It is generally accepted that a compression strength of 2500psi is acceptable with fence installations, according to building code C387 which is the building code specification for packaged, dry combined matter for concrete. Premixed concrete such as Sakrete have 7 day compression strength of 3000psi and a 28 day strength of 4200psi. Site mixed concrete will yield similar results.
How do we at Sterling Fence Inc. decide when to mix concrete and when to use a premixed product? It almost always comes down to which method will give us the greater efficiency which translates to a better value for our customers. At Sterling Fence Inc we pour concrete footings on every post with our residential fence installations. We dig an 8" diameter footing for our Western Red Cedar wood fence installations and a 6" diameter for our 4' tall chain link fence installations. Many projects require 30 to 70 concrete footings to be poured. Using the premix products is not as practical as site mix for us in terms of cost or efficiency although the resulting quality is the same.. On the other hand, in doing repairs where we may be replacing 4 or 5 posts, the premixed products work well.
In summary, premix concrete products and site mixed concrete both have there place at Sterling Fence Inc. The main focus is to do the installation correctly with the fence looking straight and good over its lifespan.