Styrofoam blocks in a concrete slab?

Hi All,

I've been watching some houses being built around my area recently and noticed something I haven't seen before. After setting up the formwork for the slab they had mostly filled the space with sytrofoam blocks (about 1m square and 30cm high). The blocks were solid on top, but underneath were honeycombed. They left about 15-20cm of space between the blocks, and it looked like the thickness of concrete above each block would end up about 10-15cm (based on the height of the formwork).

Is this standard practice nowadays, or a cheap shortcut? Seems like it would save a heap in concrete costs (although i on't know how much the styrofoam would be).

John.

johnnyb said:

Hi All,

Is this standard practice nowadays, or a cheap shortcut? Seems like it would save a heap in concrete costs (although i on't know how much the styrofoam would be).

John.

Click to expand...

I don't know if it's standard practice or not, but I have noticed quite a few of the McMansions are built like that.

I'm no expert, but it is common practise. I think its called "waffle pod"

It is however an inferior slab. Wouldn't have one in my house.

Can be quite funny though when the wind picks up and youve got concreters chasing them across new housing estates.

Yes it's called a 'waffle pod' or 'waffle cone' slab and is believe it's common these days.
The house being bulit next to mine was done the same, they also use the more traditional boxing method, which is how my slab was done.
I agree with Andy, some of the waffle cone slabs I have seen in my area look a bit untidy to be honest, and not the best.

How long have they been building like this? I can't help wondering how long the waffle material will last before it breaks down.

They used to use a foam product in upholstery but it would break down into crumbly dust after some years. This was years ago but I wouldn't like to think that this could happen under a house!!!
cheers
blossomoz

http://www.hunterpodsupplies.com.au/faq.htm

These are known as waffle slabs and are more efficient than traditional slabs. The styrofoam works as formwork allowing a slab with great rigidity (due to depth) as well as large savings in concrete costs.

Hunter pod supplies also recycle styrofoam packaging and members of the public can just drop in styrofoam waste to be recycled - was on the news a few weeks ago.

Win, win all round I say

Cheers
Pulse

Engineers do not specify large blocks of concrete without rio, so the builders wont be ripping anyone off, but quarry products such as sand and cracker dust are expensive so that would be what is being replaced.

As for longevity, foam would last 100yrs out of the weather so as Pulse said "win-win".

Waffle pod slabs are used when the soil gives a very bad foundation. If the soil is deep, soft and highly reactive, it is the way to go. I'm not sure why, but it must work. My parents built a house with this slab ten years ago on deep black soil, and the house is still OK. Few others about too. As the soil was some of the worst possible to build on, this slab was a metre thick. [soil grows good crops though].

I think the foam is just to fill in the gap, and serves no other purpose. This leaves big gaps under the foundation, and as the soil wets and swells, it can move up into the gap. As it drys, it retracts. The foam would also make the slab lighter, which would be a good thing on this soil type. I'm no builder. I think this is how it works.

One unforseen benefit of this is insolation. This house is amazingly well insulated and a big part must be the slab. It always feels cool in summer and warm in winter.

See ya's.

I was looking at having a house built a few years ago in an area where waffle pods were being used - I had never seen them before.

On making enquiries with various building companies I was told they were a cheaper option than putting piers down in less than ideal ground. As usual there were various opinions on their use, but the majority of builders with a well known reputation did not favour them. I was told that sometimes if not laid properly things can move resulting in a reduced thickness of concrete over pod which is not obvious until problems/cracks occur later.

From memory the idea is that they are laid on the level ground leaving gaps - which are to become the footings. Reo is laid in these areas (just as you would if laying reo in dug out areas for footings) and reo is set out over waffle pods for the slab. When concrete is poured it fills in between pods to form footings and then the balance is laid over this and waffle pods to the thickness of the slab. The waffle pods are basically just for framework.

There are bound to be builders on this forum who will correct me if my memory is wrong.

Waffle pods slabs are better in reactive clay soils as they sit on the ground not in it and the clay is free to expand and contract. The other advantage is less excavation as they tend to level out the site with crushed rock. The slab is thinner 85mm insted of 100mm thick. Most volume builders are using them these days. Like anything they have there advantages and disadvantages, I would recommend though the garage not be waffle slab as one good hit with a hammer can put a hole in it.

wow - talk about timing. thanks for all that info guys as my husband rang from work to say the builders are laying the steel for the slab of our latest ip. when i went to look the formwork was a network of styrofoam - steel underneath, styrofoam in the middle and more steel on top, with gaps in between the foam blocks. if i hadn't read this post i would have been head scratching and unable to explain it to my husband and ex-builder dad-in-law. needless to say, i sounded intelligent and knowledgable. none of us had seen the foam used before but it makes sense. the layer of cement above the foam is still around 10 inches or more so is pretty thick.

lizzie

I would recommend though the garage not be waffle slab as one good hit with a hammer can put a hole in it.

My brother in law brickie tells me he has seen a car jacked up in the garage punch a hole thru the slab and into the foam.