best way to simulate "concrete"

If you want to simulate a carrier flt deck the easiest way would be to select a sheet of 'wet and dry ' sanding paper suitable for the scale of your kit
ie 400 grit for 32nd scale ,6 or 800 for 48th etc and attatch it to a base with
3M spray glue.
Spray it grey for concrete and weather it .
You can even mix different grit sheets like 400 with 600 to simulate different textures in the concrete and a big plus is your model will never slide off of your diorama.

In addition to the above, before shading and weathering, use the solvent based Floquil concrete as the base color. It is perfect. Don't use the Floquil water based acrylic concrete. It dries too green.

The Floquil primer is also a good shade to use. It is slightly grayer than their concrete.

Pete

I'm working on a display base at the moment, depicting some concrete runway and grass on the border.
I've used the thick card from the back of A4 note pads, various shades and slightly different thickness to simulate the concrete settling over many years. I've used pastel colours to shade the edges and I'm going to use railway scenery for bushes and greenery.

Artist matte board, suitably painted, looks good in larger scales, and if you want to get crazy you can cut them into individual panels, join them together to vary the "grain" and run a bead of black acrylic (tube) paint from a syringe in the joints.

Why not plaster?

You may also want to check what model railroaders have to say... they frequently incorporate roads into their displays.