[TowerTalk] Mixing your own concrete?

[Alan NV8A (ex. AB2OS)]

The spec. for the foundation for the tower I intend to put up (AN 
Wireless 60' Light Duty) at this location (80mph winds) calls for close 
to 10 cu. yds. of 4000lb concrete.

How many bags of premixed concrete is that? (They are sold by weight 
rather than by finished volume). Is the stuff in bags up to that 
strength, and is it possible to mix that quantity quickly enough (even 
with an electric- or gasoline-powered mixer) to avoid having the 
first-poured stuff set already before the last lot is mixed and placed? 
I've worked in testing laboratories for construction projects, and I 
know how important it is for concrete to be poured within a certain 
period of time, or for the structure to be designed for discontinuous 
pouring.

Alan NV8A