A concrete slab that size would be a no-no in my opinion.
Next time you are in a big city and parking in a multi floor parking garage, take a look at what the floor is made of. Yea, concrete.
Thank you all.
Kienbien.
you can get engineered prestressed concrete panels for this application. i do not know about the span and weight limits though. your engineer shoudl be able to clue you in though.
the nice thins about the prestressed panels is they assemble like 'planks" and have a v groove joint to use mastic between to make tight for the gas fumes and water.
after rereading you posts, i think you are expecting a miracle carbon fiber, fiberglass epoxy solution to help you out weight wise, and i am not sure of any such solution.