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Slab on grade

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I am writing this not as an engineer, but as an owner/contractor.  My
experience, both as SE and contractor, is exclusively with steel.  All I
know about concrete is that it cracks.

I am acting as engineer/contractor/owner/banker/inspector/anything-else-
anyone-can-think-of on a 54'x105' hangar.  (I've always heard that
anyone who acts as their own attorney is employing a fool, does the same
apply to engineers?  It's fun wearing all the hats!)  

I plan the SOG to be 6" 3000 psi concrete with #4@12" both ways in the
middle, and a 12"x12" thickened edge, tapered to normal slab thickness
at 2:1, on 4" clean gravel.  The footings are not cast integrally with
the slab.  Soil is well compacted red clay common to eastern Tennessee. 
Use will be for planes less than 3200#, plus dump trucks, backhoes, and
any other toys my wife acquires.  The finisher is just that, a
finisher.  He knows nothing about the stuff we're supposed to worry
about - rebar placement, crack spacing, mix, etc.  Three questions:

1)  How often should I cut control joints.  I will form and pour the
slab in two pours on seperate days, each 27'x105'.  I want the minimum
number of joints, but I also don't want any random cracks.  Suggestions?

2) What about fibers?

3)  Is there anything in the above that jumps out at those experienced
with concrete and shouts "This guy is an idiot"?  Anything I should do
different?  Again, suggestions?

By the way, if there are any pilots out there comfortable on 1900' of
grass, The 100 Aker Wood private field is at N85d46' W84o46'.  Drop in
someday.

TIA

Phil Hodge
phil(--nospam--at)joistdesign.com