Can I connect two floating slabs?

[Roddy73]

Hi guys, I have a 24X32 shop on a thick monolithic slab and I want to build a 24X12 addition.

I know a seperate slab with a breezeway between them would be best but it gets mighty breezy up here when it's -40!!

Is it possible to drill re-bar into the existing slab and pour the new one so they're joined?

Thanks

 

[Big-Foot]

Hi guys, I have a 24X32 shop on a thick monolithic slab and I want to build a 24X12 addition.

I know a seperate slab with a breezeway between them would be best but it gets mighty breezy up here when it's -40!!

Is it possible to drill re-bar into the existing slab and pour the new one so they're joined?

Thanks

Yep and that is exactly how I did mine... I pinned them together with rebar that was embedded in the old slab 6 inches. I slathered in a bunch of JB Weld Epoxy into the holes after I blew them out. Was probably overkill on my part.

Worked well!!

 

[Roddy73]

Right on! I'll do it the same way, thanks Bigfoot!

 

[larry_g]

Check with the experts, but I believe that there are some specific products designed to glue the rebar in the holes. JB would probably work but I found some specific stuff for this work that is dispensed and mixed with a caulking gun.

lg
no neat sig line

 

[Roddy73]

Yeah I'll over do it for sure, I was just curious if anyone else had done it. I'm getting a lot of conflicting advice from the other tradesman around here. My
footing is 24"X30" around the edges and a 9" slab so she's pretty solid, I just want to run services into the addition so I can live in it for awhile.

 

[DekeT]

Hi guys, I have a 24X32 shop on a thick monolithic slab and I want to build a 24X12 addition.

I know a seperate slab with a breezeway between them would be best but it gets mighty breezy up here when it's -40!!

Is it possible to drill re-bar into the existing slab and pour the new one so they're joined?

Thanks

Yes, it is very possible and a good idea, but it is no substitute for an excellent base.

 

[Falcon67]

Haven't used it personally, but I'd think this would do the job:
http://www.strongtie.com/products/anchorsystems/adhesives/set/index.html

 

[cheechi]

Why wouldn't it work to drill the holes bigger than the rebar and fill them with concrete? I know it's vertical but I'm sure you can make some conical holes and fill them, then add a board or something to keep the concrete from falling out as it dries.

 

[Charles (in GA)]

Why wouldn't it work to drill the holes bigger than the rebar and fill them with concrete? I know it's vertical but I'm sure you can make some conical holes and fill them, then add a board or something to keep the concrete from falling out as it dries.

Not as strong as snug fitting holes and the proper trade accepted epoxys for doing this.

Charles

 

[2manytoyz]

When I added a patio extension to my last house, I dug the dirt around the perimeter of the existing slab, the drilled/installed 3 3/4" Tapcon bolts. Put them in about 1/2 way. The patio was then poured, and ceramic tile was eventually added. Almost a decade later, the slabs haven't moved - relative to one another. None of the tiles have cracked.

 

[ConCretin]

Pinning the two slabs together should work fine. I'd install two rows of rebar an inch and a half or so from the top and bottom - something like #4's at 12" on center. Definitely use an epoxy product designed for the purpose - JB Weld may make one but I've never heard of it.

 

[cheechi]

Not as strong as snug fitting holes and the proper trade accepted epoxys for doing this.

Charles

Thanks, I didn't know that.

 

[buening]

If you ever drive by road construction where they are putting in concrete pavement and curb and gutter, you may notice spray paint marks half way up the slab or curbs. These are locations for dowel bars, which holes are drilled and the rebar "dowels" are epoxied in. You are doing something very similar. In my opinion you don't want the two slabs moving independently if the addition walls are also tied to the original structure, thus the need to dowel the two slabs together. Ideally the two slabs would be floating with the footings tied together, but it doesn't sound like yours is constructed like this (slab not connected to footing/stemwall)

Make sure to use compressed air to blow out debris from drilled holes prior to using the adhesive. Hilti is another brand that makes chemical adhesive for the bars.

