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Void Forms for Expansive Clay

Void Forms for Expansive Clay

Void Forms for Expansive Clay

(OP)
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.  

RE: Void Forms for Expansive Clay

general rule of thumb for these is that you can assume about 60% of original depth. I spec http://bildavoid.com.au/main/page_bildavoid_supavoid.php, they have some test data in the techincal section.  

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

RE: Void Forms for Expansive Clay

(OP)
No, that's not what I'm looking for. The 60% of the original depth is the requirement of the form to keep its shape during the formwork phase, before the casting of the concrete.

I'm looking for data where the compressive strength of the form has been tested after casting of the concrete and water has soaked the form.

 

RE: Void Forms for Expansive Clay

vandede,
You need to read again.  The link rowingengineer gave states that the void after flooding is 60% of the original height of the void former.  It doesn't collapse 60% due to the wet mass of the concrete.

RE: Void Forms for Expansive Clay

I usually use one of two products.  One by Beaver plastics called Frost Cushion.  They have two products, one with a higher compressive strength and one with a lower one.  The former is used for beams and walls and the latter used for forming slabs where there is a larger area. The other company is Plastifab and they market similar products.  I've spec'd void form to use against a backfilled wall where the adjacent soil (other than the fill) is rock salt which creeps and exerts a lateral pressure.  By having the voidform 'crush' the lateral loads on the cantilevered wall were minimised. It was the biggest sump pit I had ever designed... about 18' wide, 70' long and 7' deep...

Dik

RE: Void Forms for Expansive Clay

GeoSpan, manufactured by Plasti-Fab is an excellent voidformer.  The compressive stress at 50% Deformation is 50 kPa (7.3 psi).  

Edmonton clay is expansive and I usually specify a 4" thickness of GeoSpan under grade beams.  This seems to work reasonably well in most cases, but there are times when a greater thickness is warranted.

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

BA

RE: Void Forms for Expansive Clay

BAretired:

Curios and want to learn more on this.

So, the soil has a potential to exert 2000 psf pressure (after DL) on the structure, then there are two scenios:

1. use 4" form to obsorb 50% of pressure, and design the grade beam/slab to resist the rest (7.3 kpa approx = 1000 psf).
2. use 8" form to relief entire pressure.

I don't know if I have worded my question correctly or not, you can see I am lost in somewhere.

Apreciate your comments and further explanation.

RE: Void Forms for Expansive Clay

(OP)
cntw, I think what BAretired means by 50% deformation is similar to the 60% of original depth after flooding mentioned above.

Basically the question is "how much compression can wet cardboard trasmit?"

In my head, I'm thinking of it as the linear-elastic / perfectly plastic stress strain curve we assume for steel. The wet void form will carrying the swelling pressure, transmitting an increasing amount of compression to the structure, until it finally crushes and continues to crush while applying the same load.

BAretired says his product is 7.3 psi.  That's the number I was looking for.  The geotech eng said the metal product crushed at 500 psi (which shocked the crap out of me) but I think he may have meant 50 psi. I saw an online brochure of the product and it's nothing more than corrugated chicken wire. There's no way that stuffed with-held 500 psi.

RE: Void Forms for Expansive Clay

(OP)
7.5 psi (50 kPa) is about 1000 psf.  Are you telling me you design foundations for 1000 psf applie uplift?

RE: Void Forms for Expansive Clay

vandede427:

Quote: Basically the question is "how much compression can wet cardboard trasmit?"

That's exactly the question I have in mind.

No kidding, 1000 psf uplift? I think you need to follow up on BAretired's lead to select a form with proper thickness to further reduce the pressure to a more manageable level. Keep me posted on your findings. Thanks.

RE: Void Forms for Expansive Clay

(OP)
I'm just really having a hard time believing that a 12" x 12" piece of soggy wet cardboard is able to withstand 1000 pounds.

And if it is that strong, why is the product designed to be so strong when all that it is need to carry is the self weight of the concrete; 100 psf for an 8" slab or 400 psf for a grade beam?

Am I missing something here?

RE: Void Forms for Expansive Clay

I think it is to be investigated that while the form will collapse 50% of its original volumn under 1000 psf load, will, or how much, the load remains on it. I don't quite get it.

and, as the question you have raised, what is the form of complete collaspe, by what pressure?

