Concrete platforms with electrowelded net are being realized in a substation renovation, see the photos. Do you share this solution?. I have some doubts about those because the earth grid becomes inaccessible.
The floor has to be paved with concrete plates on a sand bed-except the equipment basis and the access way. You need this to repair or extend the Grounding Grid and cable conduits [or pipe].
Also handholes [or manholes] have to be provided. This is what in a good design it is required.
I have only our experience in Power Station design [about 10 fossil-fuel power stations].But if it is about a large substation you have to fill the area with crushed rock as per IEEE-80 –for instance-in order to produce an insulated layer. In this case you don't need concrete plates. The reinforced concrete presents a low insulation resistance. See: IEEE-80/2000
Ch. 7.4 Effect of a thin layer of surface material