concrete/soil mix

[devildog]

Im wanting to use p.t. wood post for my foundation, and Im curious to know the reasoning behind the adding 5-10 parts soil mixed w/concrete Post and Pier Foundations found here on this site. Is it to get more material for less cost? will this weaken the strength of the concrete?
  In the company I work for we build small retaining walls and just use bag concrete "dry" and pack it around the post. It already has rock in it. and since im familiar with doing it this way I was wondering if this would be sufficient. Iwould definetly do the 6" of crushed rock at the bottom. thanks for your help.

[MountainDon]

If you mean using a soil/cement mix for packing around the PT piers/posts in a foundation, I don't do that and I don't think that is necessary. Our cabin piers were placed in the hole, plumbed and the same plain old soil, dampened slightly to pack better, rammed into place with an iron packing tool. The only time I've used concrete around posts was if I was in a hurry to get the post installed and wanted a quick method to firm it up. Pouring dry mix in the hole and wetting it works well for that.

However, all that irregularly shaped mass clinging to the post supplies more area for a freezing uplift force to lift the post, IMO.

Dirt in cement/concrete makes for poor strength concrete, but I suppose cement in dirt makes for more firm dirt.

[John Raabe]

In loose sandy soils a mix of soil and cement can add stabilizing mass to the lower area of a post. You do not want concrete or soil/cement up near the surface as it will hold moisture and encourage wood eating organisms. Below about 8"-12" they do not have access to oxygen and the moisture won't hurt the post.

Bottom line: If you need concrete or soil/cement for stability and mass don't carry it up to the surface. Use well draining material at the top of the hole. If you have solid well-packed soil that is normally dry you can just use that and forget the cement. Painting a tar wrap around the post on either side of the soil surface can also improve the lifespan of the post.

[MountainDon]

There ya' go... and I did the tar, 10" above and below ground level. Also mounded the dirt to promote water running away from the post.

[devildog]

thanks mountain don and john. I am going to put some type of tar around the post like recommended. I am also concerned about frost heave. my frost depth is 18", but for stability of post, im putting them at 24" depth to bottom and ill do 6" of crushed rock below that. and  Im thinking bout not filling whole to top w/concrete but leaving 12". thanks again, If it wasnt for the help of people on this forum, I wouldnt attempt this