Monolithic slab montreal

[Mike_72]

I know this topic has been beat to death but after having read countless posts on this forum and numerous articles on the web I have yet to come to a decision on what details to include in my slab construction. I’ll give you an idea of what I’ll be building.....

18x26 monolithic
32 mpa concrete (4700 psi)
12x12 thickened edge with 4 runs of #3 bar
6” center with #3 bar on 16”
6” curb wall
16” compacted gravel in center
10” compacted gravel around perimeter
10 mil vapor barrier

I do intend on heating the garage year round, cooler when I’m not using it but always above freezing. I’ve read conflicting information about the use of insulation under monolithic slabs in garages that will be heated. Some say not to insulate if you’re heating so that the heat from the garage can pass easily to the substrate below to prevent frost heaving, others say the opposite.

My main goal here is having a dry garage floor and minimizing cracking of the slab, is there anything I am missing thay may help me meet those goals?

My lot is very level and quite high compared to the water table. Sump pit in the house hasn‘t ever had water in it. Landscaping around the garage will be graded properly, roof will have eavestrough with downspouts well away from the structure.

I would appreciate any input you folks might have that will help me come to a final decision.

 

[strutaeng]

For nearly crack-free you need 10x10 saw-cut joints. Look in the flooring sub-forum...

4,700 psi? That's pretty high! 3,000 psi is considered standard for a slab on grade around here. 3,500 is high!

Generally, the higher the cement, the more shrinkage and more cracks!

 

[Mike_72]

For nearly crack-free you need 10x10 saw-cut joints. Look in the flooring sub-forum...

4,700 psi? That's pretty high! 3,000 psi is considered standard for a slab on grade around here. 3,500 is high!

Generally, the higher the cement, the more shrinkage and more cracks!

Yes, I forgot to mention I was going to put 2 saw cuts perpendicular across the slab. This would leave me with 4 9x13 sections. Would the fact that they aren’t perfect squares create any issues?

As far as the 4700psi, I went with the higher strength because I intend on installing a lift so I figured stronger is better and gives some more margin for error. The standard concrete they use is 30mpa (4350 psi). Maybe I should stick with that since it’s still well above the lift manufacturers minimum.

Thanks for your input!

 

[finn]

Not sure what you gain by going from 4” to 6” either.

The slab in my shop is 4” and the original owner parked logging trucks and forestry equipment inside, including bulldozers.

The cracks all seem to be shrinkage cracks and seem to be about the same as the 6” slab I had poured for my home garage.

I think I wasted money going to 6” for my home garage.

Both are monolithic slabs with pex.

 

[850xpeps]

Insulated under the slab. Monolithic Slavs can have frost in and around them. Keep your heat in the building. The slab is meant to be on its own and can move somewhat.

I would be ok with 5 or 6” thick. More so because it allows the peg piping and rebar to share the space and not be too close to the top or the bottom. How big of a building? I usually like my thickened edges a little bigger than 12x12. And I’m assuming you are sloping the inside edge of the thickened edge to come up and meet the slab at a 45?

 

[ConCretin]

While strutaeng is correct that higher cement content does minimally increase shrinkage, I believe it's a good decision. The higher compressive strength isn't really necessary but a lower water cement ratio will provide a more durable surface. If it isn't already in the mix design, I'd also consider a mid range water reducer to provide workability and minimize mix water.

As strutaeng goes on to mention cracks are primarily the result of shrinkage. This is even more relevant in a mono slab because the haunched edges restrain the slab from contracting. To avoid random cracks, control joints will be required. I'd do one the long way and two across. Increase the depth of the cuts as much as you can as you cross the thickened edges. A 1" cut in a 12" haunch might not be enough to keep the crack where you want it.

You could consider a 15 mil vapor barrier such as Stegowrap. This material stands up to construction traffic much better than poly.

Finally on the matter of insulation. The important thing when dealing with frost is uniformity. While it's true that your heated floor will keep the frost out in the middle, it might not be enough to keep the frost from pressing in around the perimeter causing differential movement.

Personally, I would install rigid insulation under the slab and extend it out 4 feet around the perimeter. This will make it a little more efficient to heat and the insulation will retain the neat from the ground and keep the frost out even if you stop heating the garage.

