Tying In Ajacent Concrete Floor Slabs

[Rich Goeken]

I have a question about extending a Lanai that has a concrete floor on grade, by placing a footing around the perimeter of the extension, pouring the slab, then floating the slab level with the existing lanai concrete floor. When asked if he planned to drill into the existing slab to install rebar pins to tie the slabs together, he stated that it was not required because of the footings.

I have gone through the Florida codes and can't find anything that discusses this topic. However, I have seen it done and the rebar was epoxied into the existing slab to prevent movement.

My thoughts are there could be movement at the joint that would put limitations on finish materials.

Any comments, thoughts, experience with this?

[John Kogel]

I think site prep is super crucial, maybe more than a bit of rebar. If the new slab doesn't sink, it will be OK, just the way a new section of sidewalk butts up to the old. Assuming there is not a lot of load on the slab.

[Scott Patterson]

Rebar will not keep the concrete from cracking where the old and new join. If it was me I would have an expansion joint designed into the new slab where it joins the old.

[JeffGHooper]

I have a question about extending a Lanai that has a concrete floor on grade, by placing a footing around the perimeter of the extension, pouring the slab, then floating the slab level with the existing lanai concrete floor. When asked if he planned to drill into the existing slab to install rebar pins to tie the slabs together, he stated that it was not required because of the footings.

I have gone through the Florida codes and can't find anything that discusses this topic. However, I have seen it done and the rebar was epoxied into the existing slab to prevent movement.

My thoughts are there could be movement at the joint that would put limitations on finish materials.

Any comments, thoughts, experience with this?

Is the Lanai covered? If so and you are trying to eliminate the crack that will be present, or more importantly the differential settlement, then absolutely needs to be tied, not at the floor level but at the mono footing. The bars need to be drilled in and epoxy filled. The Engineer will write a letter for the slab tie in as required in Florida. He will want the two slabs tied.

Regardless of the method, you are going to be limited to exterior final finishes. All sub soils in FL are required to be compacted to 95% prior to any pour.

What is on the floor now?

If you are using tile you will have to isolate the joint with a 36" wide membrane set in thin set prior to installation. Pavers of course will not pose any problem. All spray decks and the like will show the cold joint no matter what you do.

I have done many of these in the Palm Beach area and surrounding Counties here in FL.

[JeffGHooper]

You will not specifically see it written in the code because a drawing by a design professional is required. That is where you will see the requirement. They will require the footing on all 4 sides in our soil.

[Mark Reinmiller]

Even with footings at the perimeter of the new slab, slabs on grade often settle. Rebar dowels is a good idea. Regarding the joint between the two, the new concrete will shrink and there is also some expansion/contraction of concrete due to changes in moisture or temperature. If you install a flooring that is bonded to the slab and crosses this joint (like tile), expect a crack.

[JeffGHooper]

Even with footings at the perimeter of the new slab, slabs on grade often settle. Rebar dowels is a good idea. Regarding the joint between the two, the new concrete will shrink and there is also some expansion/contraction of concrete due to changes in moisture or temperature. If you install a flooring that is bonded to the slab and crosses this joint (like tile), expect a crack.

That is why you have to span the crack with isolation. Agreed.

[Jerry Peck]

Any contractor worth their salt (so to speak) will install the dowels, drilling properly sized holes (yeah, right), properly cleaning out the drill holes with an air compressor, brush, and repeat (yeah, right), and properly epoxy the rebar into the holes (yeah, right) ... okay ... back to reality ... they will likely drill the holes and shove the rebar in an inch or two ...

Every one I have seen which was engineered has the dowels specified in there.

VERY FEW contractors have any idea of the proper drilling, cleaning, and epoxying requirements for the dowels.

[CoronadoBruin]

Any comments, thoughts, experience with this?

I deal with this all the time. Every engineer (or architect) would insist on dowels, though I don't know why an engineer would be involved in this project. Actually very simple and several of the posters have already hit the mark. Even the slightest amount of heaving would be noticeable, and it will happen, particularly in Florida, which is why I always insist on slab and temperature steel even in sidewalks here in California and we have great soil with minimal water (earthquakes can be a bear). I've never seen a new pour next to an old pour look good without significant effort (read time and money), expansion joint or otherwise, and any tile or topping would need a robust isolation membrane.

