Hi There;
I use Polys light grey on styrene & then rub it with carbon paper. Dose’nt show up as good as I would in pics but looks real good in the flesh.
If you can find a craft store or a department store with a craft selection that stocks Delta Ceramcoat craft paint, their “lichen grey” color is very close to a new concrete shade and you won’t have to mix paint. Or you could use Polly Scale or Floquil concrete.
To get a rough textur when painting with a brush try using a technique I saw in a FSM mag. from the '80s. The tip was used for representing cast iron by stippling or poking at the wet paint with a paint brush that has bistles the same length or is flat across the botom ( a Testors plastic brush with the white handle works great ). If you are using an airbrush or rattle can, try, on the last coat, spraying from a little further away from the surface ( giving the particles of paint a chance to dry before the hit the surface, thus giving the surface a rough texture ). These techniques have given me good results, after some practice, for years.