Monolithic slab with inverted beam perimeter 4

[Buildwell]

Greetings all,

I have searched tirelessly for a methodology for producing a shallow sunken foundation for a small single storey structure. Frustratingly I have uncovered nothing other than an apparent dearth of information on the subject.

The proposed is a 4m x 6m reinforced slab of 150mm thickness and 225mm thick at the footing with an integral perimeter up-stand.

I have attached a section detail for clarity.

The problem I am having is that I want to avoid cold joints and place the concrete at once. The soil retaining up-stand is not tall but I am not confident that even with a relatively stiff 100mm slump, the concrete will not displace out into the slab pour if I attempt to cast both at once. Water ingress through any joints in the slab is not a concern as the foundation is fully protected by double layer 1200gm visqueen and EPS insulation. There will be 150mm clean gravel under the slab for drainage and thermal decoupling. Also, the water table in these parts is well below the point of concern. The soil is very heavy clay, and there are some small (non-threatening) trees within meters of the proposed structure, the 150mm reinforced slab thickness is merely a precautionary measure in case the trees and clay become troublesome in years to come.

Does anyone here have experience in such a task? Is a cold joint inevitable? Will I have to cast one element prior to the other? If so, which should be first? All rebar will be completed so sequential pours will be tied together regardless.

I thank you in advance for any light you can shine on this issue.

(I usually like a challenge but this one has got me on the ropes)

 

[HTURKAK]

I just write ( inverted bowl foundation ) and searched the web.. One of the probable outcomes which may fit your requirements i copied and pasted below,

 

[Buildwell]

No no, I think you've misunderstood the problem. I don't want an inverted bowl, I want a bowl. Casting it monolithically is the challenge.

But thanks for replying.

 

[LittleWheels]

Pouring a kicker, then the wall, is trivial but pouring a slab and a 550mm upstand in the same pour is a big ask. I can see why head-scratching is happening.

 

[Buildwell]

Yes indeed.Head scratching and soul searching...

I have looked into gunite and shotcrete methods which are often used for swimming pool forming, but I understand there can be mix and dimensional inconsistencies which are way outside the acceptable tolerances of this project. It does seem that I will have to revisit this idea as a probable solution though.

 

[phamENG]

You explain all the reasons why a joint is NOT a problem, but you don't explain why it is you want to pour it all monolithically. Can you expand on that a little?

 

[Buildwell]

PhamEng, apologies for my incomplete explanation. Though water ingress should not occur if I were to introduce a joint, I am concerned that the stress handling capacity of the complete foundation will be reduced significantly by the two elements not being homogenized through the placement and curing process. The upstand, acting like a ring beam in this instance, will be unbonded and connected essentially by the reinforcement only and therefore not providing maximal shear strength at the slab edge where the inner face of the wall intersects the floor.

With the possibility of inconsistent ground heave beneath the slab, the resistance to deflection would be optimal if the whole unit is truly monolithic.

Does that answer your question?

Also, due to limited access and having to utilise a pump truck, two visits are not cost effective.

 

[jayrod12]

An alternative to pouring just the upstand, would be to pour the gradebeam with upstand in one pour with appropriate keys/dowels for the slab, then pour the slab as a second pour.

I've seen guys pour integral upstands on thickened edge slabs, however they always complain, and the straightness of the inside face of the upstand is always questionable as they don't have anything substantial to fasten the inside face formwork to.

 

[Buildwell]

So in this case, would a vertical joint (wall 1st slab 2nd) offer any advantage over a horizontal joint (slab 1st wall 2nd)?

I am thinking the greater shear will be on the steel through the vertical joint since the soil retaining height is very small and the vertical ground heave forces should be greater. Thus, horizontal joint should be preferable?

 

[phamENG]

Buildwell - thanks, that does.

Are you designing this as reinforced or plain concrete with some nominal rebar to minimize cracking?

If reinforced, then I don't think I agree with your integrity concerns. If the slab is poured first with the thickened edge, you just have to come back and roughen the surface to promote aggregate interlock and modify your reinforcement as needed to promote shear friction. In the US codes, optimal shear friction is achieved at a 1/4" amplitude surface roughness.

If plain concrete, then I would agree with your point, but would then suggest you consider making it reinforced...

 

[Buildwell]

Yes the entire unit is reinforced and tied-in.

A393 mesh in the slab (10mm bar on 200mm centres).
16mm bars in the bottom of the thickened edge, also midway vertically and at the top of the up-stand. 4 No in total, like ribs all around the foundation.
12mm vertical bars on 400mm centres locking slab to wall (L-shaped) tied to the flat mesh.

This thing should go nowhere.

 

[Buildwell]

I intend to specify C30 concrete.

 

[JoelTXCive]

I think there is significant shear capacity at the base of the upstanding leg. As long as the reinforcement is continuous across the joint, then any driving force will engage shear friction between the concrete sections. We do 30ft+ retaining walls and rely on that shear friction all the time.

 

[Buildwell]

 

[Buildwell]

Thank you all so much for your contributions to this thread. It is wonderful to have a place to discuss such things with other curious minds.

I am an order of magnitude more confident that a 2-step process (slab then wall) should not impact the structural integrity of the final unit.

The above detail shows the steel reinforcement arrangement.

 

[SwinnyGG]

Pour the slab with your rebar in place. Roughen the area where the 'upstand' is going to be poured to give yourself some shear interaction between the slab and upstand. Form and pour the upstand second. This is box stock standard practice for an assembly like this, minimal hand wringing is necessary.

If you need to protect against water, you should be waterproofing the outside of the wall anyway; if you aren't for some reason, use cast-in-place waterstop and a shear key between the wall and slab.

You'll get pretty much the same strength you would with a monolithic pour as long as a shear key or sufficient slab roughening is in place prior to placement of the wall. You'll get the same moment strength too, since where you're putting a cold joint would've been a crack location anyway if the wall ever sees moment. Your reinforcement should and will carry the load in either case.

 

[haynewp]

Yes, it's just like a retaining wall with the wall poured separate from the footing with a roughened interface and add a waterstop if needed. What's the big deal?

 

[bookowski]

buildwell - for what its worth it is very common in my area to pour elevated concrete slabs with a monolithic upturned perimeter beam and/or curb, it's done all the time. yours is on grade but that wouldn't matter. the formwork is a pain, they need to "float" the inside vertical bulkhead but not the end of the world. i agree with the other comments above so if the contractor doesn't want to do your monolithic preference i wouldn't sweat it, but it is definitely doable without worrying about the concrete migrating down (within reason for wall height and pour rate).

 

[Buildwell]

bookowski - Thanks for providing balance (that it is possible to do monolithically). I will however proceed to cast the slab first, since the extra work involved otherwise seems both unnecessary and impractical from an efficiency standpoint.

Interestingly, though a few contributors to this thread have alluded to the fact that this is a relatively straight forward and standard practice, I found no useful examples or descriptions of such anywhere on the web. Hopefully this thread will survive and serve others who may be in quandary with similar design.

Again, many thanks to you all.

"Go forth and build well!"