 

[ConCretin]

If you ever drive by road construction where they are putting in concrete pavement and curb and gutter, you may notice spray paint marks half way up the slab or curbs. These are locations for dowel bars, which holes are drilled and the rebar "dowels" are epoxied in. You are doing something very similar.

I'd elaborate on this by noting that highway dowels are designed to restrain vertical movement but allow horizontal movement. They are typically smooth bars that are not anchored on both sides. in the OP's case, I think he wants them tied together both ways hence the use of deformed bars and epoxy.

 

[K13]

Make sure you check you local building codes as to the maximum size they will allow to build on a floating slab. I was going to do the same to expand my garage and the city here said it would be considered one building and it surpassed the maximum allowed size for slab construction and it would not have passed inspection. I am in Alberta so things may be a bit different in B.C.

 

[buening]

I'd elaborate on this by noting that highway dowels are designed to restrain vertical movement but allow horizontal movement. They are typically smooth bars that are not anchored on both sides. in the OP's case, I think he wants them tied together both ways hence the use of deformed bars and epoxy.

Correct. I didn't want to go too in depth and confuse the OP. May be a bad analogy though. Deformed bars and epoxy is definitely what the OP should use.

 

[Zeke]

When I dowel concrete with rebar, I make the bars look like this:

/— \ —/— \— / — \ — /

 

[ConCretin]

When I dowel concrete with rebar, I make the bars look like this:

/— \ —/— \— / — \ — /

Just curious. What are the — ?

 

[Zeke]

— trying to show the edge of the existing concrete. Not too good, huh?

 

[ConCretin]

— trying to show the edge of the existing concrete. Not too good, huh?

Actually it seems rather obvious now that you've explained it. Apparently I'm a bit slow on the uptake tonight. I like the technique. I may steal it.

 

[ConCretin]

Correct. I didn't want to go too in depth and confuse the OP. May be a bad analogy though. Deformed bars and epoxy is definitely what the OP should use.

I'm not happy until I've confused almost everyone including myself.

 

[teamextreme]

Make sure you check you local building codes as to the maximum size they will allow to build on a floating slab. I was going to do the same to expand my garage and the city here said it would be considered one building and it surpassed the maximum allowed size for slab construction and it would not have passed inspection. I am in Alberta so things may be a bit different in B.C.

Digging up an old post, but I'm searching for info on this exact topic. I'm adding an addition to my existing monolithic slab garage and the addition puts me over the city's code limit for monolithic pours. I asked what they recommend and they said "we don't do that", you need to get an engineer. Gee, thanks. What did you end up doing K13? How does one construct a stem wall foundation and attach it to a monolithic poured slab. I've been doing a bunch of searching and haven't found any answers to this. If I do the rebar dowel approach, like I've seen before and described in this thread, wouldn't I then be tying a floating mono slab to a fixed stem wall? I don't see how that would work.

 

[K13]

Digging up an old post, but I'm searching for info on this exact topic. I'm adding an addition to my existing monolithic slab garage and the addition puts me over the city's code limit for monolithic pours. I asked what they recommend and they said "we don't do that", you need to get an engineer. Gee, thanks. What did you end up doing K13? How does one construct a stem wall foundation and attach it to a monolithic poured slab. I've been doing a bunch of searching and haven't found any answers to this. If I do the rebar dowel approach, like I've seen before and described in this thread, wouldn't I then be tying a floating mono slab to a fixed stem wall? I don't see how that would work.

I eventually gave up because I started getting into other issues with a pre-existing gas line under my garage that no one could give me a straight answer on whether I would have to move or not that would have added close to $5000 to my build if I did have it moved. There was so much "we are not sure" and "no one's ever asked that" surrounding the whole project I just decided it wasn't worth the headaches that were sure to ensue had I proceeded. Sorry I can't be of more help.

