On a set of engineering drawings for a project on which you are...

On a set of engineering drawings for a project on which you are working (Central Supermarket), a reinforced concrete floor slab spans over a grid of steel beams spaced at 6 metres in each direction. The drawings say that the slab is 120 mm thick and has N20 bars running in two directions both top and bottom of the slab (two-way spanning and continuous). In the specifications, cover to the reinforcement is noted as 30 mm top and 20 mm bottom and the concrete strength is noted as 32 MPa. The slab is an internal slab with no external exposure

please explain according to australian standards

why slab isn't thick enough and why covering should be equal (top and bottom of steel )

what should be done for this slab , is N20 and 32mpa enough ?

thanks