This is a close-up of the "crack" that I noticed that most large, concrete floors have in RL. To make the crack, I crafeully tore the mat board, glued it back down, and used charcoal to highlight the crack line.
This is the floor that's going on the first level. Again, I ussed mat board. I made grooved indents with the non-serrated part of a plastic knife instead of laying out tiles and using grout. Then I used chalk pastels and charcoal to create the look of gray concrete.