 

[buening]

Digging up an old post, but I'm searching for info on this exact topic. I'm adding an addition to my existing monolithic slab garage and the addition puts me over the city's code limit for monolithic pours. I asked what they recommend and they said "we don't do that", you need to get an engineer. Gee, thanks. What did you end up doing K13? How does one construct a stem wall foundation and attach it to a monolithic poured slab. I've been doing a bunch of searching and haven't found any answers to this. If I do the rebar dowel approach, like I've seen before and described in this thread, wouldn't I then be tying a floating mono slab to a fixed stem wall? I don't see how that would work.

You don't. A monolithic pad moves with the frost heave, whereas a stem wall is typically built down to the frost line and avoids movement with heave. Combining the two will result in cracking and movement at the joint and part of your garage will move while the other is in the original location, causing all kinds of issues. In my area mono pads aren't allowed so its all stem walls down to frost line with a floating slab between the stem walls.

If it is outside the limits of a mono slab, things will either get expensive or won't look very good once done. Either jack up the garage, saw cut the two walls that won't touch the garage extension, dig from the outside and form/pour a stemwall. Then dowel into the side of the existing slab where the new slab will joint, and pour the new slab with stem walls on the other walls. The other alternative is to have an expansion joint where the new slab meets the existing, but you will have to have separate walls and not have the roofs jointed (expansion joint if that is even made). Essentially two garages next to each other. A beam could be used to open up the wall area between the two garages.

You may check with your local structural engineers. I don't mess with mono slabs much since they are banned in my area.

 

[Gary S]

You can try to join them, but it defeats the purpose of expansion joints and relief cuts. The purpose of those joints it to allow expansion without breakage. If you join them, you defeat that, and increase the risk of cracking your concrete.

 

[teamextreme]

Thanks Buening. I couldn't see how it would work, but honestly figured there had to be some way to do it. Damn. This is not good news. I searched for almost a year before I bought this place because it had everything I wanted, including the elusive garage expansion capability. Now it's looking like it's not going to be possible short of tearing down the existing garage or major structural foundation work. I plan on contacting a local structural engineer, so we'll see what they say.

I did talk to my dad last night who was in concrete construction for huge projects all his life and he told me that a monolithic slab was no different than a stem wall in terms of "float". He said it doesn't float, that the footers are dug down to solid earth just like a stem wall construction, it's just poured in one piece. He said the slab in a stem wall does float, but I wouldn't tie into that, only dowel the stem wall to the mono and the new slab is independent. I don't think he is correct though.

 

[buening]

The monolithic slab, at least what I call them, are slabs with thickened edges (at the perimeter) that are typically a foot or so thick when a 4" or so slab is used. The bottom of the thickened edge does not go down to the frost line, so when the ground heaves so does the slab. These are popular in the northern regions when the frost line can be 10' or deeper. I'm not sure how attached garages are done with monolithic slabs however, as I would think the garage would move and would affect the flashing/roof where it joins the house. Maybe they aren't connected, this I am not sure. Is your garage attached to your house, and does your house have a basement?

A stem wall garage is a strip footing around the perimeter (excluding the door opening wall) that is poured first, then a slab is poured adjacent to the stem wall and is not joined to the wall. Preformed joint board is typically used, but sometimes its just pour against the stem wall. The slab itself "floats" and heaves when the ground moves, but the stem wall stays put since it goes down to frost. The intent is to keep the structure from moving but allow the floor to move to minimize cracking. There are some slabs, like commonly used in industrial or warehouse buildings, where the perimeter stem wall is attached to the slab using #4 rebar. This is commonly done when you have those prefab steel buildings that have the columns and roof beams all one piece and which vary in depth (get thicker as you go up to the roof). The columns try to push out on the stem wall/footings due to the roof forces, so the #4 bars are put there to utilize the slab in preventing the stem wall from moving due to these forces.

