slab on grade restraint

[joncle10]

Good day.

I don't have that much experience with slabs on grade.
What I am facing is the following
The design drawings are indicating a slab on grade for a residential house with ground beams to tie the columns. The slab on grade is separated from the ground beams and the columns by a separation joint of 20mm. the thing is that the client doesn't want that much joint and would like to remove them.
Removing joints will result in restraining the slab from contracting due to drying shrinkage which will increase the stresses due to shrinkage / temperature.
I was reading the ACI360R which deals with the design of slabs on grade. There is the formula of subgrade drag which depends on the joint spacing. A joint is defined as the length between the free ends of the slab. In my case, I have no free joint since it will be connected to the ground beams.

In my case, I doubt that I can use the subgrade drag procedure since no free joint is in this structure. What is an alternate solution ?
I have already sent the request to the design office but I would like to hear your thoughts about the above. Thank u

 

[hokie66]

There are always tradeoffs with thickenings in slabs. With houses, shrinkage cracks often don't matter, as floor coverings conceal them.

 

[joncle10]

thank u hookie.
So for residential slab on grade, u are saying that the presence of the joints isn't that critical. therefore, it can be deleted

 

[ATSE]

hokie66 is a good engineer. I think we can agree (as reasonable engineers) that thin stable cracks in a non-structural slab are of little consequence, except for special circumstances, like high ground water and sensitive floor coverings.
However, put on your attorney hat for a minute. In the US, claims and lawsuits regarding slab cracking are in the top 3 of all construction claims.
The uploaded image does not quite have the clarity needed.
Regarding your question, can you minimize the joint thickness without eliminating it? Say, 10mm?
For concrete, I prefer to either tie two members together, or separate them. If you kind of connect them, you get unpredictable results.
If you butt two relatively brittle members together that contract and expand occasionally, you risk spalling at the interface edges in the long term.

 

[Ron]

The width of the joint seems to be the sticking point for the owner. You should isolate the slab from the grade beams, but it doesn't have to be with a 20mm joint. A simple piece of felt attached to the grade beam will allow sufficient isolation. As slab shrinkage occurs, you will get a widening of this separation. By the time your construction is ready for floor coverings, you can go in and seal the joint with a flexible sealant or even a semi-rigid epoxy.

 

[DETstru]

For the middle beam, why not place your ground/grade beam completely below the slab? The slab can run over the top of the beam and there will be no isolation joint.

Assuming the perimeter beams are an exterior wall, you should isolate the slab for shrinkage and thermal reasons. As Ron said, a 20mm joist is quite large. I've typically seen 3/8" (10mm) joints, which are filled with either a backer rod and sealant or a true flexible isolation material with sealant.

On the other hand, I have very often seen house foundations where the slab was poured on top of the perimeter footings (separate placement) all the way to the edge of the building line. (similar to my suggestion for the middle grade beam above)

Side note: Are you planning on cutting control joints in this slab? Control joints will minimize random shrinkage cracking, which is not a huge concern unless there is an exposed slab (like a garage). I would still suggest it.

 

[hokie66]

Are joncle10 and chekre the same person, or do you work together? It is confusing when you use two handles in the same thread.

 

[joncle10]

Not the same person. We work together. My colleague deleted his post.
I wanted to ask if when a sawcut is introduced to the grade slab, will it be ok to covet this sawcut by floor finish ?
Thank u

 

[hokie66]

It depends on the floor finish. If it is a floating wood floor or carpet, the joint can be ignored. If it is ceramic tiling, there should be a joint in the tiles at the slab joint. But in houses of modest dimension, joints are often unnecessary, with tiling continuous. My house is fully tiled, and there are no slab joints, although I am sure there are some slab cracks...no tile cracks.