Need Help / Opinions on Base Compaction Flatness for Insulated Slab

[87BuzzinHalfDozen]

Hey all,

LONG time reader, first time poster! I’m overthinking and concerned about the flatness of my compacted base below my soon-to-be insulated concrete slab, and need some opinions. First a quick background before I get around to posting a build thread; I’m building a 30x38 stick built garage myself in Michigan, and I’m an over-thinking industrial engineer who stresses over stupid things! The actual building is already completed sans siding and garage doors. It sits on a 48” concrete trench foundation, so the slab will be floating obviously. Slab will be 6” thick 4000PSI fibermesh, ½” rebar in 16” grid spacing on 2.5” steel chairs , Pex for future in-floor heat tied to the rebar all over 2” Foamular 250 on top of a 10 Mil Vapor Barrier.

I bob-cated’ the crappy Michigan clay and topsoil out of there and brought in a gravel train load of 40 tons of crushed ¾” natural with fines I compacted in 4” lifts. I strung the floor every 2 feet with mason line and measured to the base with a ruler, marking the high and low spots with spray paint and striking it with boards as needed. I don’t know how I can get it any flatter. It’s been compacted enough times I had to stop because my vibratory compactor was starting to jump up and down and I was afraid of damaging the thing. My pickup with an engine in the bed does not even leave tire marks! I actually had my compaction inspection yesterday and the guy passed me with flying colors, saying it was better than 99% of the stuff he sees.

With that said, there are ½” gradual undulations I could chase for the next 20 years, and I’m going to be out of time to stress about it as I have my buddies lined up for foam and rebar labor help this coming weekend.

The problem is, places say you need ¼” of flatness, which I don’t know how I will achieve!? Am I just being **** or will the foam have these ½” deep voids in places, the floor with crack, my Jeeps will fall to the center of the earth, and life will cease to exist? All kidding aside, is this something I need to worry about or not? My math says the 25PSI stiff foam will NOT conform to the low spots during the pour with concrete that weighs only 0.5 Lbs per sq. inch. The internet is sparse with similar questions, but I have found some (one even on the garage journal) where they said it was no big deal for minor flatness variations, but the original posters never commented back on how their garage was holding up.

Any insight on the flatness would be greatly appreciated!!

 

[Lonnies Performance]

Sounds like you have already installed it way better than 90% of the contractors doing this work. I can guarantee mine is not installed that flat & I do not have any cracks a year later, while running an 11,000LB forklift & 24,000LB truck on it.

 

[87BuzzinHalfDozen]

Sounds like you have already installed it way better than 90% of the contractors doing this work. I can guarantee mine is not installed that flat & I do not have any cracks a year later, while running an 11,000LB forklift & 24,000LB truck on it.

Great to hear it's holding up, thanks

 

[p_mori7]

You are good to go.

 

[Chris705]

Your overthinking will give you a wonderful floor! The base compaction is wonderful and your foam over the 10 mil VB is ideal! I do hope your using micro fibers and not the long fibers ? Good luck in putting your rebar & pex in our place and may you have a great slab pour! Make sure your contractor saw-cuts that night or early the next morning.

 

[brownbagg]

square point shovel

 

[matt_i]

I used a laser level on the "coarse" range, I was using 3/4" washed/crushed stone, and basically found I could only get to a precision of "1 stone".

I didn't insulate although I kinda wished I did now. But I think using the Jack is excellent. The foam is going to help you, where there are sharp points engaging the flat foam its going to exceed the 25psi rating in small micro-areas, crush ever so slightly right there until there is more bearing, and then be fine forevermore. The concrete naturally distributes loads so once its placed you are all good.

 

[ConCretin]

It sounds to me like you've done things the right way. I wonder if you're grading job isn't better than you think. A floor graded to a 1/4" tolerance will vary from 1/4" low to a 1/4" high, which is what it sound like you have achieved. It's difficult to do much better but the flat side of a 36" loam rake works as good as anything when it comes to fine grading.

To be honest, I've always been mildly uncomfortable with laying rigid insulation under a slab for the reasons you mention. It's counterintuitive to prep a rock solid base and then leave voids under the slab. The literature says it's fine and it's done all the time with worse sub grades than yours so it must be OK, right? I've never experienced a problem or read about anyone who has so I guess it must be. Good luck with the placement and the rest of your project.

