Polystyrene foam under concrete floor

[bigegg]

As an addendum to this, I went with using the soil I removed mixed with c.5% Portland cement[1] - seems to be a standard construction technique in the states.

[1] one shovel of cement to 20 of soil.

I'll be pouring a four inch reinforced slab of normal concrete over the top

 

[Rocketman]

As an addendum to this, I went with using the soil I removed mixed with c.5% Portland cement[1] - seems to be a standard construction technique in the states.

[1] one shovel of cement to 20 of soil.

I'll be pouring a four inch reinforced slab of normal concrete over the top

You said it was a veg plot. So loads of organic matter is in the soil from improving it while it was a veg plot. It's just going to slowly sink over the years I would suspect.

 

[bigegg]

You said it was a veg plot. So loads of organic matter is in the soil from improving it while it was a veg plot. It's just going to slowly sink over the years I would suspect.

It's only going to be a storage shed, then an underfloor for a patio/deck.

I've read a fair bit about it - apparently as the organic rots, you end up with a reasonably strong matrix - like I say tho, it's not going to take any huge weight

 

[bigegg]

Oh - dad died 17 years ago, so I'm guessing most of the rotting will have occurred already?

 

[mangocrazy]

Bit late now, but there are different grades of extruded (not expanded) polystyrene to be used under concrete slabs. They vary from domestic to heavy industrial grades that will take heavy static loads once they are embedded under the slab. It's a preferred construction method, apparently. The boards have high compressive strength and protect the DPC. See here:

http://www.knaufinsulation.co.uk/products/polyfoam-eco-xps/polyfoam-eco-floorboard

 

[Henrik]

As an addendum to this, I went with using the soil I removed mixed with c.5% Portland cement[1] - seems to be a standard construction technique in the states.

[1] one shovel of cement to 20 of soil.

I'll be pouring a four inch reinforced slab of normal concrete over the top

I used soil for part of my she'd floor mixed with sharp sand and cement, and apart from looking brown compared to grey concrete it's been fine so far for three years.

I also poured some of it for a section of wall as a test, and that's also "stood" the test of time, so to speak

 

[awemawson]

Henrik, I'm very surprised that it 'went off' ok. I excavated a solid kitchen floor in my previous house (In Bromley as it happens!) to make it all suspended, and under the Edwardian concrete slab was what appeared to be a 6" layer of ordinary 'ballast' ie aggregate up to 3/4". So I used a bit to knock up some concrete, which never went off properly. Turns out that it was 'unwashed' and had too much clay fines in it.

 

[Henrik]

Hmm, my soil is also very clayie (is that a word?), but it might have had to do with the sand... I reckon i had about half sand and half soil in the mix, which would have gone some way to sorting the clay issue (I think)

 

[123hotchef]

Iam in the same dilema, I was going use the 50mm polystyrene sheets i have lay them over the hard core then lay the DMP on top of this. Is the the done thing for a shed base?

 

[mangocrazy]

Iam in the same dilema, I was going use the 50mm polystyrene sheets i have lay them over the hard core then lay the DMP on top of this. Is the the done thing for a shed base?

I'd say to blind the hardcore with sand, to give the insulation a better chance of remaing unscathed. Is the polystyrene you are using expanded or extruded? Big difference in toughness and insulation performance.

 

[123hotchef]

I'd say to blind the hardcore with sand, to give the insulation a better chance of remaing unscathed. Is the polystyrene you are using expanded or extruded? Big difference in toughness and insulation performance.

It is the lighter stuff so expanded I think

 

[Sean]

loads of quite innovative methods out there using polystyrene beads or recycled packaging bound in a cement slurry.
https://www.youtube.com/results?search_query=polystyrene+concrete

 

[Burdekin]

I was going to say use you polystyrene and add hardcore to it for the base. Compact it down and then pour concrete over the top.

What you did sounds ok for what it is. I've done similar with a deck foundations to reuse the dirt rather than get it removed. Concrete blocks laid on a load of soil/cement and then joists on the blocks. For the main load bearing ones I used concrete.

 

[Burdekin]

Iam in the same dilema, I was going use the 50mm polystyrene sheets i have lay them over the hard core then lay the DMP on top of this. Is the the done thing for a shed base?

Sounds overkill for a garden shed unless it's going to be a work area as well. Hardcore, concrete blocks and then a timber floor I imagine would be about the norm for a shed.

 

[123hotchef]

I may have the pillar drill in there and the fly press and a few other bits! anyway I have ripped up half the garden path kangoed it all up enough hard core just about now all tamped down with some dirt on top to protect my DPM will be mixing the concrete next weekend.