Is my 24x24 slab good enough?

[GlenW]

I'm in the process of building a new 24x24 single story garage in NS, Canada. I've been using local contractors that I've been introduced to through our local Rona lumber yard. Ground prep work seems to have been done very well, needed a lot of topsoil removed and good gravel fill due to uneven ground. B gravel build up and compacted at multiple stages and 6" A gravel on top compacted as well.

Issue is, Slab was put in just after I caught covid, so I wasn't directly involved, literally too sick to go outside to talk with them.

The 24x24 slab was put in as a monolithic pour, but with no footers. The entire slab was poured ontop of 2" xps with a 6"x6" curb in the same pour (included within the 24x24, also on the flat foam (Outside of slab is 10" (4"+6" curb). Heavyduty wire mesh was used on the entire slab. I don't believe rebar was used anywhere. I believe it was 3500psi concrete.

Should I be concerned about this slab cracking under the weight of the walls? It's not how I would have done it and had several (non pros) question the slab, but is it "good enough". I'm looking to hear thoughts on if it's almost guaranteed to fail this way, or if it will most likely be fine for 20+ years. I haven't started the build yet, so I want to fix anything at this stage should I need to prior to spending $40-50K CAD to build on top of a bad pad. Hoping to hear it's good to go, but it is what it is.

The lip you see sticking out around the entire pad is approximately 4" wide of the 2" thick foam.

Thanks,
G

 

[Garcky]

The contractor who did that must know what is required for that slab, I'd think. I certainly can't comment on it. Have you talked with the contractor since the slab was finished?

 

[GlenW]

Yes, I've spoke with him previously and he said "we do it all the time". I've just had several people comment on the way it was done since then and looking for more qualified input prior to saying anything further.

He seemed to know what he was talking about, but with all of the contractor horror stories you hear about. I'd just like to get more than his word.

 

[jack stand]

Your first few comments about removing the topsoil leads me to think that you don't have to worry.

 

[Garcky]

Yes, I've spoke with him previously and he said "we do it all the time". I've just had several people comment on the way it was done since then and looking for more qualified input prior to saying anything further.

He seemed to know what he was talking about, but with all of the contractor horror stories you hear about. I'd just like to get more than his word.

I'm betting it will be just fine. I assume there are building codes and inspections where you are, as there are here. Reading what they did to prepare the site before pouring the concrete seems to reinforce my idea that it's OK, as well. In some ways, it's a lot like the 12' x 24' slab I poured for the free-standing workshop I built many years ago. The reinforcement with a wire grid (mine was 6" x 6" x 6-gauge wire. The slab never cracked at all in the years after it was built.

I'd just go ahead and build your structure and enjoy it. Worrying about it won't change it, anyhow.

 

[GlenW]

Where I'm at, any "accessory building" 24x24 or under and one story is only a pre site inspection to ensure it follows setbacks and sizing requirements. All they want is a location drawing, size and height. No post inspection necessary after that.

I think where you're located has similar frost to me, so I hope the comparison to your 12'x24' is a sign of what mine will be like. I'm just worrying about the slab now, in the hopes of preventing worrying about my entire garage later. Even though I don't know what I could do at this this point.

Cheers and thanks for your input!

 

[kbs2244]

this is how it should be done

 

[K'ledgeBldr]

this is how it should be done

I wouldn't be so fast to say "this is how..."-
Even though the OP has no other inspections, every AHJ has "their" way of how's it's to be done. So, there are many ways to build an acceptable foundation. Building the right one for the soil, climate, structure, is "what's best"- BOP.

 

[readhead]

Sounds like a version of an Alaskan slab and they have been proven for years. The lack of rebar would cause me some concern.

 

[Moss]

It seems good from what you described, the only thing that stands out as "different" to me for my area is the over pour outside the curb but I don't think it will have any negative impact. What are your areas of concern? You mention comments from others and that you wouldn't have done it the same but don't tell us what you think the potential shortcomings are?

Edit* maybe you are concerned about rebar in the curb/wall? You don't seem to know for sure there wasn't any used though.

 

[billconner]

Codes in US exempt accessory buildings 600 sf and under from frost protection. I join those that believe it will be fine. I am curious how thick it is at edge just 4" plus upturned curb?

If the base is good, it will be fine.

 

[ConCretin]

In geotechnical and concrete terms, the weight of a wood structure is inconsequential so that's not really an issue if everything was done properly. If there's any subsidence of the stone around the perimeter, cracking is likely so keep an eye on that. It looks like there was a quite a bit of fill under the back and far side.

I wouldn't be too worried about the lack of rebar. In this application, it wouldn't do any more than the wire mesh they put in.

