Planning a Post Frame Building Upon Sandstone
Reader GEORGE in FARMINGTON writes:
“Mike, in the early stages of planning a pole barn 32’x48’x12′ on a newly acquired piece of property. We’ve cleared the land and found the area we want to erect the building is 100% sandstone. Everywhere we’d have to drill for poles is sandstone as the ground in-between so the sandstone would be the base for the slab as well. Drills real easy but I’m getting conflicting stories from concrete contractors regarding the slab. One says we should “de-couple” the pad from the sandstone by spreading a layer of crushed stone between the sandstone and the slab while another says we should drill into the sandstone and epoxy rebar verticals into it then tie that rebar into the slab rebar. Really don’t know how to proceed as both of these guys appear to be authoritative in their opinions. The de-couple guy claims the sandstone could “heave” the slab up and crack it whereas the other guy says by epoxying vertical rebar to the horizontal rebar, we’ll avoid any issues. Help!”
Mike the Pole Barn Guru responds:
Thank you for reaching out to me. I have never before been faced with building upon sandstone, so I will give you a logical answer and then advice as to what I would do personally.
In regards to our “de-couple” guy’s advice, if the sandstone would heave your slab up on its own, placing a layer of crushed stone between sandstone and slab seemingly would not prevent slab movement in a “heave event”. Rock types such as bedrock, limestone, sandstone, shale and hard chalk have high bearing capacities. These are very strong and good for supporting foundations because of their stability and depth. As long as sandstone is level your slab will be well supported. My question to him would be more of, “What could possibly cause such a heave event?”
From my research, it appears concrete adheres well to sandstone as long as lime is not added to concrete mixture. This seemingly precludes epoxying rebar verticals into it to tie them together.
Now – what I would do. I would search out a Geotechnical Engineer in your area and broach this situation to him or her. This way you get an expert opinion and it should be a reasonably priced solution.
Concrete: Cured or Dried?
Recently I posted an article in regards to finishing a concrete slab-on-grade. I admit to knowing slightly more about concrete than I do about plumbing. Muy poquito – one of the few Spanish terms I can actually pronounce (and have used all too frequently when visiting South America).
For those of you who missed my previous article (https://www.hansenpolebuildings.com/2019/12/how-to-finish-a-concrete-slab-on-grade/) I made reference to concrete drying out. I was corrected, as one reader felt I should have used ‘cured’ rather than ‘dried’.
Being fairly ignorant and having relied upon wisdoms (and terminologies) from actual professional concrete finishers, I broke out Google and went on a search. www.cement.org seemed to be a likely prospect for correct language and here is what I found:
“The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs. The following definitions clarify these terms.
Curing of concrete is defined as providing adequate moisture, temperature, and time to allow the concrete to achieve the desired properties for its intended use. This would mean maintaining a relative humidity in the concrete of greater than 80 percent, a temperature greater than 50 degrees Fahrenheit, and for a time typically ranging from three to 14 days depending on the specific application. When these recommendations are properly specified and performed in the field, the final properties of the concrete mixture will be achieved.
Drying of concrete is defined as providing the proper conditions to allow the concrete to achieve a moisture condition appropriate for its intended use. The moisture condition of a concrete slab is of significant importance for the application of moisture sensitive floor finishes such as vinyl composition tile, linoleum, wood flooring, and non-breathable coating like epoxy. The moisture condition is specified as a maximum relative humidity by percent or a vapor transmission rate in lb/1000 ft2/24 hr. A typical value specified for relative humidity may be less than 75 to 80 percent to assure the successful application of the flooring materials, while a commonly specified value for vapor transmission rate may be 3 lb/1000 ft2/24 hr.”
Personally I can live with these terms being used interchangeably, just don’t try to do it with cement and concrete (read why here: https://www.hansenpolebuildings.com/2019/01/cement-versus-concrete/).