Insist on dowels. I've worn out multiple 50 series Hiltis just drilling dowels for horizontal work.

[Dirk Jeanis]

I I don't know why an engineer would be involved in this project.
This is Florida, pretty much all work needs a professional stamped plan set. They wont even accept a federally stamped standard ADA standard bathroom for a commerical job (from the book), it must be wetstamped by an engineer for $1200 or whatever. The plumber still has to do the actual work underground regarless of what is drawn.

With regard to epoxy, if called out on the dwgs the contractor had better use it and properly. It is CHEAP to use, one self mixing tube goes a long way and injects into the hole easily. Be sure and blow the holes out with 120psi dry air (wearing a mask) before injecting or it does not meet the spec for install. Epoxied pins are required by many building/planning departments and inspected for by city inspectors as well in pre-pour inspections.

Also if you are pouring new perimeter foundations or know you will be adding a slab that needs to be captured from heaving, there is a "keyed" system that looks great to me. It creates a break in larger slab pours and lock the segments from heaving but allows expansion. It looks like a hat section (possibly available in several materials, sets the thickness of the fondation by its design, and possibly can be used at perimeter walls facing the slab pours as well.

- - - Updated - - -


I I don't know why an engineer would be involved in this project.
This is Florida, pretty much all work needs a professional stamped plan set. They wont even accept a federally stamped standard ADA standard bathroom for a commerical job (from the book), it must be wetstamped by an engineer for $1200 or whatever. The plumber still has to do the actual work underground regarless of what is drawn.

With regard to epoxy, if called out on the dwgs the contractor had better use it and properly. It is CHEAP to use, one self mixing tube goes a long way and injects into the hole easily. Be sure and blow the holes out with 120psi dry air (wearing a mask) before injecting or it does not meet the spec for install. Epoxied pins are required by many building/planning departments and inspected for by city inspectors as well in pre-pour inspections.

Also if you are pouring new perimeter foundations or know you will be adding a slab that needs to be captured from heaving, there is a "keyed" system that looks great to me. It creates a break in larger slab pours and lock the segments from heaving but allows expansion. It looks like a hat section (possibly available in several materials, sets the thickness of the fondation by its design, and possibly can be used at perimeter walls facing the slab pours as well.

[Jerry Peck]

Be sure and blow the holes out with 120psi dry air (wearing a mask) before injecting or it does not meet the spec for install.

Blowing it out with 120 psi air is not the requirement, not that I have seen, and is but only one step in the requirements:
- blow it out with 80 psi oilless air compressor
- brush it out with a non-steel brush
- blow it out again with 80 psi oilless air compressor
- (I think brush it and blow it again, just not sure)

[Mark Reinmiller]

Personally, due to the light loads involved, I would be satisfied if the contractor drove the rebar into a close fitting hole. And if they do use epoxy, I would not be worried too much about how clean the hole is since the purpose is for shear transfer and not transfer of lateral loads.

I realize what should be done, but I'm usually happy when I can get a residential contractor to come somewhere close to doing things correctly. And since we know that men don't read instructions thats not easy.

I put all kinds of notes on repair drawings and always state that I should be notified regarding any changes to materials or methods. They are often ignored. I sometimes thing I should add a final note "BILTFD". Then if they call to ask what that means I could tell them in a way that they may understand.

[JeffGHooper]

Personally, due to the light loads involved, I would be satisfied if the contractor drove the rebar into a close fitting hole. And if they do use epoxy, I would not be worried too much about how clean the hole is since the purpose is for shear transfer and not transfer of lateral loads.

I realize what should be done, but I'm usually happy when I can get a residential contractor to come somewhere close to doing things correctly. And since we know that men don't read instructions thats not easy.

