Q.: Where should we place rebar in a floor that's a slab on grade? I've heard it needs to be near the top. But to increase tensile strength, shouldn't it be near the bottom?
A.: In most plain concrete slabs on grade, reinforcing steel or welded wire fabric is used to limit the width of any surface cracks. To be most effective for this purpose, the reinforcement needs to be near the top of the slab. For corrosion protection, however, most specifications require a minimum depth of concrete cover that usually ranges from 1 inch to 2 inches.
If the floor is designed as a reinforced-concrete slab requiring rebar in the tension zone, the designer should specify the correct rebar position. In that case, a layer of rebar near the bottom or at both the top and bottom of the slab will be required.