Tag Archives: Modified Proctor Density

How to Pour a Slab on Grade

How to Pour a Slab on Grade in an Existing Barndominium

Reader PAUL writes:

“I have an opportunity to purchase a barndominium that has the posts set in 20” wide 40” deep peers. Unfortunately the county where this is located does not require a footing. All city codes in this area require an 8”X 36” footing. What solutions do you recommend for pouring the slab now that the shell has been erected?”

Most post frame buildings have shells erected then slab poured, so this should not be an issue. A pressure preservative treated splash plank should be in place around this building’s perimeter. It will become forms for your slab. Snap a chalk line on the inside of splash planks up 3-1/2″ from bottom, this will be top of your slab.

In Climate Zones other than 1 through 3, you will need to frost protect the building perimeter. This can be done by trenching around the edge of the building to required depth – 24″ in zones 4 and 5, 48″ in 6 and greater. It is usually easiest to install R-10 rigid insulation on the inside of the splash plank, with top of insulation even with top of slab to be poured. This also precludes any need to UV protect vertical insulation.

Depending upon how the site was prepared, you may need to excavate inside of this building. 

If in “frost country” a sub-base 6” or thicker should be first placed across the site. To maintain frost-free soils sub-base should be such as no more than 5% (by weight) will pass through a No. 200 sieve, and it is further desired no more than 2% be finer than .02 mm.

Prior to pouring, 2” to 6” of clean and drained sand or sandy gravel is spread below where concrete is to be poured. Mechanically compact fill to at least 90% of a Modified Proctor Density, otherwise slab could sink.

In areas prone to subterranean termites treat prepared soil with a termiticide barrier at a rate of one gallon of chemical solution per every 10 square feet.

Install a good, well-sealed 15mil vapor barrier below any interior pour, to stop moisture from traveling up into the slab through capillary action. Overlap all vapor barrier seams by a minimum of six inches, then tape. Vapor barrier should extend up column sides and to splash plank top. 

Minimum R-5 (R-10 being preferred) insulation shall be provided under full slab area of a heated slab in addition to required slab edge insulation R-value for slabs as indicated in International Energy Conservation Code (IECC) Table R402.1.2 Footnote (d).

In most instances, over properly compacted fill, 15 psi (pounds per square inch)  EPS (expanded polystyrene) or XPS (extruded polystyrene) insulation has adequate compressive strength to support a five yard dump truck on a nominal four inch slab on grade.

Consider this: 15 psi equals 2160 psf (pounds per square foot), making this greater than assumed compressive strength of most soil types.

If not using fiber-mesh or similar reinforcement additives to concrete mixture, place rebar (reinforcing steel rods) in slab center to add rigidity to concrete to aid in minimizing cracking.

Is #57 Crushed Gravel a Good Choice?

Thank you to reader KEN in BRECKSVILLE who writes:

“How do you secure the posts in the ground when you have a steep slope requiring longer posts in the back? 

The rear of a 48′ deep pole barn has about a 6′ drop in ground level from the front end. A retaining wall and 57 stone fill are planned to be used to level the site.  The wall height is 16′ and width is 36′.  The rear posts will need to be 6′ longer than the front to secure the posts in virgin ground not filled in with leveling stone.   At a 16′ wall height I will need posts at least 20′ long for the front and 26′ long for the back.  Do you provide posts this length or is it better to use shorter posts in the back to secure in the ground and attach additional posts on top of these posts for the wall heights.”

Before answering, I (knowing not nearly enough about stone) had to research 57 stone fill.

Contrary to popular opinion, there’s more than one type of gravel and selecting proper size and style is crucial to project success. One most commonly used and versatile type of gravel is #57 crushed.

When considering different gravel or crushed stone types, a number indicates sieve size a material is screened through.

A #57 sieve produces gravel materials approximately 1” – 1.5” in size. To put this into perspective, you can expect gravel in this size family to be around the same size as nickels and quarters.

Depending on where it was manufactured, #57 crushed gravel may be comprised of granite, limestone, trap rock. Because this type of gravel is so common, it is highly affordable.

Without using DGA (Dense Grade Aggregate comprised primarily of ¾” minus crushed stone aggregate combined with a careful and precise mixture of stone dust as a means of reducing void contents), or a similar material, you will find this is not an ideal sized gravel for creating an extremely firm, compact surface. 

My answer to KEN:

We can get columns over 60′ in length (I have them up to 50′ long on my own personal shouse in South Dakota). Attempting to attach a post on top of a post is not a good structural solution, so I would eliminate it as a consideration.