 

[Mike_72]

Not sure what you gain by going from 4” to 6” either.

The slab in my shop is 4” and the original owner parked logging trucks and forestry equipment inside, including bulldozers.

The cracks all seem to be shrinkage cracks and seem to be about the same as the 6” slab I had poured for my home garage.

I think I wasted money going to 6” for my home garage.

Both are monolithic slabs with pex.

6” seems to be the standard floor thickness that the contractor pours. I didn’t have an issue since the lift requires minimum 4” floor. Figured more is better.

 

[Mike_72]

While strutaeng is correct that higher cement content does minimally increase shrinkage, I believe it's a good decision. The higher compressive strength isn't really necessary but a lower water cement ratio will provide a more durable surface. If it isn't already in the mix design, I'd also consider a mid range water reducer to provide workability and minimize mix water.

As strutaeng goes on to mention, cranks are primarily the result of shrinkage. This is even more relevant in a mono slab because the haunched edges restrain the slab from contracting. To avoid random cracks, control joints will be required. I'd do one the long way and two across. Increase the depth of the cuts as much as you can as you cross the thickened edges. A 1" cut in a 12" haunch might not be enough to keep the crack where you want it.

You could consider a 15 mil vapor barrier such as Stegowrap. This material stands up to construction traffic much better than poly.

Finally on the matter of insulation. The important thing when dealing with frost is uniformity. While it's true that your heated floor will keep the frost out in the middle, it might not be enough to keep the frost from pressing in around the perimeter causing differential movement.

Personally, I would install rigid insulation under the slab and extend it out 4 feet around the perimeter. This will make it a little more efficient to heat and the insulation will retain the neat from the ground and keep the frost out even if you stop heating the garage.

Thanks for the input, I have a couple of questions... how would the control cuts be done when there is a 6” curb wall? Do I continue the cut vertically up the side of the curb wall? Also, due to proximity to the property line I would only be able to able to extend the insulation out on 3 sides. Could this cause even more issues than no skirt insulation at all?

 

[850xpeps]

While strutaeng is correct that higher cement content does minimally increase shrinkage, I believe it's a good decision. The higher compressive strength isn't really necessary but a lower water cement ratio will provide a more durable surface. If it isn't already in the mix design, I'd also consider a mid range water reducer to provide workability and minimize mix water.

Cracks are primarily the result of shrinkage. This is even more relevant in a mono slab because the haunched edges restrain the slab from contracting. To avoid random cracks, control joints will be required. I'd do one the long way and two across. Increase the depth of the cuts as much as you can as you cross the thickened edges. A 1" cut in a 12" haunch might not be enough to keep the crack where you want it.

You could consider a 15 mil vapor barrier such as Stegowrap. This material stands up to construction traffic much better than poly.

Finally on the matter of insulation. The important thing when dealing with frost is uniformity. While it's true that your heated floor will keep the frost out in the middle, it might not be enough to keep the frost from pressing in around the perimeter causing differential movement.

Personally, I would install rigid insulation under the slab and extend it out 4 feet around the perimeter. This will make it a little more efficient to heat and the insulation will retain the neat from the ground and keep the frost out even if you stop heating the garage.

Good advice here. What are you suggesting for a water reducer? My experiences have kept me away from water reducers on slab because I’ve found some keep the cement workable but then for lack of a better word “snap” on you.

 

[Mike_72]

Insulated under the slab. Monolithic Slavs can have frost in and around them. Keep your heat in the building. The slab is meant to be on its own and can move somewhat.

I would be ok with 5 or 6” thick. More so because it allows the peg piping and rebar to share the space and not be too close to the top or the bottom. How big of a building? I usually like my thickened edges a little bigger than 12x12. And I’m assuming you are sloping the inside edge of the thickened edge to come up and meet the slab at a 45?

Building is 18x26 and thickens edge meets slab floor at 45 degrees.

 

[850xpeps]

Thanks for the input, I have a couple of questions... how would the control cuts be done when there is a 6” curb wall? Do I continue the cut vertically up the side of the curb wall? Also, due to proximity to the property line I would only be able to able to extend the insulation out on 3 sides. Could this cause even more issues than no skirt insulation at all?

Can you not pour the curb later? As it’s annoying to try hang a form and keep it nice and straight and do a nice finish job on the slab at the same time.

