We have a project where the contractor has used the wrong voidform under a slab.

Two types of plastic foam voidform were specified, one for gradebeams and one for the slab. Voidform for gradebeams has a higher 'crushing' strength than that for the slab.

The contractor erroneously used the gradebeam void under the slab. Two slab areas are involved. One, well within the building plan and the other on a corner of the building.

Soils in this area are highly plastic (swelling with moisture) clays.

My initial thoughts are that it is unlikely that the interior one will be a problem. Experience is that the soil within a building dessicates and that the soil shrinks away from the void anyway. This same dessication occurs around the perimeter to some extent with the dryer soil tending to remove moisture from the perimeter soil.

There are no pipes within the void space that can leak.

I'm of the opinion that it is OK to live with the situation and that uplift damage is unlikely (not impossible <G>). Repair is difficult and costly to the contractor.

I can stress that the perimeter drainage in the corner area must be 'flawless' in operation to eliminate perimeter moisture.

I can check the slab (which is reinforced both top and bottom continuously) to determine what amount of uplift the slab can accommodate; this detracts from my argument that the soil will shrink away from the slab.

Does anyone have information on the dessication of soil within a building or other suggestions? Using a Tiger Torch to melt the void would not likely be a good idea.

Dik

Water rises through clay soil by capillary action over a long period of time. If there is any groundwater below the slab, eventually it will come to the top and expand the clay.

I don't know what can be done to correct the situation after the slab has been poured other than hope for the best.

How thick is the voidform? How much pressure is required to crush it to half its thickness? Is this a structural slab on grade?

BA

Thanks, BA...
No groundwater issues we have about 40' of laclustrine highly plastic clay all over... I'm still digging up info on the compressibility of the material to see what load-deformation characteristics will be loading the slab. 6" void so I would be looking at approx 1" to 1-1/2" deformation.

Any excavations of old slabs/buildings with voids have shown large shrinkage cracks and soil has shrunk away from the structure. Some 30 or 40 years ago, I've done many slabs with loose granular fill and no void form... not sure why I've gotten away from that other than becoming older and more cautious <G>.

Difficulty of fixing is the reason for putting the added effort into stating that the problem is reasonably OK to live with.

Dik

Void form crushing strength should be irrelevant once the concrete is placed and set. Use of a stronger than required void is just more expensive for the builder. If you are not using a form which deteriorates, you aren't getting any value out of it.

If there is any swelling of the soil, the voidform used under beams permits a higher pressure to be transferred to the slab. They manufacture plastic voidform that has a lower crushing strength for that purpose.

I've used foam under gradebeams for 40 years (used to be aerofoam skins which had a slightly lower compression strength. I've done several reports (years ago) with the cardboard voids collapsing and the void filling with water and causing heaving of gradebeams.

As I noted, it's my experience that the void increases due to dessication of the plastic soils during the lifetime of the structure.

Dik

I have investigated many buildings where a 6" void closed, In one case, it also lifted the tilt walls up to 4" above the piers before we were called. You mention over excavating and using granular backfill - which is a recipe for disaster in this area. It ends up providing a reservoir of water to swell the underlying clays.

Just goes to show that its always location, location, location.

The failure of cardboard voids at the time of placement is well known and can be easily avoided by several means (some expensive, some not so much). I have not seen a foam void system intended to allow soil swelling, only for isolation to allow movement and thermal insulation.

I am in the process of writing language for ACI 301 to include void forms. If you have specific materials and methods you want us to look at for inclusion, please let me know. I do not want to accidentally exclude materials or methods currently in use.

John Turner CSP PE
CRSI Greater Southwestern Regional Manager

thread507-262448: Void Forms for Expansive Clay

BA

We don't have the same water problems I guess... There is no free water in most shallow excavations.

With my projects there is little overexcavation... I’m not one for large amounts of consolidated fill under slabs even SOG's, just enough to provide a relatively flat soffit, but the aggregate placed for structured slabs was not compacted. The manner of construction was chosen on the basis of the soil shrinking away...

Products I've used numerous times and had good results with:

Geovoid by Plastifab for slabs
Dynavoid by Beaver Plastics for slabs
Geospan by Plastifab for beams
Frostcushion by Beaver Plastics for beams

Dik

I would not use a plastic void former. But I am in an area where freezing does not occur. Swelling soils are a big problem here, and cardboard void formers work well, with only an occasional problem due to unpredicted rain.

I have used all of the ones mentioned by Dik above. I have also used cardboard voidform. They come in waxed and unwaxed form. I would never again use the unwaxed variety since I witnessed a 10" thick liquor store slab collapse during pouring because the unwaxed voidform got wet in the rain prior to the pour, then got wetter due to the wet concrete. Never again!

What do you have against plastic, hokie? It has always worked well for my projects.

BA

Well, I am really only starting to formulate a prejudice against plastic void formers, as I haven't used any, and don't even think they are available in Australia. Maybe I am wrong. But to me, and where I live and practice, a void former which is strong enough to support the plastic concrete but doesn't deteriorate is not what I am looking for. The old way was to support the slab on sand and wash out the sand later. Cardboard formers here are common, and a good use for discarded box material, so I like.