab to neutralize some of the tendency of the upper concrete to cure and shrink before the concrete below it.
As with all concrete, the goal isn’t generally to eliminate all cracking, it is to control it.
Larger reinforcing rebar, including multiple layers of reinforcing is typically only used in expansive soils, or in mat foundations.