If I'm being honest, it isn't how I would build it but it will very likely work just fine.

 

[Zeke]

Garage? No slope? Huge birdbath? That wouldn't stay long on my property.

 

[bluedog225]

No way to know without a soil analyses and engineer’s report. Probably fine.

 

[Leaflessshadetree]

It doesn't look very flat with the water pooled on it. At least it isn't pooled along the walls. I'd expect rebar in the curb (especially around the corners) but they could have bent and tied the mesh which would serve the same function. Around my area (central Illinois) they usually make the edges thicker (deeper) but flat is common elsewhere.

 

[klassenl]

I'm in Alberta. Mine was poured 26x36, 6 inches thick with a 16x16 thicked edge, no curb. This is what was required. Talking with the general contractor and the concrete guys we all agreed it was a bit ridiculous and 4inches would have been enough and probably a 12inch thickened edge.

Prepared and poured well it will be fine. If you don't think you can trust your contractor why did you hire him? Are you really going to trust "some guys" who don't do concrete for a living?

Finally there are only 2 guarantees with concrete slabs.

1. It won't get stolen.
2. It will crack.

 

[murd]

Don't have anything to add, just wanted to welcome another Caper!

Which Rona are you dealing with? I couldn't get Don Ray to give me any prices after 3 attempts.

 

[GlenW]

Hi All,

Thanks for all of your input. I'll try to hit on all of the commented points.

Some have commented on how thick the edge was and the overpour. You may not have caught that there is no thickening of the edge and no overpour (aside from a few leaks around the form). A pad approximately 25'x25' was setup with 2" 40PSI foam panels. The 24'x24'form was made on top of that. Pad is 4" and curb goes to 10" (6" inside). What looks like overpour is just the 6" of 2" foam on the outer edges.

Fill (amount and quality) - This appears to have been done very well. Yes, the far corner was raised about 3 ft. There are multiple 1+ ton rocks around the outer edge to help hold it all in. The small excavator couldn't even lift them, just slid them into place. They were ground rocks from the original site, just previously used as landscaping. The gravel stones in that area are a concern and I'll do what I can to stabilize them. Likely shoot a bit of landscaping foam under the edges to lock in the small stones and then put some larger stone on top to prevent help prevent erosion.

What I would have done? - I would have followed more what was in the guide that was provided by KBS2244. Main difference would have been to add something along the lines of a 12"x12" footer around the edge of the mono slab. It's really the absence of the footer that I was concerned with. There are some aspects of this flat slab that I would think would actually be a benefit. It will allow a bit more lateral movement with less stress on the slab.

Level - It is fairly level. seems to have a bit of a ring where the center and the edges are same height and a ring that may dip 1/2 or so. It seems like it may just have high part in the front keeping the water in.

Trusting contractor - As the saying goes, trust is earned. His name was provided by the builder and excavator that Rona put me on to. This guy is a local concrete guy. Does garage pads and home foundations. I've just moved back home to NS from ON. I'm just used to a much different way of doing things. A 24x24 here is treated like you bought a 10x10 shed kit at Home Depot. No drawings, engineers, etc. needed. Just where are you building it. Ok, go ahead. I think most rural areas in ON are more strict with the 10x10 shed. ;-) At this point I have no reason to trust, or distrust him. He could be a concrete genius and I'll be dead before this slab even gets a hairline crack. I'm just looking for other opinions.

Hi Murd! - Surprised to find a fellow Caper on here. I'm in Sydney, so dealing with the Rona on Townsend St. It still takes a few tries/reminders to get anything from there as well. Nice guys though. Prices are stupid, but I think it's jut the standard now. Everything is adding up to way more than I expected.

 

[K'ledgeBldr]

From one pic it’s hard to see/say…
But, is the embankment just covered with gravel? If it is, I wouldn’t consider that a “permanent” erosion control. Whatever is a native ground over up there I’d be planting within the gravel.
“For sure, eh?”

 

[GlenW]

Yes, just gravel. I was just thinking of putting larger flagstone or something on the embankment to hold it all in eh! The native ground cover is another good idea. May do both. Tanks eh! ;-)

 

[andyvh1959]

I'd say nothing to worry about. In Oct 2019 (quite wet here in Green Bay) my slab contractor poured the edge lock (12" edge) 4" thick floating slab for my 24 x 28 shop. Site prep: contractor scraped back the top soil, very sandy underneath, topped that with about 6" to 8" of reground compacted laser leveled asphalt, topped that with 2" of pink stiff insulation. Pour was done in mid October over rebar. Two rows of 8" block went around the perimeter, 16' OH door and 3' man door on the front 24'. 5x12 pitch rook with front and back hips. Now three winters later I have yet to find any stress cracks.

