Void Forms for Expansive Clay

[vandede427]

I'm designing a building that is on a site with expansive clay soil. Deep foundations (drilled shafts) and grade beams will support a structural slab at the ground level. I've looked at several different products for this application; the more typical "cardboard" type and the "rainy day" metal type for grade beams and slabs.

What I'm looking for is whether or not void forms completely degrade and absorb all the soil swelling pressure or if they still transmit some of the upward swelling pressure. I can't find this information on any of the products' websites.

The geotechnical report for this site says that swelling pressures could be as high as 2000 psf if no void form is used. It recommends void forms to help "reduce uplift forces" but it doesn't say what that reduced force will be. I understand it probably differs based on each product, but I can't find that info.

 

[tadeng]

Void forms are designed to completely degrade so that eventually you are left with a complete void beneath the grade beam. We use them all of the time here in Texas. We use Surevoid products, as they are wax coated and will break down more quickly than wax impregnated boxes. We also put retainers on either side to keep soil from sloughing into the void once they break down.

 

[vandede427]

That 7.5 psi at 50% deformation is a dry compressive test.

The numbers we really want are wet compressive tests.

 

[BAretired]

The manufacturer claims the properties are not affected by water, so it is a dry test and a wet test.

BA

 

[tadeng]

Surevoid has some testing data that might be helpful here:

http://surevoid.com/product/testing.php

 

[vandede427]

I like the testing done by Surevoid. Although it is under lab conditions, at least it proves that the product does degrade.

I spent 30 min on the phone with one of their owners. He knew this product inside and out. Basically the strength reduction is a function of the moisture absorbed as a percent of the void weight.

At 20% moisture absorbtions, their product lose 60-70% on average. Some products lost up to 80% strength.

But it really all depends on the moisture levels of the site and the soil of your project.

Based on their testing, I feel comfortable designing my grade beams for an applied swelling pressure of 500 psf. This is a conservative estimate that a 1000 psf form will degrade 50% of it's strength. That probably corelates to about 10% moisture gain.

 

[BoutDat]

There are six Important Values that have not been stated in these reccomended considerations for either of the proposed void products. Especially the cardboard.

(1) Length of time between initial void form placement, cover and section casting.
(2) Effects of Soil Poision Treatment moisture.
(3) Effects of Ambient Air/Soil Moisture.
(4) Capillary Moisture Rise, especially so at grade beam sections.
(5) Deflection strengths of the Void form Cover Sheet.
(6) The most important... FOOT TRAFFIC Degradation!!

All of these six things I list have a direct influence and correlation as to how much void section height remains. After subtracting these considerable losses, what actual void heights can be expected at the Time of casting?

 

[dik]

BArt... <G>

Plastifab also manufactures Geogrid which is a lower strength void form that can be used under slabs...

Dik

 

[hokie66]

I can't see the point in using plastic for void former. You want the stuff to disappear after placing the concrete. The cardboard type turns to mush, while the plastic type will still exert a pressure which is undesirable.

 

[BAretired]

Dik,
Here is a link to Geovoid by Plasti-Fab:

http://www.plastifab.com/applications/geotechnical/compressible_fill/geovoid.html

Another slab voidform by Beaver Plastics is Dynavoid:

http://www.beaverplastics.com/beavercurrent/dynavoid.html

hokie,
The upward pressure is not a problem if the slab is designed to resist it.

I had a nasty experience with cardboard voidforms a few years ago on a 10" thick structural slab for a liquor store. Two types of cardboard voidform were available, waxed and unwaxed. The contractor chose unwaxed.

Before he could pour the slab, the voidform was saturated with rain. It collapsed under the weight of the wet concrete. What a mess! The entire slab had to be removed.

I suppose the waxed type would have worked adequately, but since then, I have avoided cardboard voidform like the plague.



BA

 

[hokie66]

We only use cardboard former in Australia, and the contractors know there is a risk involved. If rain happens, all the reinforcing comes out and the void former is replaced. No exceptions. But then, this is the driest continent.

 

[vandede427]

Do you live anywhere near Melbourne? If so, have you been to any of the Open matches this week.

My wife and I went to NYC this past Sept and got to see the Federer - Del Potro mens singles final at the US Open. Besides college football, it was probably one of the best sporting events I've ever been to.

 

[dik]

Geovoid... not Geogrid... thanks for the correction... I've used it under slabs...

Dik

 

[Bobber1]

For what it's worth: I used to design drilled piers in Louisiana. Even though we used void forms, we designed the piers with bells at the bottom to resist the full Geotech provided uplift. This was for industrial facilities where the cost of belling the piers was justified.

 

[rowingengineer]

here is something I never would have thought would happen.

http://www.cfa.vic.gov.au/business/documents/polystyrene_advice.pdf

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

Just to clarify RE's post, those polystyrene blocks are not there to provide a permanent void. That is a ribbed slab on ground. The blocks just provide depth in the ribs and save concrete.

 

[rowingengineer]

Hokie is right, this is a waffle slab, didn't mean to give the impression otherwise, but I thought it was of interest how this material could have some unexpected problems onsite.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[Bobber1]

I can see a contractor setting the styrofoam on fire then hurrying up his concrete delivery to put the fire out before he gets caught.

 

[southard2]

lol, Bobber1 is right about that. Thanks to this board discussion. I never even knew this stuff existed. I had always relied upon either undercutting the clays or using stiffened slabs which at best really just reduce the cracking. This stuff is pretty cool. Great idea

John Southard, M.S., P.E.

http://www.pdhlibrary.com