I put all kinds of notes on repair drawings and always state that I should be notified regarding any changes to materials or methods. They are often ignored. I sometimes thing I should add a final note "BILTFD". Then if they call to ask what that means I could tell them in a way that they may understand.

Build it like the f%^&ing Drawing.

If it is used as a military spec it reads:

MIL-TFD-4AC.

Make it like the $%^&*( Drawing for a change.

:p

Mans got to know his specs.

[Marc M]

I'm adding 400 sq ft to the back of my place and there was no requirement to core into the (E) slab. I event went as far as to ask my engineer and architect both had no such requirement

[JeffGHooper]

I'm adding 400 sq ft to the back of my place and there was no requirement to core into the (E) slab. I event went as far as to ask my engineer and architect both had no such requirement

I think I would consult another Engineer and Architect if I were you. Or you could just do what they say and risk deferential settlement with a future potential trip and fall hazard. Or you could ask them to design it with a tie in, remember, they work for you.

All Codes are Minimum Requirements. They are the very least you can do. Good construction practices generally exceed the codes. You can build junk, (meet the code), or the Taj Mahal, (exceed the minimum code). Choice is yours.

[Jerry Peck]

I think I would consult another Engineer and Architect if I were you. Or you could just do what they say and risk deferential settlement with a future potential trip and fall hazard. Or you could ask them to design it with a tie in, remember, they work for you.

All Codes are Minimum Requirements. They are the very least you can do. Good construction practices generally exceed the codes. You can build junk, (meet the code), or the Taj Mahal, (exceed the minimum code). Choice is yours.

To add to what Jeff said: Code is the MOST UNSAFE one is LEGALLY ALLOWED to build something.

Or, stated differently: Code is the LEAST SAFE one is LEGALLY ALLOWED to build something.

Or, to state it as I used to state it years ago: Code is THE CRAPPIEST one is LEGALLY ALLOWED to build something.
- (But I received complaints that "crappiest" indicates "workmanship" and that the code do not address "workmanship" - codes don't address "workmanship", codes address "safety" ... so now the contractors WISH I was still saying "crappiest" instead of "most unsafe" or "least safe". :bounce:)

For some reason, unbeknownst to me, contractors don't brag about "meeting code" as much as they used to? Just cannot figure out why? :)

[Phil brody]

With 28 years experience - yes pinning it to the slab is the way to go - and undoubtedly where the two meet will be a crack. The pins will keep the slabs on the same plane.

[Mark Reinmiller]

I'm adding 400 sq ft to the back of my place and there was no requirement to core into the (E) slab. I event went as far as to ask my engineer and architect both had no such requirement

Required, no, but a lot more expensive later if the new slab settles.

[Garry Sorrells]

Agree with Mark. Rods better way for a few bucks more. But more important is to compac//tamp everything three times longer than normal. Will reduce future problems biggest time.

[Jerry Peck]

I'm adding 400 sq ft to the back of my place and there was no requirement to core into the (E) slab. I event went as far as to ask my engineer and architect both had no such requirement

Marc,

While not being "required" per se, in your shaky quaky area ... I would think that tying the two slabs together would be "required" to keep both slabs from separating from each other laterally (much less vertically).

[Phil brody]

Marc,

While not being "required" per se, in your shaky quaky area ... I would think that tying the two slabs together would be "required" to keep both slabs from separating from each other laterally (much less vertically).

While I have the utmost respect for your knowledge Jerry, why would the pins be an effective lateral tie. Vertically yes, the pins serve in a sheer capacity.It's analogous to (2) 2x4's nailed together,easy to separate along the nail length,difficult to slide perpendicular. The ridges in the re bar provide not much resistance to movement on that axis.

[Jerry Peck]

While I have the utmost respect for your knowledge Jerry, why would the pins be an effective lateral tie. Vertically yes, the pins serve in a sheer capacity.It's analogous to (2) 2x4's nailed together,easy to separate along the nail length,difficult to slide perpendicular. The ridges in the re bar provide not much resistance to movement on that axis.

