This base was a HDF plaque. I carved the joint lines for the pads and a few cracks and areas to show it spalling. Then I painted it it a sand-grey, dappling it with craft acrylics using darker and lighter colours, and a couple of washes. The craft acrylics created a little bit of surface texture as they aere stippled on. Added a few spots for fuel and oil leaks. When I was happy with the basic concrete colour, I filled the joint lines with a thin bead of white glue painted dark grey after it was dry.
I have also used some art board that had a pebbled finish. Cut squares for the pads and glued them to the base leaving a slight gap between. Painted them a grey-brown and filled the gaps with white glue painted dark grey.
Cork tile can be used. Usually one side is fairly smooth with finer cracks and the other side has much larger cracks. Cut to size, paint and add the expansion joints.
I have seen some bases done with various grades of sand paper or emery cloth for a bit rougher texture.
For a rougher, broken up surface I have used celluclay mixed with some white glue and rolled out on the display base. The more you roll it out, the smoother it becomes but usually shows a lot of `cracks`.
Spackle or Plaster of Paris can be used. Mix some of the base colour into it in case there are any unexpected cracks. It can be textured as it sets up and extra cracks and breaks added later.
For small areas like figure bases, I have used epoxy putty rolled out.
Cheers,
Rich