The concrete platforms around Fillmore are fabricated with styrene sheet.  The expansion joints are made with a scriber and straight edge following lines drawn in pencil.  Prior to painting, I scuff all the flat surfaces with 400 or 600 grit paper and wipe down with rubbing alcohol.  This is how I paint concrete…

1. I apply Polly Scale Concrete thinned as usual for painting a model for an opaque finish.  This is usually a couple of drops of distilled water added to the paint in my air brush paint cup to make the liquid a milky appearance.  To gauge this appearance, I use a 3/32” brass rod to stir the paint.  This is the base coat.  Fortunately, this color is still offered by Testors in their acrylic Model-Master line.

2. Without waiting much (patience, or lack there-of, is another one of my flaws), I then stir up ~50:50 color & distilled water mixture of a grey like CP Grey or Reefer Grey.  I spray this as a random mottle all over.

3. I do the same again with a color like Dirt.

4. With the same ratio, I apply Grimy Black following the expansion joints mainly, but will apply a little here and there.

5. For heavily tracked areas, like truck tire marks, doorways, car loading spots, I usually spray a less-thinned application of Oily Black, but straight black or another very dark grey works well too.

6. At about this time the model doesn’t really look all that good.  The trick to make it better is to apply a ~50:50 mix of thinned Concrete on top of the whole area.  Like the late-great John Pryke’s “control coat” for locomotives, this subdues the previous applications.  The more I apply, the less grungy the concrete surfaces look.

7. Finally, a spray of Tamiya TS-80 Flat Clear lacquer.  I find that this also helps to blend everything in.

To add just a little more detail, I use a very fine pointed brush to apply some white glue to areas of expansion joints where weeds may grow.  I sprinkle some Woodland Scenics Blended Turf – Green Blend onto the glue and let it dry.  The excess is then vacuumed off.