raised beds can be done on concrete or brick slabs, all you realy ahve
to do is ensure you creat drainage outlets, by not melding the raised
beds edge to the cement under, and then i would suggest put a rubble
layer down first, cover that with some old fly screen or shade cloth
so it doesn't get clogged up, then sart layering your bedding medium.
i have some pics on my site 'building a garden', and also in the blog
at http://ausgarden.com .
i suppose another analogy is that container gardening is an extension
of raised bed gardening hey chuckle?
With peace and brightest of blessings,
"Be Content With What You Have And
May You Find Serenity and Tranquillity In
A World That You May Not Understand."