A lot of concrete slabs under underpasses or along creeks are places that graffiti artists like to tag, where they are out of the way from the casual eye and like Patrick said backs of warehouses that face rr tracks or alleyways. Over the last couple of years so many local govts have taken on the idea that any graffiti is bad and they usually fine property owners who don't clean it up quickly off of their buildings and if they don't comply within a couple weeks they will send in a crew to either scrub the work off or paint over it.
If you want some ideas of what can be seen in Deep Ellum you can look here.
Art In and Around Deep Ellum Photo Gallery by pgkps at pbase.com
There have been some additional works added by the large slabs on the bridge supports that I have shot but have not had the time to upload so there is actually more in the area now than in the album. (I would bet that a few more in the dog park area will be done in the next few months).