Overbuilt Barndominium Foundation
Hi All, planning a barndominium on my property and I wanted to run the design plan by some expert eyes. Right now the guiding principle is overbuilt. I want to create a slab that is going to be stable and warm, and am trying to minimize the chance of cracks. Heres the plan, please let me know if there are elements that are going to cause a problem.
24’x40′ monoslab. I want the footers to be below the frost line, which is 18 inches. My plan is to dig footers that are 12″ wide, and 20″ below grade. The slab will be 4″ starting at grade, so the overall footers will be 24″ tall with the top of the slab 4″ above grade.
I’m going to line the entirety of the footer with sheet insulation in addition to the vapor barrier.
In from the footer, in the area of the slab, i’m going to compact 3/4- 4″, and add one layer of the one inch 4×8 block insulation on top of the vapor barrier and insulation- as recommended for the pex radiant heat.
I’m going to run the rebar in a 12″ on center grid, and lay down 4 separate loops of pex controlled by a manifold, for in floor radiant heat.
From here, pour fiber infused concrete, add the stud bolts, and when dried polish and seal.
I came up with this design by synthesizing multiple sources regarding best practice for avoiding cracks and spalling (footers below frostline, insulating the actual footer, doubling the rebar, and creating an insulated base for the pex).
I understand that this is a deviation from the standard, and is overbuilt. My hope is that by doing that, I will have a better product, but if there is a catastrophic flaw pleases let me know.
Replies
IF you really intent on avoiding cracking, use "checkerboard" placements. With your dimensions, divide the slab into 8' x 8' panels. Your first concrete placement should be every other panel, with only the corners touching. You'd have a total of 15 panels - starting at one end, you'd place the 2 outside corner panel, then the 2nd middle, the 3rd outside, the 4th middle and the 5th (final) outside corner panels.
Then wait at least 2 weeks. allowing the 1st placement to cure.
Then place the remaining 7 panels.
Be sure to cover every placement with a curing blanket for at least a week.
After about a month after the final placement, chase the construction joints with a router type blade about 3/8-1/2 inch deep and fill with a self-leveling joint sealant that never fully hardens.
Then seal the surface (or epoxy coat after 18 months or more).
In MHO, no slab on ground should be thinner than 6".
12" centers on your bars is a bit much unless you're working with #3 or #4 bars. Set all reinforcing on bar chairs to insure its properly position in the slab (about 2: below the top surface).
BTW, the proper terminology for concrete is "cured" NOT "dried."
Your radiant heat might be a challenge using the method I describe.
[licensed P.E., 50 years in the concrete construction industry]
Thanks for the response. So there won't be any heavy equipment inside, that's all going under attached lean to's. As such there won't be much of a load to worry about. With that being said, what would the benefit of a 6" vs. 4" slab be?
I posted the same question on another forum, and after the responses it sounds like the rebar mentioned isn't worth it, given that there wont be a heavy traffic load, and wire mesh will be just fine.
I'm in the Pacific Northwest, so our winters can't compare to CT. But, do you still think it's worth digging the footers below the frostline?
And I'm sure that your checkerboard plan is a valuable option, but in reality that's more than I'm willing to do. This is my first time working with concrete, and it sounds like I'm outgunned for an operation like that. The plan is to pour the slab, than cut in the expansion joint lines to divide the 24' width into three sections, and the 40' length into four sections, making 12 8x10 rectangles. Once cured, and the building complete, I will hire out to a a polishing company to have it polished, stained, and sealed.
"what would the benefit of a 6" vs. 4" slab be?"
Better cover for the reinforcing steel. Better cover for your hydronic lines, which may float up during concrete placement and run the risk of getting cut if you choose to sawcut control joints. Even filled with water they are still less dense than concrete and will have a tendency to float.
"the rebar mentioned isn't worth it, given that there wont be a heavy traffic load, and wire mesh will be just fine."
Maybe so, but you can't guarantee that the mesh will be near the upper third of the slab even if you install it on chairs or bricks. Your concrete placing crew WILL be walking on the mesh and pushing it to the bottom. Hooking the mesh up after concrete placement is a joke and offers no assurance it'll be where it's supposed to be. Rebar installed on chairs has more 'springiness' and will better return to position after getting stepped on. It also offers a more reliable platform to tie your hydronic tubing to.
"do you still think it's worth digging the footers below the frostline?" In a word - YES.
"The plan is to pour the slab, than cut in the expansion joint lines to divide the 24' width into three sections, and the 40' length into four sections, making 12 8x10 rectangles."
