How can you make large foam blocks?

Does anybody know how to (inexpensively) make large foam blocks? I need a size of approx 4ft x 4ft x 16in. I want to use them to form a
void in a concrete pour. I don't care if they decay or rot later.
Any spray type insulations are more costly than the concrete they would displace. Is there a cheap method to make a void form block?
Perhaps soemthing that is not a chemical reaction (maybe you could use compressed air to make the bubbles in the foam). Perhaps a mixture of soap and something that would harden + compressed air, inside a wooden box mold.
Any thoughts would be appreciated.
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Buy bulk styrofoam. Cut it into blocks. Tie it down so that it does not float up when you pour the concrete slush.
Any foam-manufacturing will be more expensive than concrete. Buy the foam blocks.
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Any reason why you can't just use a wooden box?
Barry
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On 29 May 2005 21:19:37 -0700, snipped-for-privacy@yahoo.com wrote:

From a sheet of 4 inch insulation foam, cut two 4 ft X 4 ft panels. Cut two 4 ft X 8 inch strips. Cut two 40 inch X 8 inch strips, Glue together as a box with hot glue etc.
If this is too expensive, you could make a box from thin luan ply with sotwood reinforcements under $20.
Brian Whatcott Altus OK
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The big boys build a box with plywood and whatever and nail it in place.
Most building supply outlets have competent staff to help you get to the point of disaster and sell books. Some, have free handouts.
How big is the pore?
Atty
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Thanks for the replies. It seems the consensus is to use a wood form. I had thought of that, but was hoping for soemthing even cheaper and easier. I hate to waste the wood. Also, I think the cost in my area would be about $30 per box. I wanted to make a 24x36 pad, which would require 54 boxes = approx 1500 bux. Plus a lot of internal bracing/work to prevent them from collapsing.
I know that everybody just builds-up underneath with gravel, but I find that the frost is gaurunteed to crack it. I like the idea of having the extra "concrete waffle" structure underneath which makes built-in beams that should keep the pad in one piece.
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snipped-for-privacy@yahoo.com wrote:

Great big Foam blocks are made for casting in concrete "floating slab floors".
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As a concrete professional I will offer these suggestions: The best option is buy styrofoam from Lowe's or Home Depot.(they Offer Various thicknesses in 4x8 sheets. Stake it in with stakes and tiewire or stringline etc. If it ever has to be removed just pour some gas over it and it will break down to nothing.
You can always use wood and wood stakes. Just build your perimeter form and fill it in with compacted dirt or sand. This is called soffit fill.(or put a wood top on your form)
Anything else would be too expensive and I build condos, bridges, flood channels and I have also poured Stallone's driveway and work at the studios. I have pouring and running Civil/ Public Works projects for 11 years from Florida to Illinois to LA and Las Vegas.

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