 

[teamextreme]

This is a detached garage. Your description confirms my understanding of the 2 types of foundations, and yes, the existing mono slab, if they did it to code here, should be about 18" below grade, which is above frost line, so there will be movement. To recap your solution above to make sure I understand it. I would basically convert my mono slab to stem wall by saw cutting the inner slab away from the footers to make it independent. Then creating a stem wall by jacking walls/building up and pouring a stem wall below the existing mono footer. Correct? Sounds like an enormous effort and expense. Almost to the point of demo-ing and starting over. I guess I'm going to look into what the zoning restrictions are for another seperate structure.

 

[K13]

Our slab construction is as buening describes a pad with thickened edges. I am in a Northern climate. Here any detached garage over 592 square feet requires a foundation of 4 feet. I came to the conclusion, like you seem to be, that the cost involved wasn't worth what I would gain as demo and rebuild was my only real option.

 

[buening]

To recap your solution above to make sure I understand it. I would basically convert my mono slab to stem wall by saw cutting the inner slab away from the footers to make it independent. Then creating a stem wall by jacking walls/building up and pouring a stem wall below the existing mono footer. Correct? Sounds like an enormous effort and expense. Almost to the point of demo-ing and starting over. I guess I'm going to look into what the zoning restrictions are for another seperate structure.

You are understanding correctly. For an unfinished garage, it may not be that much cost but if you have drywall then the jacking of the garage may bust your finished joints. I recommend talking to your local engineer and maybe get a couple Contractors to do a site visit and give you their thoughts. Sometimes good contractors get creative and do things easier than an engineer typically thinks. An experienced Contractor is invaluable.

Also, I recommend starting with phone calls to Architecture firms in your area, then go civil/structural engineering companies. Architecture firms typically have structural engineers that do nothing but building design, whereas civil/structural engineering companies can sometimes have structural engineers that mostly design bridges (like myself) and are therefore not specialized in the field and may not know all the intricacies of buildings. You may have issues getting someone to take on a project that small, but try as many as you can if this is the case.

 

[teamextreme]

I spoke with a local structural engineer who deals with small buildings/residential and he agreed, you can't tie the 2 different foundation types together. He had 2 suggestions.

1) Go with a second, separate mono slab for the new addition and have him write a letter to the City stating that this method is acceptable and the only way to accomplish the addition. Assuming the City allows this violation of their codes based on his letter, this would be the best approach. He said I could choose to dowel the two slabs together or not. Both have their pros and cons. Seems like this would be ok, since it's technically not one giant mono slab exceeding the square foot limits, but instead is 2 separate, smaller slabs below the limit. Only issue I see is how the framing connects between the 2.
2) Just build a separate structure joined to the existing garage with a breezeway. I'm not sure how this would look or work for my use. Not a fan of this option, but if it ends up being the only one, I may have to go this route.

 

[K13]

I spoke with a local structural engineer who deals with small buildings/residential and he agreed, you can't tie the 2 different foundation types together. He had 2 suggestions.

1) Go with a second, separate mono slab for the new addition and have him write a letter to the City stating that this method is acceptable and the only way to accomplish the addition. Assuming the City allows this violation of their codes based on his letter, this would be the best approach. He said I could choose to dowel the two slabs together or not. Both have their pros and cons. Seems like this would be ok, since it's technically not one giant mono slab exceeding the square foot limits, but instead is 2 separate, smaller slabs below the limit. Only issue I see is how the framing connects between the 2.
2) Just build a separate structure joined to the existing garage with a breezeway. I'm not sure how this would look or work for my use. Not a fan of this option, but if it ends up being the only one, I may have to go this route.

I will be interested to see what your city says as mine was very non committal on whether they would allow me to do the addition in manner #1 you gave. Part of the problem I had was everyone I talked to at the city gave me a different answer so one inspector said he would "probably" allow it and other said he wouldn't. Basically they said no one had ever asked so they didn't really know. I wasn't about to take the chance of being the guinea pig to try and fight through all the red tape. Good luck I hope you get it figured out!!

 

[buening]

By connecting the slabs with dowels, in theory it becomes one slab so you would build the garage addition like normal. If you get the City to allow doweling them together, pay extra for good subgrade work beneath the slab. Pay for extra rock and do the compacted lifts in 6" or slightly more.