That small of a building i would have no worry at that size edge. Not sure why I missed the size up above. Haven’t had morning coffee yet

 

[ConCretin]

Thanks for the input, I have a couple of questions... how would the control cuts be done when there is a 6” curb wall? Do I continue the cut vertically up the side of the curb wall? Also, due to proximity to the property line I would only be able to able to extend the insulation out on 3 sides. Could this cause even more issues than no skirt insulation at all?

So the curb wall will be placed monolithically? That does present a challenge. The best you can do is to cut as close as you can with your early entry saw and finish the cut with a diamond grinder. You'll be able to get about an inch away. You'll probably see a crack form the rest of the way.

It's possible the cut/crack in the slab could telegraph up the curb. If this will bother you, you could definitely cut a control joint in the curb too.

I assume you have a set back from the property line you are trying to respect. I'm not sure buried insulation would be a problem if it intruded. The other option would be to run the insulation vertically as deep as you can manage.

 

[Mike_72]

Can you not pour the curb later? As it’s annoying to try hang a form and keep it nice and straight and do a nice finish job on the slab at the same time.

That small of a building i would have no worry at that size edge. Not sure why I missed the size up above. Haven’t had morning coffee yet

I am not sure why they plan to do it all in one pour. My guess would be that it saves the crew time.

Working as an aircraft mechanic, I spend a whole lot of time on concrete but this is my first experience with having some placed and I gotta say it is WAY more complicated than I had ever imagined... I feel like my head is going to explode with the amount of information I have taken in over the past week.

 

[850xpeps]

I am not sure why they plan to do it all in one pour. My guess would be that it saves the crew time.



Working as an aircraft mechanic, I spend a whole lot of time on concrete but this is my first experience with having some placed and I gotta say it is WAY more complicated than I had ever imagined... I feel like my head is going to explode with the amount of information I have taken in over the past week.

I would be nervous of them leaving a crapping looking edge where the slab and curb meet. Ultimately to be done nicely they should be stripping the curb while finishing the slab and making the join look nice. As long as the concrete mix, weather and everything else cooperate to allow.

 

[Mike_72]

So the curb wall will be placed monolithically? That does present a challenge. The best you can do is to cut as close as you can with your early entry was and finish the cut with a diamond grinder. You'll be able to get about an inch away. You'll probably see a crack form the rest of the way.

It's possible the cut/crack in the slab could telegraph up the curb. If this will bother you, you could definitely cut a control joint in the curb too.

I assume you have a set back from the property line you are trying to respect. I'm not sure buried insulation would be a problem if it intruded. The other option would be to run the insulation vertically as deep as you can manage.

The issue with the setback is that there is a fence on the property line so I can’t excavate past that. Leaves me with 2 feet between building and fence.

 

[matt_i]

Google Frost Protected Shallow Foundation and click on the link that has a .pdf from the US Government's HUD. That has a lot of good info including a cookie-cutter designn guideline for insulating the earth. There's a lot of info on using "insulation wings" that LLWillys suggested. The idea is to trap the heat that's already in the ground and keep the foundation just over freezing temp so it does not heave.

 

[ConCretin]

Good advice here. What are you suggesting for a water reducer? My experiences have kept me away from water reducers on slab because I’ve found some keep the cement workable but then for lack of a better word “snap” on you.

What you describe isn't uncommon with high range water reducers or super plasticizers. Since there wasn't much mix water used in batching, when the chemical wears off it gave the impression of a flash set.

Mid range water reducers don't typically behave this way. They'll give you a nice workable 5 or 6 inch slump with less mix water.

 

[Mike_72]

What differences would I likely see if I decided not to insulate under the slab? I’ve had numerous people, including a couple of civil engineers tell me that I would be better off not putting insulation under the slab if I intend to heat the building. They mention that minimal heat will be lost through the slab and the heat that does pass will help prevent frost heaving. All this seems to make sense to me but wanted a few more opinions.

 

[850xpeps]

What differences would I likely see if I decided not to insulate under the slab? I’ve had numerous people, including a couple of civil engineers tell me that I would be better off not putting insulation under the slab if I intend to heat the building. They mention that minimal heat will be lost through the slab and the heat that does pass will help prevent frost heaving. All this seems to make sense to me but wanted a few more opinions.