 

[NUTTSGT]

Monolithic pour with a thickened edge or a 4" thick slab (with a 6" curb) ?

 

[dcg9381]

Sounds like a version of an Alaskan slab and they have been proven for years. The lack of rebar would cause me some concern.

Aren't some of these fiber-reinforced slabs in lieu of rebar?

 

[jerzm3]

The way I am reading this, is the contractor "prepped" the base, put down the sheets of 2" Insulation, then "formed" on top of the insulation and poured the concrete? It looks like the excess insulation from under the concrete is "wavy" around the perimeter which has me worried. I would feel much better if they formed the whole space and laid the insulation inside the forms. I just question the strength of the structure given the order of operations, if that makes any sense? Maybe I am completely wrong in the way I see it?

 

[billconner]

I think only question was how well was asphalt base compacted.

Was the foam extension purposeful?

 

[Garcky]

I think only question was how well was asphalt base compacted.

Was the foam extension purposeful?

I think that's there so they can build the forms on top of it to get full concrete depth with standard lumber forms. Once the forms are removed, the foam is still there. I think it could be trimmed off if the owner wanted. That's my take on it, anyhow. I've never poured a slab on foam insulation, so I'm not certain of that, though.

Based on what the OP wrote, where he lives accessory structures under 600 sq. ft. don't have a lot of design requirements. They appear to be treated like small sheds are in other jurisdictions. I think the floating slab will handle the building just fine.

 

[bb29510]

let look at from a differnt view. that curb going hold the whole weight of the building, you can calculate that, so pull out your calculator and start figuring. once you get that number, will the stem wall bend and break under the weight, the footer is to transfer the weight to the soil. since its gravel, its not moving. now calculate the weight of whats in the shop. will that slab support it and the weight of the building

 

[billconner]

where he lives accessory structures under 600 sq. ft. don't have a lot of design requirements

Basics IRC - no frost protection for foundations for buildings 600 sf and under.

 

[andyvh1959]

The slab for my 24 x 28 garage is a monolithic pour, 4" thick with the 12" thick lock edge formed by sloping the fill down to meet the depth of the slab framing boards. The 4" thicknes is above the 2" pink styrofoam insulation, the insulation fit inside the slab framing. When the forms were taken off, there was no insulation visible, and I pushed fill up against the slab. I laid out Pex in floor tubing for an eventual floor heating system.

 

[PoorUB]

I would have preferred a 12" thickened edge with rebar, in this city it is required.

Also that slope concerned me. My garage was built on about a foot of fill. Two sides of the garage are just gravel sloping away. I had the slab mud jacked a few years ago and I can see it will be due again in a few years. The gravel settles slowly over time and the weight of the building squeezes out the fill.

I have intended to build a low retaining wall with landscape blocks thinking it will help slow the soil slumping away.

In your case I think I would have built a foundation wall on the back sides and back filled it.

 

[jerzm3]

That's kind of my point. When the structure is erected, wouldn't it compress the 2" insulation it is poured on? I assure you the base was not 100% level and flat. Any high spots or low spots would be some trouble areas, I would think.

 

[billconner]

Most rigid foams are rated for at least 15 psi which is over 2000 pdf so no, it won't compress.

 

[wssix99]

What I would have done? - I would have followed more what was in the guide that was provided by KBS2244. Main difference would have been to add something along the lines of a 12"x12" footer around the edge of the mono slab. It's really the absence of the footer that I was concerned with. There are some aspects of this flat slab that I would think would actually be a benefit. It will allow a bit more lateral movement with less stress on the slab.

Most folks here with a monolithic slab have the thickened edge under the soil, as shown in the guide. I would build one this way because the structure on top is much easier to deal with. ... Just nail it down to the slab.

The curbs are more exacting and less forgiving for what must go on top. (But, that's your contractors problem.) Stucturally, they serve the same purpose as the thickened edge under the soil and is another form of a wall footing. So, I think you are fine in principle. I would expect that there should be rebar in the curb. If so and if the spec of the curb and slab came from a bona fide engineer or municipal spec at some point in the past, you should be set.

 

[GlenW]

Hi All!

Thanks for all of the feedback. The general view seems to be that there may be alternative and/or better ways to do it, but it should work. Key concerns are maintaining the gravel around the back and side edge.

I may shoot a bit of landscaping foam around the outside edge to lock in the top gravel which is where the erosion will all start. Then I'll likely lock it in further with either large stone and/or some native grass etc. to bind up the soil.

Next step is to cross my fingers and hope it all works out for years to come.

Cheers!
G