Arch Forms

As you may recall, my design features pre-cast concrete arches to support the heavy earth loads and let me have open spaces without requiring large spans.  I had some old posts about how these will be built, and even my own experiment to build quarter scale models.

I did get quotes on having these ribs done by professional concrete pre-casting companies. One even sent me nice faux stone concrete samples, but when the cost estimates came back, they ranged from $40K+shipping to $80K (with shipping) for the 10 ribs…  I thought that was ridiculous considering that each rib only used about 130$ worth of concrete and less than 200$ worth of rebar.  I asked the companies how many forms they would make and what they would make the forms out of… All three said that 10 was a small order, so they would just make 1 form out of wood…

Obviously, they were charging way too much and I was going to have to take this into my own hands…  I had designed the ribs to be cast easily in a 1ft deep form.  I could do this.

But first, I wanted to make a computer model to figure it all out.

My model revealed that the cost of reusable parts, assuming I went with a rather expensive Melamine base and 2 layers of 1/4 inch smooth plywood for the side walls, would be about $575.  I would probably make 2 in order to cut down on crane visits (the crane will have to come at least 5 times with 2 forms).   Then each rib would require about $350 worth of rebar, concrete, etc.  I also decided that I would need to buy a concrete polisher (wet) ($200) and a sawsall concrete vibrator attachment ($50).

All told, that would mean about $1150 for the 2 forms, plus $3600 for the rib materials, plus $250 for tools, which gives less than $5000.  If I add 20% to cover misc, it comes to $6000.   I plan to work out a deal with the concrete company to rent their crane for a reasonable cost.  They currently use it to place pre-cast septic tanks which are about the same weight (35 cuft at 130 lbs each is 4550 lbs, plus the weight of the rebar).

Along the way, I thought about things like layout, materials, form removal, etc.  For instance, I plan to build these on the front half of the garage slab.  I will build the back half of the quonset for use as a shop, and then cover the front half with a large tarp to keep the rain out.  I can then remove the tarp so the crane can pick up the ribs more easily.  I plan to use some #4 rebar to create hooks on the top of the casting.  In order to remove the form later, I will need a slot in the form that I will plug with pieces of scrap insulation during the pour.  For a base, I plan to use melamine sheets that will provide a non-stick surface.  I will then need to polish both sides of the form to get a similar finish on the trowel finished side of the concrete.  I plan to build two forms, one left handed and one right handed…  There are a few different configurations to lay these out next to each other to minimize the space needed, I think I will go with a 24×24 layout that will require 5 sheets in the top layer, 6 in the middle layer and 1 in the bottom.

Here are some pics with a few more details…