Hey everyone, I’m getting closer to the construction date of my detached garage and I’m getting more and more confused with details about the slab. The garage is being built in Montreal so will be subject to hot summers and cold winters. Garage will be kept above freezing in the winter by means of an electric forced air heater and brought up to comfortable temps when I intend to work in there. Garage is 18x26 with 9.5’ walls. My main concern is that I want to do everything possible to prevent or minimize cracks in the slab. Slab will be 12” thick around the perimeter with rebar and 6” thick with welded wire mesh in the center. 4500psi concrete will be used. I am confused about if/how/where to insulate the slab in order to get the best performance out of it. Any input would be greatly appreciated.
Help with monolithic slab design
- Thread starter Mike_72
- Start date
wssix99
Well-known member
I don't think a heated garage in Montreal mixes well with a monolithic slab. I would expect that you'd get some freezing around the perimeter, differential ground movement and lots of cracking.
I standard foundation to the frost line and a floating slab would be easier to deal with, but (obviously) more expensive.
If you want to stick with a monolithic slab, you'd need to heat the garage 24/7 and go with an insulation scheme like #5: (boards buried in the perimeter dirt, angled downward)
Other associated graphs: https://www.researchgate.net/figure...slab-on-ground-edge-insulation_fig1_320845857
I standard foundation to the frost line and a floating slab would be easier to deal with, but (obviously) more expensive.
If you want to stick with a monolithic slab, you'd need to heat the garage 24/7 and go with an insulation scheme like #5: (boards buried in the perimeter dirt, angled downward)
Other associated graphs: https://www.researchgate.net/figure...slab-on-ground-edge-insulation_fig1_320845857
Concrete cracks. It's just what it does. Controlling where it cracks is the solution. Proper placement of control joints is critical. The subbase is also extremely important. The slab itself will float around. You may want to consider upgrading to #4 bars 12 on center each way and L bars down into the footing
wssix99
Well-known member
Yea, but this situation could cause visible cracks, outside of control joints; even if the floor is reinforced.
ConCretin
Well-known member
There are two types of cracks; structural and shrinkage. A 6" slab on a properly constructed base is extremely unlikely to incur a structural crack. It's possible on a slab that size you wouldn't get any shrinkage cracks either but it's probably not worth the risk so you may want a couple control joints to hide them.
The challenge with a mono slab with a thickened edge is that a 1" saw cut isn't deep enough to ensure a crack in the deeper sections and the crack can jump the cut and become visible. It's advisable to increase the depth of the cut in these areas and even to extend it vertically on the face after you strip your forms.
As an aside, your choice of reinforcing won't prevent cracks but with a free-standing monolithic slab, I feel better with reinforcing bars over wire mesh to help hold everything together. I'd consider #4's at 18 inch centers and a couple continuous #5's in the bottom of the haunch.
Insulation won't directly affect cracking either but you want the structure to be affected uniformly by the frost. I would forgo insulation entirely and let the structure float -or- insulate under the slab and extend it out a distance equal to your the depth of your frost line on all sides to protect the structure from frost movement entirely.
The challenge with a mono slab with a thickened edge is that a 1" saw cut isn't deep enough to ensure a crack in the deeper sections and the crack can jump the cut and become visible. It's advisable to increase the depth of the cut in these areas and even to extend it vertically on the face after you strip your forms.
As an aside, your choice of reinforcing won't prevent cracks but with a free-standing monolithic slab, I feel better with reinforcing bars over wire mesh to help hold everything together. I'd consider #4's at 18 inch centers and a couple continuous #5's in the bottom of the haunch.
Insulation won't directly affect cracking either but you want the structure to be affected uniformly by the frost. I would forgo insulation entirely and let the structure float -or- insulate under the slab and extend it out a distance equal to your the depth of your frost line on all sides to protect the structure from frost movement entirely.
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I'm north of Winnipeg, my pad is monolithic. Insulation under and around perimeter as well as approximately 2 feet out on all sides. No heat in it as yet. But I put pex in the floor for it. Pad is 32X50, and it has shrinkage cracks.
I dont think you can stop them.. but like the others said controlling them would be your option.
I didn't have cuts in mine.
I dont think you can stop them.. but like the others said controlling them would be your option.
I didn't have cuts in mine.
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readhead
Well-known member
LLW described perfectly how I have been doing mono pours for metal buildings for the last 15 years. Haven't had any call backs so it must work.
One of the big factors for shrinkage cracks is the water content for the concrete. Make sure it is poured at a slump of 4. If the contractor wants to add water then add super-P to get it to flow. Use fibre along with remesh. Make sure the that to concrete stays wet on the top for 5 days. I used wet burlap on the top. This will allow the concrete to gain strength and not have shrinkage cracks. If you leave the forms on this will help to keep water in. Concrete needs to cure not dry.
Thanks for all the input everyone. Doing footings below the frost line seems like a great way to do it but just isn’t in the budget. So I guess my main concern is that I don’t want to do anything that would be unfavorable to the slab if the garage ended up not being heated for a season in the future. Even though my intent is to keep it heated above freezing year round. Are there any drawbacks (besides additional cost) of having R-10 insulation below the slab when it comes to either heating or not heating the garage.
ConCretin
Well-known member
Not really. Insulation under the slab would be of some benefit when the building is heated although in all honestly I'm not sure how much. When the building is unheated the insulation won't bother anything although it might be cooler than it would be without under slab insulation.
I have a 32 x 28 unheated garage on an uninsulated mono slab. There is a noticeable difference in temp from outside to in. I attribute this to the warmth from the ground rising into the building. Rigid insulation would block this flow. I could be all wet on this but its not really a drawback in any case.
wssix99
Well-known member
My main concern here is the potential for freezing around the edges of the mono slab and causing heaving, separating the outside edges of the slab from the stable base.
Heating year-round and proper insulation solves this issue, but heating strategies can change when the first utility bill arrives...
No, but keep in mind that the insulation along the edges of the foundation, or in your case - the mono slab, are just as important. You need to keep a "cushion" of heated ground underneath the slab, via the little heat that does get through the insulation boards. Some of the pictures I linked show this - think of your building as a heat hovercraft with the insulation around the boarder being the hovercraft's skirt.
That brings to mind... Maybe you could bury heat sensors around your slab to monitor ground temperature and drive your garage thermostat off of those inputs? You could lower the temperature at night, etc. just as long as the ground doesn't get close to freezing a few inches under your slab.
matt_i
Well-known member
I'd get the HUD design guideline for frost protected shallow foundations. Its a cookie-cutter design where you look up some stuff in various tables and it gives you the design at the end via a selection process. There are designs for unheated buildings (which would be appropriate for a sometimes-heated building). More or less one uses rigid foam and "wings" but the foundation is poured directly on top of the foam. Not a good idea you say, typical design guideline for soil bearing is 2000 lbs/sqft. A 25 psi foam (like Foamular 250) is good for 25 x 144 = 3600 psf.
I wouldn't mess around with a cheapo foundation. If it heaves your 5 figure investment into the top of it is kaput, tear down and start over....way too much risk to shortcut that stage in my opinion.
I wouldn't mess around with a cheapo foundation. If it heaves your 5 figure investment into the top of it is kaput, tear down and start over....way too much risk to shortcut that stage in my opinion.