Okay, since time is running out with the deadline tomorrow and my original idea taking too much time, I attempted to do something that is quickly realizable.
I worked on it throughout yesterday and the day before yesterday and almost totally forgot to open the WIP thread. :X
I chose a very iconic setting for UT's arenas: old, abandoned and run down industrial facilities.
As this post in the QA thread
already gave me a positive answer for my to-be-asked question if I can use own photo resources in a concept, I could just start right away and dug out old photos from my enourmous pile of pictures that I have taken to use them on top of the blockout.
I used the blockout called Shot07 as it was the most interesting one for me.
The evolution of the picture can be seen below in my attachments, this part of the process was mostly texturing the blockout with less painting involved.