Phil,

Because when they are properly ... "properly" ... epoxied into their ... "properly" ... drilled, brushed, and blown out holes and the epoxy is ... "properly" ... applied with the applicator nozzle into the hole - and, yes, "properly" is key - then the rebar will not ... WILL NOT ... pull out as you are thinking.

I watched a test at a Simpson class, the steel actually stretches and breaks, leaving the epoxied in end still in the concrete.

However ... when not done "properly" ... yeah, you can pull the steel out by hand.

[Jerry Peck]

Here is a test with a shallow embedment and an all thread rod typical of hold-down bolts which extend from the bottom plate all the way up to the top plate - again, this is a shallow embedment test: https://www.youtube.com/watch?v=EFw_oCw71gQ

[Phil brody]

Here is a test with a shallow embedment and an all thread rod typical of hold-down bolts which extend from the bottom plate all the way up to the top plate - again, this is a shallow embedment test: https://www.youtube.com/watch?v=EFw_oCw71gQ

Well I'm not sure a vertical hold down test on epoxied threaded rod would be equivalent to horizontal rebar primarily because, how much grip would fresh concrete have on the rebar or threaded rod. The Simpson system works well because of the characteristics of the epoxy. But you do bring up an idea for a very innovative anchor. A threaded rod as depicted in the video on one end and a cone shape or T on the other to mechanically anchor the two together. I could see this genuinely tying the slabs together.
What would you expect the pullout force to be on a threaded rod pushed into wet concrete-impossible to predict without knowing a great deal about the concrete.
Now if the contractor puts a nut and washer on the fresh concrete end that too would tie the slabs together.

Problem is: if the rod is properly epoxied, if the correct embedment , if the correct size and nut & washered

[Jerry Peck]

Phil,

I didn't say to use threaded rod, I said rebar* (which implies deformed rebar as non-deformed rebar is seldom used).

Epoxy one end into existing cured concrete and the other end gets embedded in the new concrete - if the rebar holding power embedded in new concrete is suspect ... then there are A LOT of "suspect" buildings out there.

The all thread video was the only video I found of a pullout test ... and it held to 7,000 pounds pull ... even at 7,000 pounds pull neither the threaded rod nor the epoxy failed ... the concrete did.

And that was with shallow embedment - looked like maybe only 3 inch embedment? Think what a typical 6 inch embedment would do.

*corrected typo 01-23-2020 was "rebates", should have been "rebar" ... have got to watch that spell checker better

[Rich Goeken]

To add to what Jeff said: Code is the MOST UNSAFE one is LEGALLY ALLOWED to build something......
Or, stated differently: Code is the LEAST SAFE one is LEGALLY ALLOWED to build something......
Or, to state it as I used to state it years ago: Code is THE CRAPPIEST one is LEGALLY ALLOWED to build something.....


I love those statements, especially the last one.

Update:

The contractor got bent out of shape when I mentioned that I wanted to put a sleeve under the slab for sprinkling and soaker piping. "The code won't allow it! The community board won't allow it!! I won't do it!!!" I said "The code allows it. The County Plans Review says it's OK. The community board has authorized it."

And, he says that I would have to pay the permit fee if decide to go elsewhere. Except, that all his documentation says that they will not apply for the permit until they get all he approvals---but they applied a month before they received the final approval. So that is BS.

Then it was left to "I won't do it." So he calls the board president, who calls me all excited. I calmed her down, told her she was getting misinformation, and said that I have the approvals. She said she will call the contractor back.

A few days later the contractor calls me back saying the will proceed if I give them a "No liability" letter for the pipe.

BTW, I have a copy of the PE"s sealed drawing in which he does not specify any method to tie the slabs together. Go figure.

Thought that you guys could use a laugh today. You just can't make this stuff up!!!

[Marc M]

Marc,

While not being "required" per se, in your shaky quaky area ... I would think that tying the two slabs together would be "required" to keep both slabs from separating from each other laterally (much less vertically).

Yea I agree, we had that in our original drawings but they were marked out in plan check for what ever reason. Also, the (E) slab is a PT slab and we went as far as to have it X ray'd to core.