Originally posted by: skyking
It sounds like a fun project. I toured the plant at concrete tech in tacoma, and I have been on projects using massive bridge beams they made, 120,000# beams 120' long.
The hollow core units they make would be ideal for your application. They are used in parking garages, with fairly high PSF ratings.
http://www.concretetech.com/hcloadtables.htm
Check out the double tees also.
You can build it out of parts and then apply a sealer, and also grade and work the soil above to direct and manage rainfall/runoff.