If you choose to not use dowels, then you have two separate slabs that will allow the them to move independently with heave...which can wreak havoc on your drywall (assuming it will be finished). With proper/extra subgrade work, they may not move much differently. Its a crapshoot on the soil type below the subgrade and if it is the same or if you have pockets of highly expansive clay.

I'm not sure if they make them for residential use, but there are expansion joints used in commercial buildings and roofs. Emseal is an example of one manufacturer. I've never used them in residential though. There are also drywall expansion joints, like these: http://www.hamiltonnw.com/AP_TrimTexExpansionJointsRevealBeads.asp

 

[teamextreme]

I spoke with the City again and they completely shot down the structural engineer's idea of pouring a second mono slab next to this one. They said they didn't care who stamped the drawings, they don't allow additions to mono slabs. The ONLY way they will allow it is to pour a "grade beam" down to below frost line (36" here) below the existing mono footer. So I guess that would mean jacking the whole building up, etc, as described above. Next step is talking to some concrete contractors to see what this will cost. I haven't given up, but it's not looking good for my dream shop.

 

[rclassic]

Could you build it like a pole building ie drill holes down below frost line for the wall post to sit on and let the floor float..

 

[ADSR]

When I dowel concrete with rebar, I make the bars look like this:

/— \ —/— \— / — \ — /

This is the way i do it as well. I never go smaller than 5/8 rebar. I also try and not use epoxy. 5/8 rebar is a big 11/16's on the ribs, so i drill a 5/8's hole and sledgehammer them in 6". IMO its stronger than epoxy.

 

[buening]

this may be a crazy idea, but why not pour your strip footing in sections and excavate under the existing mono slab? Essentially, dig from the outside down to the bottom of the existing mono slab and keep going till you go down to your local frost level. Do this for a 10' width or so without any cars in the garage and without snow on the roof. Use the soil below the slab as your back form (or put up plywood forms that will be left in place), throw in some rebar along the length of the beam and allow it to stick out past your pour length a couple feet, put up a form a foot or two away from the face of your existing slab edge (to allow concrete to be poured and consolidated), then pour concrete in down to the bottom of your existing slab edge, finally backfill with sand once the forms are removed. I mention doing this in 10' lengths or so, as to minimize the length the slab is hanging. This assumes it is a fairly small garage and not some 40' wide garage with huge trusses. The only challenge would be doing this under the garage door if you have a poured concrete drive. Below is a picture of what I describe. It might be labor intensive, but should be less than jacking the garage up and saw cutting your slab.

Legend for the picture: green is soil line, red is the forms and kicker brace, purple hatch is new concrete footer limits, red circles are rebar, light blue is existing slab, and brown is finished grade after backfilling.

 

[teamextreme]

Great info Buening. I was wondering exactly what the City was referring to when they wanted a grade beam UNDER the existing mono footer. Your drawing makes it clear. I assumed they meant what you show, but I couldn't understand how you would do it that way without compromising the slab. Your 10 ft suggestion confirms my concerns. Now I have a better feel for what is involved. The existing garage is 23 x 30 with a gravel driveway, so excavating the door won't be an issue.

As an alternative, would pouring the new footer beside the existing one and doweling them together provide similar effect? This approach would eliminate the need to piecemeal the pour, and any potential slab damage from removing the support.

 

[buening]

That alternative may indeed work and be less laborious. Talk with your local structural engineer and run it by him. The only problem I see is the slab on a strip footing is typically not attached to the footing....allowing the slab to heave and not crack. Slab that are attached to footing typically have more rebar in them to handle the heave stresses. With your method the slab couldnt move and would likely crack. My method may also need a saw cut on the inside face of the walls, so that the inner slab can still heave....otherwise cracking occurs. The sawcut idea on your alternative may not work as the load from your walls won't be centered over the footing. This may not be an issue since light loading is involved. Maybe make the footer beefy enough so it isn't an issue.