Maybe I missed something, but that's why I am sticking to it :)

RE: Void Forms for Expansive Clay

Bear in mind that the geotech's estimate of uplift pressure is with essentially zero upward movement of the soil surface allowed.  It shouldn't take a whole lot of movement to relieve the uplift pressure to close to zero.

Last month, some were installed under some new grade beams at my house (on swelling clay) with the trade name AVOID - clever, eh?  I believe I could have stomped them flat without much trouble.  They were just cardboard boxes with internal stiffeners, designed not to crush under the weight of the concrete, about 4 psi in this case.  They could not have been very stiff in compression.

RE: Void Forms for Expansive Clay

Good point.

So the question should be "what is the anticipated volumn change of the soil", a pure geotechnical question, am I correct?

RE: Void Forms for Expansive Clay

(OP)
In doing my calcs, I just found something in the ASCE load combos that's a little disturbing. If H (earth) is a downward or lateral force then everything is fine. But if H (earth) is an upward force then there's a problem with the load combos.

For checking pile reactions worst case downward, there isn't an ASD load case that has D and L but not H. For designing the grade beam's worst case downward, there is a case with D and L but not H, but that case is only 1.0L and not 1.6L.

If I blindly went by the combos listed I would be assuming that the uplift swelling pressure occured 100% of the time (which isn't true) and would never get a worst downward case.

Does anybody agree with this?

 

RE: Void Forms for Expansive Clay

(OP)
I think IBC 2003 saw this issue.  The pulled H, F, T, and S out of the combos and just said "apply as necessary."

But IBC 2006 pulled all load combos and reverted back to ASCE 7.

RE: Void Forms for Expansive Clay

"So the question should be "what is the anticipated volumn change of the soil", a pure geotechnical question, am I correct?"

That's part of it - how fast does the pressure reduce with swelling allowed, versus how fast does the void form pick up the load with compression?  Seems it would take a significant amount of upward movement of the underside of the beams to get the piers (normally in compression) into tension.  (Don't ask me to quantify "significant.")

RE: Void Forms for Expansive Clay

dgillette:

I think a leason is learned today. Thank you very much.

RE: Void Forms for Expansive Clay

GeoSpan is not soggy cardboard.  It is an expanded polystyrene product whose properties are not affected by the presence of water.  It is very easy to indent the material with finger pressure.

The attached graph shows Compressive Stress vs Percent Deformation.  

The magnitude of heave varies from place to place.  It may be caused by swelling clay soil, frost heave or both.  If you experience a 2" heave and use 4" GeoSpan, the Percent Deformation will be 50% and the pressure would be about 50 kPa.  If you get only 1", the deformation is 25% and the pressure would be about 40 kPa according to the graph.

If you are expecting more than 2" heave, then you should use a thicker voidform.

Quote:

7.5 psi (50 kPa) is about 1000 psf.  Are you telling me you design foundations for 1000 psf applied uplift?
The answer to this is "You bet!"  A 12" x 24" grade beam weighs 300 plf so the net uplift is only 700 plf.  You likely wouldn't have less than 3 - 15M top and bottom, so the uplift is carried by the grade beam spanning between piles.

BA

RE: Void Forms for Expansive Clay

(OP)
I don't mind designing a grade beam for 1000 psf applied uplift, but not an 8" slab spanning between grade beams.

But it appears you don't have to.  It appears that properly designed cardboard products will on average degrade down to 30% of the initial strength.

Read this

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

it is testing done by a manufacturer to determine strength reduction.  they show that strength reduction can reach 80-90% based on the amount of moisture absorbed.

RE: Void Forms for Expansive Clay

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.

RE: Void Forms for Expansive Clay

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

The numbers we really want are wet compressive tests.

RE: Void Forms for Expansive Clay

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

BA

RE: Void Forms for Expansive Clay

(OP)
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.  

  

RE: Void Forms for Expansive Clay

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?      

RE: Void Forms for Expansive Clay

BArt... <G>

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

Dik

RE: Void Forms for Expansive Clay

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.

RE: Void Forms for Expansive Clay

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

RE: Void Forms for Expansive Clay

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.

RE: Void Forms for Expansive Clay

(OP)
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.

RE: Void Forms for Expansive Clay

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

Dik

RE: Void Forms for Expansive Clay

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.

RE: Void Forms for Expansive Clay

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.

RE: Void Forms for Expansive Clay

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

RE: Void Forms for Expansive Clay

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.   

RE: Void Forms for Expansive Clay

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

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