I would use a concrete saw to cut out two foot by two foot squares at each column location remove the concrete auger holes as appropriately designed by the building engineer set the columns in the hole per the plans then form and fill with premix around the top of the column to complete the slab.
Pouring concrete floor in pole barn.
I recommend a standard 4 floor for most residential pole buildings.
If a thicker floor is desired excavate below skirt board bottom by any slab thickness greater than 4.
Since there are no foundation walls in a pole building a floating slab can be poured inside the poles using the skirt board for forms.
There can also be a delivery fee of 60.
Time lapse video of my 40x60 pole barn floor being poured.
If you have a cherry picker for motors or you plan on storing something very heavy then 5 or 6 floor will hold up better.
The cost of labor to pour concrete is 45 per hour.
The concrete is considered to float because it is totally independent of the building.
This can prevent damage caused by a pre mix concrete truck.
It is best to have the shell of your pole barn completed before you pour a concrete slab.
If your pole barn has solid walls they will serve as forms for pouring the slab.
If you are just storing toys lawnmowers and cars in the pole barn then 4 of concrete will more than suffice.
Labor cost for concrete slab.
Concrete slabs are poured either off or on the job site.
Time lapse video of my 40x60 pole barn floor being poured.