In Robert J. Hoyle, Jr. and Frank Woeste’s 1989 textbook “Wood Technology in the Design of Structures”, in Chapter 21 (Post-Frame Building Design) on Page 336 is the following:
“Concrete encasement of the post in the ground contact area enlarges the friction surface and can generally be credit with a wood-to-concrete bond strength of 30 psi.”
Now what does this mean in plain language?
Let’s take a look at a very small pole building column, a nominally sized 4 x 6 (3-1/2 inch by 5-1/2 inch surfaced). Placed suspended into a 40 inch deep hole, so the bottom of the column is eight inches above the bottom of the hole, an 18 inch thickness of redi-mix concrete is poured around it. This poured concrete is known as a bottom collar.
The distance around the 4×6 is 18 inches. With 10 inches of concrete in contact with the sides of the column, the total surface area of the column, in contact with the surrounding concrete, is 180 square inches. Applying the 30 psi wood-to-concrete bond strength from Hoyle and Woeste, the union of the two materials results in the ability to resist 5400 pounds of force! Obviously bonding of a pole barn post in concrete is nothing to discount.
One of the forces which a pole building column must be able to resist is uplift – the columns literally being sucked out of the ground by the wind.
There are some who would believe the column is going to somehow shrink away from the concrete encasement. Keep in mind, the top of the 18 inch thick encasement is 22 inches below grade, which makes it unlikely surface forces will influence any possible column shrinkage.
Ignoring totally the bond strength between wood and concrete, I’ve seen cases where either numerous large nails have been driven into the column (in the collar area) before the concrete encasement has been poured. As well, I’ve seen holes drilled through the same area, and short pieces of rebar placed through the holes. Neither of these solutions offers the same amount of vertical resistance, as what is naturally provided. They won’t hurt anything, but it does seem to be a waste of both time and materials. The bonding of any pole barn post in concrete will provide the uplift resistance needed to keep your building…where you intended it to be.