The mesh should be about 1/3 of the way up from the bottom, not in the middle. The point of the reinforcement is to resist tensile stress, and the stress in the middle is 0.

Tensile stress increases linearly to the bottom of the slab, but you can't put the metal all the way at the bottom because you need enough concrete under it to embed it properly.

(I realize you probably didn't mean "middle" literally, but just in case...)