at the shop i work at we moved a bunch of machines last year and this is what we did.
Poured new 12" thick floor for all the lathes, mills, and threaders.
Poured a 3' deep pad for our grinders and used expansion board to keep it seperate from the 12" floor.
I don't know what the standard is but our floors were only 3" to 5" thick in some spots thats why we did what we did, and most grinders need a good pad and its better to separate it from others. Hope this helps some