 

[87BuzzinHalfDozen]

Thanks everyone for the input and guidance, it gives me a lot more confidence in doing this slab! Part of the reason I'm building the garage myself is two fold: I can't afford to have someone else do it and it's fun to build things, and second, I've seen so much shoddy work from people I wanted to have full control of everything so at least if I make a mistake, it's mine to live with ; )

Your overthinking will give you a wonderful floor! The base compaction is wonderful and your foam over the 10 mil VB is ideal! I do hope your using micro fibers and not the long fibers ? Good luck in putting your rebar & pex in our place and may you have a great slab pour! Make sure your contractor saw-cuts that night or early the next morning.

Thanks Chris705, I had not specified the fiber yet, any input you may have? There are a lot of opinions on it and I can't tell what's valid or not. This will be the first time I've dealt with fiber in concrete.

 

[87BuzzinHalfDozen]

I used a laser level on the "coarse" range, I was using 3/4" washed/crushed stone, and basically found I could only get to a precision of "1 stone".

I didn't insulate although I kinda wished I did now. But I think using the Jack is excellent. The foam is going to help you, where there are sharp points engaging the flat foam its going to exceed the 25psi rating in small micro-areas, crush ever so slightly right there until there is more bearing, and then be fine forevermore. The concrete naturally distributes loads so once its placed you are all good.

Good to know, I have a cheaper laser level but I don't trust it much. I'm used to dealing with micron precision machined equipment, so wood, concrete and stone level accuracies drive me nuts, haha.

I had the same thought on the individual high stones deforming the foam well.

 

[ConCretin]

Thanks Chris705, I had not specified the fiber yet, any input you may have? There are a lot of opinions on it and I can't tell what's valid or not. This will be the first time I've dealt with fiber in concrete.

The truth of the matter is that properly timed control joints mitigates the need for fiber. If you are paying for saw cuts anyway, insist they are done immediately after finishing and you don't need fiber at all.

The sole purpose of fiber is to delay early age shrinkage cracks. Cut your control joints quickly enough and it will relieve the tension from drying shrinkage and eliminate the possibility of random shrinkage cracks.

 

[kbs2244]

The Willys guy knows what he is talking about.

Fiber is often sold as a replacement for rebar.
It does not come close to being a replacement.

 

[87BuzzinHalfDozen]

The truth of the matter is that properly timed control joints mitigates the need for fiber. If you are paying for saw cuts anyway, insist they are done immediately after finishing and you don't need fiber at all.

The sole purpose of fiber is to delay early age shrinkage cracks. Cut your control joints quickly enough and it will relieve the tension from drying shrinkage and eliminate the possibility of random shrinkage cracks.

The Willys guy knows what he is talking about.

Fiber is often sold as a replacement for rebar.
It does not come close to being a replacement.

Thanks Gents, I was actually thinking of renting an early entry saw and doing the cuts myself that evening, so it sounds like I can just skip the fiber then and save some money. I feel my 16" grid of 1/2" bar should be sufficient for the rest of the shrinkage Now I just need to find out how to split up the pad for the cuts, but that's a whole other debated topic ; )

 

[ConCretin]

Thanks Gents, I was actually thinking of renting an early entry saw and doing the cuts myself that evening, so it sounds like I can just skip the fiber then and save some money. I feel my 16" grid of 1/2" bar should be sufficient for the rest of the shrinkage Now I just need to find out how to split up the pad for the cuts, but that's a whole other debated topic ; )

Up here saw cutting costs about a buck a foot, which is pretty tough to beat by the time you make a couple trips to the rental company to rent one. (actually that cost just doubled with the new silica rules but that assumes your finisher is on top of things) It's not rocket science but a little experience also helps achieve straight cuts. Talk to your finisher about doing it before you take it on yourself.

Control joint spacing isn't all that debatable. 10-15' is a pretty safe range. Keep the panels formed by the cuts reasonably square and you'll be fine.

For the record, your rebar is a great but it won't help with shrinkage. In fact it will increase the likelihood of shrinkage cracks by restraining the slab from contracting. Limiting your mix water and curing the slab properly are your best bets when it comes to limiting shrinkage.

 

[Lonnies Performance]

My cuts are 10-12' apart on average depending on the area.

Rebar was used at the door openings & heavy mesh sheets (10ga ?) everywhere else.
No fiber, as I did not want random pieces in the surface of the floor.

Saw cuts were made the next morning & worked out well.
Tried one area late the same day & the edge was noticeably rougher.