[TowerTalk] Concrete base spec info requested

[Ray]

I am pouring two tower bases on Tuesday. I am looking for some general info
on the concreted needed for the bases. The towers are older, US Tower -
MA770 and a Tristao - CZ 471. Both engineering details (1999 and 1977) state
the concrete Fc is 2000 lbs. I think this means the compressive strength of
the concrete. I thought the typical strength is 2500 lbs, and if I remember
correctly, on another 70 ft tower I poured 30 years ago, I used 3500 lbs
strength.
 
Can anyone tell me if I should be using 2000 lbs concrete? And are there any
other additives that should or could be used? 
 
Ray, 
N6VR
 
(Near Prescott, AZ)