First, they are control (contraction) joints, not expansion joints. Concrete shrinks as it cure because it loses volume as the water is consumed by the chemical process of hardening. The initial shrinkage begins to happen even before the concrete is hard enough to sawcut. Ask your concrete contractor/placer how many projects he/she has revisited 18+months or more after completion to confirm that no random cracks have appeared. You will have random cracks even if the sawcutting is done with a SoffCut saw as soon as possible after placement. Too many contractors come back a day or more later to sawcut - by then the random cracks have already started to propagate - you may not see them initially as they're too tiny to be noticed.
Post back here 2 years after completion and prove me wrong.
So you're saying its better to create the control joints in wet concrete with the trowel tool that makes lines (no I don't know it's correct name) rather than cutting with any type of saw?
I think rebar on chairs with a 6" slab sounds fine, but I'll probably just on 24" on center.
A tooled control joint is fine as long as it's deep enough. The recommended depth of a control joint is 1/4 slab thickness.
See: https://www.cement.org/learn/concrete-technology/concrete-construction/contraction-control-joints-in-concrete-flatwork
As far as bar spacing goes, the are 2 differing recommendations:
One is based on bar size and the other relates to bar spacing for crack control.
#3 bars can be spaced at 18" c/c, #4 bars could be as much as 32" c/c. Both based solely on bar size.
For crack control, smaller bars at a more frequent spacing works better. HOWEVER only 50% of the reinforcing should be continuous through a control joint. Essentially this means that every other bar should stop just short of the control joint. This then requires that control joint locations be known and laid out BEFORE placing the reinforcing steel.
Old timers might argue that "we've never had to do all of this and never had a problem!" IMHO it's a case of That they know of.
My retort to them is as I stated above: "... how many projects he/she has revisited 18+ months or more after completion to confirm that no random cracks have appeared."
BTW oversizing reinforcing is never a good idea. It might make you feel that you're making the end product stronger, but you're actually enhancing the probability of a brittle (sudden) concrete failure rather than the warning observed from slower yielding of the steel reinforcing. This not so much a critical issue for slabs on ground, but why spend more in reinforcing than you have to?
It's also similar (but different) for concrete strength. While 5,000 psi concrete seems better than say 3,500 psi concrete, in my experience the higher the concrete strength (i.e. higher cement content) the more prone it will be to shrinkage cracking.
Interesting. So #3, 18" cc, only 50 percent crossing control joint lines, and 3500 psi concrete to hopefully minimize shrinkage cracks.
This was helpful. Thanks. Right now I'm just in the planning stage so I can start getting an idea of cost. I'm not going to break ground until May. I will likely return with questions regarding optimized curing and insulation.
I also received the following response:
"where the inside of your 12" footer conjoin your 4" slab you will have a interior Crack around your entire perimeter when you get footer settle without slab settle."
Is he right, and if so how is this issue addressed?
Hello! Your plan for the barndominium slab sounds well thought-out and focused on creating a stable and crack-resistant foundation. It's great that you're prioritizing insulation, preventing cracks, and incorporating radiant heat.
Your approach of placing the footers below the frost line, using insulated footers, and adding vapor barriers shows a good understanding of the challenges that can arise due to frost heave. Similarly, your use of 3/4-4" compacted material, block insulation, and rebar in a tight grid demonstrates a commitment to minimizing the risk of cracking and enhancing the strength of the slab.
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"where the inside of your 12" footer conjoin your 4" slab you will have a interior Crack around your entire perimeter when you get footer settle without slab settle."
That will depend upon the soil bearing capacity that the footing is bearing on.
If you excavate down below frost depth and determine that it is virgin (never disturbed) soil AND you thoroughly compact that soil, the prospect of differential settlement (footing vs slab) will be minimized.
All of this assumes that the foundation wall & slab will be a monolithic placement.
Given the frost depth you're dealing with, it is more likely that you will be constructing this in 3 distinct steps, i.e. 1) footing placement, 2) foundation wall placement and finally, 3) slab placement.
If this is indeed what you will be doing, it is almost always wiser to butt the slab up against the interior face of the foundation wall with a bond-breaker between them. If (say at a door opening) you over-pour the slab onto the foundation wall you will more than likely find a crack in the future due to slight differential settlement and/or shrinkage of the slab away from the wall. A slab over-poured onto the top of the foundation creates restraint of the slab as it shrinks as over time. Something has to give.
There are 2 primary factors associated with this condition - slab shrinkage and/or slab settlement. Both can result in a crack in the slab.
IMHO, it is always wiser to separate a slab on ground from the foundation to allow for independent movement regardless of the contributing factor(s) that can result in later cracking.