VOID FORM DECOMPOSITION

[Structural Eng 7]

Hey guys,

My office has recently installed a structural slab over expansive soil using a cardboard void form. Our drawings strictly state not to place a ground poly sheet (so the cardboard will breakdown) and we didn't catch that the contractor did happen to place a ground poly sheet. The slab has been poured already and now we are looking into ways we can get them to decompose the void form by coring multiple holes through the slab in certain locations....

Has anyone ever dealt with an issue similar to this?! If so, what solutions did you come across?!

Thanks in advance.

 

[manstrom]

I have never had the issue, but I have heard the concern from builders.

Drill some holes and fill it with enough* water?



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[MikeHalloran]

Look at the aggregate in the test cylinders.

If it has sharp edges, there's a fair chance the plastic is already perforated.


Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Why do you care it decomposes now or 50 years from now?

 

[Structural Eng 7]

Thanks for the quick replies guys...

The whole point of the decomposing is to create an air void to allow for soil expansion beneath the slab so that heaving does not occur ( it is supported on piles). We have thought about water, but are scared that if we dump too much beneath the slab, the soil will become saturated and heave, which can potentially lead to cracking and movement.

The poly is laid above and below the voidform. The poly below the void is the main issue. Also we have a protection board over the void form, so the aggregate from the concrete will not come into contact with the poly ground sheet.

 

[haynewp]

When I used these before there were crushing loads I got from the mfr and didn't count on decomposition under soil expansion.

 

[Archie264]

I vote for leaving it alone, but, of course, I don't know all the parameters involved. In any case here are a couple of links of articles that you might find slightly relevant, though certainly not directly relevant so don't spend too much time reading them unless it interests you.

http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

http://www.buildingscience.com/documents/published-articles/pa-mold-explosion-why-now

 

[JAE]

If the soil heaves upward, which will fail first, the cardboard crushing or the slab failing in shear or flexure?

I would think the cardboard would go first as it is really only intended to support the wet weight of concrete.

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[Structural Eng 7]

Well that's what I would think, but the product specs state a compressibility capacity of 1000psf, which I'm sure our floor slab does not have. Also we have seen cases where the void form has not broken down properly and heaving has occurred as a result.

The specific brand of void form available here is designed to breakdown once moist creating a void space, as far as I understand. We were trying to find someone who can steam beneath the slab but are having a tough time tracking someone with the required tools down, which is why we are looking for alternative solutions.

 

[jike]

Can you drill holes in the slab and then puncture the vapor barrier? Maybe spray a bit of water thru each hole at 360 degrees to wet the cardboard and advance its deterioration.

 

[haynewp]

That rating is a lot more than I remember ours being, that's a problem. Is this a slab and beam system with forms under the beams too?

 

[SrVaro]

Can you dig a small pit next to the slab? If the poly is not broken, use this open pit break it. Depending on the size of the slab, maybe do one on each face. You good even introduce a small about of water to create a weak point (start the breakdown). If there is a heave load, hopefully this weak point will be the first to fail and allow the rest to crush.