Insulate under it. You will loose heat there. They are insulated all the time here in Manitoba. Use the proper insulation rated for the weight of cement and compact your full properly. Why would you want to have a cooler floor and pay more to heat?

They are built both ways around here but my advice would be to insulate. And you can run insulation around the building outwards to keep frost away if it bothers you. I personally wouldn’t worry about it if you slope the soil away.

 

[Mike_72]

Insulate under it. You will loose heat there. They are insulated all the time here in Manitoba. Use the proper insulation rated for the weight of cement and compact your full properly. Why would you want to have a cooler floor and pay more to heat?

They are built both ways around here but my advice would be to insulate. And you can run insulation around the building outwards to keep frost away if it bothers you. I personally wouldn’t worry about it if you slope the soil away.

Picked up a bunch of formular 300 for under the slab this week. They only had the 2x8 sheets with shiplap edges but I assume that will still work okay. Also picked up some 10 mil poly to lay down on the XPS boards, 20x100 foot roll so I’ll be able to do the whole slab with one sheet.

 

[850xpeps]

Picked up a bunch of formular 300 for under the slab this week. They only had the 2x8 sheets with shiplap edges but I assume that will still work okay. Also picked up some 10 mil poly to lay down on the XPS boards, 20x100 foot roll so I’ll be able to do the whole slab with one sheet.

The shiplap edges should work fine. It helps ones final grade is packed to use sand as a thin layer to screed flat and provide an even surface for the insulation.

 

[spudley]

No problem cutting joints to the end of the slab if you use block as a curb wall.

 

[Mike_72]

No problem cutting joints to the end of the slab if you use block as a curb wall.

Now it looks like they are going to do the curb wall as a second pour. I really don’t mind if I can’t get right up to the edge with the cuts, my goal is to control them around where my lift will be going. I’m okay with some minor uncontrolled cracking near the walls.

 

[maxpat82]

15mil vapor barrier is really the best for not much more then the 10mil.

I went 5" 32mpa for my slab in drummondville. (But I have a full fondation). 5" just to be 100% sure not to have thin spot with a 4" slab.

I also did 3'x3'x12" deep section were the futur lift post will be.

Insulation under slab is a must if you plan to heat it.
You should think about making a frost barrier too. (4' deep trench all around with 2" foam in it.)

I have a small section of 5x10 that is only a slab on insulation (attach to my garage). It's heated to 5C. I have put 4' of foam in ground on a 3/12 pitch all around it, and it still have moved(lift) this winter.

 

[Mike_72]

I had another company come and quote for the slab and the prices aren’t too far off. However, this guy said they would do an 18”x18” thickened edge with #5 bar vs. the 12”x12” with #3 bar from the original quote. What are the benefits and/or drawbacks to the beefier thickened edge?? Both slabs will be 6” thick and I’m located in Montreal so it will see freezing temps.

 

[maxpat82]

Bigger edge can juat be better. But 6” floor is way overkill. I went 5” and it was already overkill

 

[850xpeps]

Bigger edge can juat be better. But 6” floor is way overkill. I went 5” and it was already overkill

5” to 6” shouldn’t be more then $250 in cement on his slab so I would say who cares.

But a bigger thickened edge shouldn’t hurt anything.

 

[67ghiaTIV]

I built an 18 x 35 Monolithic just outside Toronto, ON, Canada 3 yrs ago. No cracking, heaving etc.
2" insulation under the pad, 3' wings at a depth of 2' around the perimeter. Only 2' wing on the fence side. Heated by a ceiling mount furnace.
Pad is 6" in the middle and 18x18 around the edges, rebar etc.

All good and no issues.

 

[Mike_72]

I built an 18 x 35 Monolithic just outside Toronto, ON, Canada 3 yrs ago. No cracking, heaving etc.
2" insulation under the pad, 3' wings at a depth of 2' around the perimeter. Only 2' wing on the fence side. Heated by a ceiling mount furnace.
Pad is 6" in the middle and 18x18 around the edges, rebar etc.

All good and no issues.

Thanks for the info. Did you only have rebar in the perimeter and welded wire mesh in the slab or rebar throughout?

