How can you make large foam blocks?

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.

Any reason why you can't just use a wooden box?

Barry

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

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

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.

Now is that a smart idea, given the constraints, or is that a smart idea?

Brian Whatcott Altus OK

Wood is probably one of the cheapest concrete form materials available.

Foam is rather expensive unless it's "surplus" material

How about old tires?

cheers Bob

Great big Foam blocks are made for casting in concrete "floating slab floors".

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)