I have a small base that will form the mount for a 88mm Flak gun. I would like to paint it to look like concrete and I'm a bit stumped as to how to proceed. I have it primered in gray.

Suggestions?

This is what I am going for:

For concrete I use Polyscale Aged Concrete with a wash of brown,then dry brush with Polyscale Concrete.

A while back I built a concrete apron for a plane model, and I used spackle. Put down a few layers just like you would do on a wall, sand down with sanding block to ensure a flat surface. The surface has the same porous look that concrete does. Take a sharp point to scribe out and patterns (hence why you need a few layers to give it depth). Paint accordingly. It came out perfect. Also if you are really hardcore, you can lay down brass/aluminum rod in the final layer or two of spackle and when you sand it down it gives it the rebar just below the surface look and texture.

Ditto. That's what I do. Sometimes, I give it a wash of Paynes Grey.

Excellent idea!