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…
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Rib assembly. The brackets are screwed into the 2x# forms, which are bridged together with plywood scraps and all of that is screwed down to the melamine base… but the sidewalls are not actually screwed to the brackets…
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A detail of the hanger. The PVC pipes along the spine will make it easier to set the vertical rebar and feature ridged insulation wedges. The PVC is held to the form by a screw which is removed before the forms.
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Rebar skeleton, mostly #5 rebar tied together with #3 rebar loops.
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The loops are mostly spaced by 8 inches, but the engineer specified that the spacing be reduced to ~4″ near the high end of the lower spandrel.
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Closeup on the high end spandrel. You can see the PVC holes that will help tie the rib into the central tower.
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Low end of the form. The white PVC pipe will allow the rib to slide over a 1″ steel post in the foundation. The green “hangers” are 2×6 boards that will keep the form from bowing and help me suspend the rebar skeleton.
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Table to calculate costs of the forms and rib material
The architect drawing of the rib. Note the construction circles… The rib was designed to be drawn easily on the shop floor
This is a rough rendering of the inside (living room)…
My unusual umbrella design uses some “Euclidean egg” shaped ribs erected around a central tower. vaults will span between the ribs forming an umbrella like structure that will distribute the earth loads down to the ribs. These interesting architectural features allow me to have an open plan layout without needing to span the space in one large vault.
The idea is to precast these flat on the ground and erect them into place. If I can find a precast concrete company to do the work for me at a reasonable price, I will gladly pay. But I suspect it will be expensive and I will need to do this work myself.
The engineers drawing of the rib showing the required reinforcing
If I can order from an architectural precast company, I can relax knowing that they have lots of experience working with precast concrete. They know about the mixes and the finishes and they have all the equipment to cast the pieces and move them around, as well as the space to store them until I am ready to position them. We we are ready, they will deliver them at what ever rate we can handle assembling them (all on one day I would guess).
If I precast the ribs myself, the first concern is having the space to setup the molds. The ribs are heavy and cranes are expensive, so it would have been nice if I had the space to setup the mold, pour the rib and then just leave the >5000 lb rib right where it is sitting. I would move the mold to another spot on the floor, pour again, etc. Then I would only need to call the crane out for one day to erect and position all the ribs at once. I have a lot of ribs to make, so if I only used one mold to make them one at a time, it may take me 6 months while the rebar is placed and the concrete cures in sequence. I am thinking that I should probably make 3 or 4 molds to speed up (parallelize) the process. I could fill several molds at once from a single cement mixer delivery or I could be setting up the rebar in one while the previous one is curing. Laying on their sides, The ribs are only one ft deep, but cover an area about 13′ x 16′. I could fit only 4 at a time on my garage slab. I don’t plan to pour the floor for the rest of my house until much later, so I will probably need to move one set out of the way before I can pour the next. This may mean several additional crane rental days at probably close to $1000 per day.
I also must consider that if I do end up using my garage slab to pour the ribs, I won’t be able to erect the Quonset hut and close it in for storage purposes until all the ribs are done.
Unlike the bridge example below, my ribs will be architectural elements of my home. This keeps them in a a more gentle environment in terms of temperature swings and corrosive elements like salt or acid rain. However, the arches will be under much closer visual scrutiny by my guests (some of whom may be concerned about the tons of concrete and earth over their heads). I am sure any flaws will be even more visible to my critical eye. So the final finish and other small details will be important.
Visually, I am most concerned with the difference in the finish between the steel troweled side and the the other three sides that will be poured against the mold. Most precast architectural components are designed so that the steel troweled side is the “back” of the piece and never seen. Speaking to various experts, I am told that the difference will be minimized if I use a lighter and finer cement. I actually got some samples (shipped in a 7 lb box) from one of the architectural precast companies and Sherri and I compromised on “sand stone”. I also plan to sand blast the all sides of the final piece to remove that poured concrete look and simulate something that looks more like the texture of sand stone.
Structurally, I have the engineered drawings for the rebar spine that should be more than strong enough once the rib is in position and loaded as expected, but I am a bit concerned about the process of putting it into place. I will need some sort of rebar hooks in the spine for the crane to lift it by. I asked the engineer to position these, but he is under a fixed price contract and declined. he said that was something the precast shop should work out. I plan to cast #5 rebar loops right into the spine of the ribs which will complicate the mold a little so it can be demolded after the hooks are set in place. I have some ideas. ;^) The hooks will end up helping to tie the ribs into the vaults and will not be visible in the final home.
Crane for 600$/day
Placing the precast rebar will take a crane. I have already shopped around for crane rentals. I can rent one to use myself for about $600/day or $4000 for 4 weeks. I will also look for one that comes with a skilled operator. The cranes will lift 25 tones 45 feet into the air, so it shouldn’t have much problem positioning the ribs. I had hoped to use the Articulating Manlift that I will already be renting to help setup the rest of the arches and put the shotcrete crew in the right positions, but it will only lift 1/10th of what the ribs weigh.
I will illustrate how it was done on the Hang Tua Bridge in China. Sorry for the quality of the images, but the process was the best documented of any example I could find.
Like many other bridges, this one is made from precast open spandrel concrete arches.
The process starts with a steel form. Steel is great when you want to pour a lot of arches, but I will make mine of less sturdy stuff, most likely poly-carbonate and plywood. This form is very similar to mine, including the open spandrel.
Next, the rebar spine of the arch is wired together. This looks just like what the engineer specified for my ribs.
The steel form is placed over the rebar spine and bolted to the steel base of the mold. In my case, I plan to bring the form in several manageable sections and then cross brace to hold it all together.
Concrete is poured into the mold to encase the rebar spine. The rebar spine is lifted off the bottom with hooks so the concrete can wrap around all sides of it. Vibration is also used to ensure that the voids are removed from the concrete.
The concrete is steel troweled off and allowed to cure. My form would have a similar process, which means that the finish of the steel troweled side would look a bit different than the other 3 sides poured against the form. Unlike the bridge components, my arches will be right in my living room. I will use a light color and sandblast the final concrete to try and hide this difference.
After the forms are demolded. The rebar sticking out of the spine will help tie it into the deck above. It looks like these may have been added after the form was removed (drill and grout). Since my forms will come away horizontally instead of vertically, I can wire some directly to the rebar skeleton before I pour.
In this sequential process with a single steel form, cost is reduce but time to manufacture all the ribs is increased. The concrete arches are stacked to wait while the next one is being poured and cured. Concrete continues to gain strength even after it is removed from the mold, so this time is not wasted.
Finally, erection day comes around and they are delivered to the job site and a crane is used to carefully position the ribs.
Assembly is fairly rapid and the bridge quickly takes shape.
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.
