Hi Glen
I said earlier that my new hangar has been built without a concrete floor, exactly along the lines of what Nev has suggested. I would echo this as a genuine alternative to a slab, depending on what and where you are building.
I have built plenty of slabs. My wife and I built our whole home including outbuildings, workshops, sheds and now hangar entirely with our own hands. We have only had outside assistance with electrical wiring and our off grid solar system. All our slabs have been hand mixed and poured on site, because we are too far for a concrete delivery (even with retardant) and at any rate our home site is inaccessible to trucks.
In our experience the key issues are the mixture, quality of cement and materials but most of all the base and screeding.
My wife is an excellent screeder. I personally wouldn't worry to much about polishing unless you're aiming to get onto Grand Designs.
But even then we are now opting for a gravel /metal dust compacted floor. Can't face doing another 144 square metre slab!
But seriously, throwing up a shelter structure isn't a big deal and can be done quite cheaply if you do it yourself. No arcane knowledge required. My wife and myself have hoisted our fabric roofed arched structure
unassisted, with total budget of under $10,000. Would accomodate two C172s comfortably..
Alan