Is the existing concrete base just a thin layer such as a pathway, or something more substantial? If it's a reasonable thickness, say 200mm or more, and spread over a wide area, then loxins set in the concrete will do a decent job if they're done right, though with the height you're after you'll need as wide a flange as you can get your hands on. I got my flanges from Blackwoods, and they were about $200 each.
Have a look at my recent pier build:
http://www.iceinspace.com.au/forum/s...ad.php?t=79998 for the steel pier with flanges,
and
http://www.iceinspace.com.au/forum/s...ad.php?t=58252
for the hole that it's set in to.
Cheers,
Jason.