My recommendation would be to build your retaining wall first. Place fill in maximum six inch lifts, compacting each layer to a minimum of 90% of a Modified Proctor Density before adding the next layer (you will need to do this sort of compaction to support a slab on grade properly anyhow). You can then treat your fill as if it is undisturbed soil and would not require longer columns. Your idea of using 57 stone fill is probably not going to yield a good result, as it is not ideally sized to create an extremely firm, compact surface (what you want).

Post Frame Construction On Clay Soils

Many years ago, when I first went to work at Lucas Plywood and Lumber in Salem, Oregon I was given a quick tour of some areas where new construction was prevalent. Having moved from sandy/gravel soils of Eastern Washington, I was totally unprepared for bright red clay soils in this Willamette Valley region. When wet walking across these soils would add huge and heavy red clay mud balls to work boots.

Post frame (pole) building construction, or indeed any type of building, can become problematic when dealing with clay soil.

Reader JEFF in GAMBIER writes:

“I have a high water table, a 24 in diameter 5 foot deep augured post hole in clay soil will fill up in 3-4 days with water. Will a CCA treated 0.60 retention 3 ply glu-laminated post survive in these conditions or is this a good place for the concrete “perma-columns”.”

Mike the Pole Barn Guru says: Let’s take a step back – to site preparation:

At a minimum, site preparation includes:
· Remove all sod and vegetation.
· For ideal site preparation, remove topsoil and stockpile for later use in finish grading. In frost prone areas, remove any clays or silty soil
from within future building “footprint”.
· Replace subsoil removed from around building with granulated fill to help drain subsurface water from building.
· Distribute all fill, large debris free (no pit run), uniformly around site in layers no deeper than six inches.
· Compact each layer to a minimum 95% of a Modified Proctor Density before next layer is added. Usually, adequate compaction takes more than driving over the fill with a dump truck, or
earth moving equipment.

Why would clay be an issue to build upon? Clay expands and contracts depending upon amount of moisture present. When wet – clay expands, when dry it shrinks. These movements will cause buildings to move as well – not a good thing.

You might also add a french drain beyond the building perimeter, in order to direct water away from your site. Make sure to slope the ground away from your new building, no less than a 5% slope. Downspouts should discharge water at least five feet away from building.

Whether your site is adequately prepared or not, properly pressure preservative treated columns should provide more than a lifetime of use. Your real question to be answered is if you want your building to be stable and straight, or if you are willing to accept it moving up and down, in and out (and perhaps randomly) with time.

How to Re-level a Garage

Auntie Em, Auntie Em My Garage Has Lifted 

Well, it wasn’t from a twister and this article has nothing actually to do with Auntie Em or actress Clara Blandick who played Auntie Em in 1939’s film classic The Wizard of Oz. For trivia buffs, Blandick also played a part in 1937’s original A Star Is Born.

Reader GEORGE in LAGRANGE might be wishing a twister had hit his garage, so insurance would pay for a replacement. George writes:

“Due to the freezing and thawing cycle my pole garage has lifted about 7 inches since it was built 12 years ago. You can now see the outside grass from inside the garage. And it has not lifted evenly so the garage is unlevel.”

George’s post frame garage has some challenges, none of them ones with an easy fix. How did his garage get this way? There are three possible major contributors to this garage’s current situation. These would include:

Inadequate site preparation

At a minimum, site preparation includes:
· Remove all sod and vegetation.
· For ideal site preparation, remove topsoil and stockpile for later use in finish grading. In frost prone areas, remove any clays or silty soil
from within future building “footprint”.
· Replace subsoil removed from around building with granulated fill to help drain subsurface water from building.
· Distribute all fill, large debris free (no pit run), uniformly around site in layers no deeper than six inches.
· Compact each layer to a minimum 90% of a Modified Proctor Density before next layer is added. Usually, adequate compaction takes more than driving over fill with a dump truck, or
earth moving equipment.

For more details on proper site preparation please read: https://www.hansenpolebuildings.com/2011/11/site-preparation/

Column Depth

Bottom of column encasement needs to be below frost line. This is a no-brainer.

Water

Read more about what causes frost heaving here: http://www.hansenpolebuildings.com/2011/10/pole-building-structure-what-causes-frost-heaves/.

There is going to be no easy or inexpensive fix to George’s situation. An investment into a geotechnical engineer who could provide a thorough site evaluation along with solutions might be money well spent.


Building could be brought back to level by excavating at each raised column to well below frost depth. Cut off columns at base of splash plank (while supporting building from falling), then remove embedded portion of column. Place an appropriately sized sonotube in excavation with top of tube at grade. Pour premix concrete into tube and place a wet set Sturdi Wall bracket – expertly placed to receive upper portion of column. https://www.hansenpolebuildings.com/2013/11/sonotube/

If all of this sounds daunting (it would be to me), a consideration could be demolition and start over from scratch.