I use a sheet of styrene, and scribe in the expansion panels. Then airbrush a mixture of gray and black to make it look like concrete, and accentuate the panels with black, being a bit sloppy so it looks like the asphault filler (at least I think it’s asphault, maybe somebody else out there knows what the black stuff is [:)]). Add some grime with pastel colors, and the occasional oil stain and you’re done. I did the tarmac in the diorama below using this technique.
i cut sheets of balsa wood (the ones that are about 3"x1/8") into perfect squares, glue them down onto your base, then spray on some of the “stone” finish spray paint, then sand it down so it isn’t so rough. After that i use putty to fill in the cracks and then paint it black (it’s OK if a little runs over the edges–it looks like Tar) After that, i just airbrush a mix of black and some brownish colors onto the cracks to make it look a little more weathered.
Example:
Note: I made my cracks too big–don’t leave that much space in-between