 

[Mike_72]

Just had a third company come quote on the slab and again I’m faced with very different thoughts on how things should be done. This one is a huge fan of using rebar and thankfully said everything I wanted to hear (installed on chairs and no pulling into the concrete). However, he said that he uses only washed 3/4 on the undisturbed soil and that it wouldn’t be necessary to compact it. This is a reputable company who has been around for decades and has done lots of work in and around Montreal.

Any thoughts on 3/4” not requiring compaction?

 

[850xpeps]

Just had a third company come quote on the slab and again I’m faced with very different thoughts on how things should be done. This one is a huge fan of using rebar and thankfully said everything I wanted to hear (installed on chairs and no pulling into the concrete). However, he said that he uses only washed 3/4 on the undisturbed soil and that it wouldn’t be necessary to compact it. This is a reputable company who has been around for decades and has done lots of work in and around Montreal.



Any thoughts on 3/4” not requiring compaction?

Clean 3/4 rock doesn’t need to be compacted per say. Because it has no fines. And I agree to an extent. But it would be good for a plate packer to run over it to put it in its “place”. We used to use lots of pea gravel for that reason. But generally speaking clean rock with no fines doesn’t require packing. The contractor sounds like he has an idea what he is doing.

 

[Mike_72]

Clean 3/4 rock doesn’t need to be compacted per say. Because it has no fines. And I agree to an extent. But it would be good for a plate packer to run over it to put it in its “place”. We used to use lots of pea gravel for that reason. But generally speaking clean rock with no fines doesn’t require packing. The contractor sounds like he has an idea what he is doing.

16” of washed 3/4 under the slab is what he is proposing. Says that it works great on monolithic slabs because the thickened edge holds it in. Also says that due to its excellent drainage characteristics there is a far lower chance of frost heaving.

 

[850xpeps]

16” of washes 3/4 under the slab is what he is proposing. Says that it works great on monolithic slabs because the thickened edge holds it in. Also says that due to its excellent drainage characteristics there is a far lower chance of frost heaving.

Id agree. Last 2 I did I used 2” clean because they needed more fill. Then 3/4 clean or washed as you call it.

 

[Mike_72]

Id agree. Last 2 I did I used 2” clean because they needed more fill. Then 3/4 clean or washed as you call it.

It seems to me it would be more difficult to lay the XPS down well on “self compacting” gravel... I would image every foot step would disturb it?

 

[850xpeps]

When it finds its place it’s stays put. That’s why running the packer over it helps

 

[Mike_72]

I really don’t understand how these quotes can differ so vastly from one contractor to the other. It makes it very difficult to determine who is right...

One says 18” of compacted 0-3/4 under the slab and the other says excavate down, assess the undisturbed soil and place 6” clean 3/4 (uncompacted), XPS, vapor barrier and concrete. That’s a huge difference in the amount of fill required!!!

He claims that they have had better luck with the clean 3/4 because of its drainage abilities which prevents frost heaving...


I am more confused now than when I started...

 

[maxpat82]

You need to find undisturbed soil with not vegetal in it and fill back
But normally it shouldn’t be that far depending how long the soil haven’t been touched.

3/4 net would be way better then 0-3/4. Not a fan of 0-3/4 for fill

 

[Mike_72]

You need to find undisturbed soil with not vegetal in it and fill back
But normally it shouldn’t be that far depending how long the soil haven’t been touched.

3/4 net would be way better then 0-3/4. Not a fan of 0-3/4 for fill

I think I am going to go with the last guy who quoted (the one who wants to use 3/4 clean). He seems the most experienced of the three. I think the benefits of the capillary break that the 3/4 clean offers will be very helpful in preventing frost heaving from the wild temperature swings we seem to get in the winter and spring. I am going to do 14”x14” thickened edges with #4 rebar @16” throughout. Probably overkill, but I’ll feel better about it lasting a long time like this.

 

[maxpat82]

I'ved put a vapour barrier under my slab (It has 2" styro insulation and tubing for the hydronic heating in the futur.)
Mine is a full fondation with compaction sand.

A friend did a monolitic 22x26 in pointe-claire and used the compaction sand too

 

[Mike_72]

I’m unfamiliar with compaction sand. Where is it placed in relation to the gravel?