These concrete ribs were designed to give me more of an open space feeling without needing to build a wide and tall vault. For more about the design of the ribs or how I made the forms, you can read this earlier post.
This was really an epic part of the earth sheltered home build project, spanning (no pun intended) much more time than I would have liked. The costs for the forms and concrete were pretty minimal, the majority of the cost was actually in hiring the crane to move them around and set them up.
Side note: since I am so far behind… I will probably go and improve the section on the timeline next. Thanks to those of you who wrote to inquire if I died. Nope, just really busy and didn’t have time to put a video together. Speaking of which…
The Pump truck did not make things easier. Pump trucks are expensive and I needed it to come out for the floors anyway, so the first few rib pours needed to be coordinated with other jobs. This complicated the planning and made the pour days harder. It also slowed down the progress on the ribs by delaying the pours. And after all that extra planning and delay and expense, it was just plain harder to fill the ribs from the pipe than from the chute because it was really difficult to move it around. I guess my advice here would be to talk to the concrete guys and ask them how they would recommend you handle it. It was probably obvious to everyone except me.
Originally, I used silicone caulk to seal the bottom edges of the forms against the floor. However, this was a pain to clean up later. For the 2nd set, we used play-dough that my wife got cheap on clearance somewhere. The play-dough came up easily enough at the end, but it was such a pain to roll out and put into place that we ended up going back to the silicone for the later ribs.
The first set of ribs took about 4 hours per side to polish. I would start with a diamond cup wheel and then follow up with successive polishing pads at 50, 100, and 200 grit. I also used a special wheel to put a 3/4 inch round on the edges. For the second half of the ribs, I got a larger, more aggressive diamond cup wheel. It worked so well for the first step, that I quickly did all the other ribs. It was only when I got to the 50 grit pads that I noticed the diamond cup wheel had made deeper scratches than the previous one and it was much more difficult to remove them. I even went back to try the less aggressive cup wheel once I realize that the 50 grit was not working well. Overall, this mistake cost me several extra hours for each rib.
The moving dollies were supposed to be able to handle 1000lbs each, but that was clearly an exaggeration. I used more than 10 for each 5000 lb rib and still we had crunching sounds as their ball bearings exploded out all over the floor. The tires on some of them shredded completely. Eventually, I learned that most of the damage happened as each wheel rotated into the correct position to roll forward. By the time I got to the 3rd set, I had learned to point the wheels all in the right direction before lowering the rib on to them. This increased the survival rate considerably. I also salvaged partially damaged dollies by consolidating the less damaged castor wheels on to other dollies.
The rubber form liner molds were an interesting part of the build for me and I like the final look on the ribs, but again, not the best idea. More details below.
Liquid Rubber Form Liners
Originally, I planned to use the Styrofoam ceiling tiles directly, but after handling them a bit, I was worried that they were too fragile and wouldn’t last thru multiple uses. I also thought it would be a bit tricky to place them in the form so they would be centered because they were a bit narrower than ideal. However, if I used the ceiling tiles to form durable rubber form liners, I could get longer pieces that would be reusable and would be the inverse of the tiles. I could carefully center them on a board of the right width so the full depth forms could be easily placed, etc. I did some math to find the volume that I would need and found that I could get the PolyTek 75-75 ingredients for about 175$. That seemed reasonable enough to me at the time. The box of foam tiles was about $40. However, I soon discovered that mixing carefully was critical. My first few attempts were mostly good, but 95% isn’t good enough to cast concrete with. I only ended up with enough decent panels to do a small section of the first few ribs.
For the living room ribs, I decided to make the panels a little thicker (these were the ones I showed in the video). I would need to order more liquid rubber. This time, it was more like 225$, so I was in for roughly $400 worth of liquid rubber. That would have been enough to buy new Styrofoam tiles each time, so not the smartest move in hind sight.
I didn’t quite use up all my liquid rubber ingredients on the panels because I decided to try the Styrofoam ceiling tiles directly on each alternate rib. This way the pattern inverts, positive/negative for each rib.
Cost and timing
See the other page about costs for the forms, but they were just a few hundred dollars and were reused for all the ribs. So divided by 11 ribs, that is just about 30$ each.
The rebar was fairly affordable also. We used about 60$ worth of #5 and about 50$ worth of stirrups, so about 110$ per rib. There were also some steel plates that I built into the ribs and I think I paid about 10$ each for those from the scrap yard. Tie wire and welding costs are hard to guesstimate, but lets say it is less than 5$ per rib.
The concrete was about 1.3 yards per rib, which would be less than $185, even after some waste (Concrete costs about $100 per yard, delivered, but there are a couple other charges).
The molds and ceiling tiles were about 450$ total, so about 40$ each.
Then I bought about 20 of those little moving dollies and some other miscellaneous stuff for about 220$ total, so 20$/rib to move them out of the garage.
The grinder and all the pads were under 220$, but I still have the grinder many of the pads, so I am just going to leave that stuff out.
So lets say the total was $400 per rib. Not bad considering the quote to have it done by someone else was about $8,000 each.
Unfortunately, the crane and welding the ribs to the ring just about doubled that cost. The guys from RTC were great to work with and I appreciated their help, but I had not budgeted enough in that area.
Timing varied as I got better at each task, but here is the rough break down in man hours.
Form prep took about 2 hours per rib and includes cleaning the form segments, fixing any damage, re-assembling the form and attaching it to the floor, and then caulking the bottom edge.
Rebar was next and was taking about 8 hours on average, including getting all the rebar in, tying and welding.
After the rebar was in, I needed to call for an inspection, which didn’t take much time, but did delay the next step.
Next was the pour. It actually took less than half an hour to pour each pair of ribs, but then we would spend at least an hour or so troweling and finishing it off. Lets be generous and say 2 hours per rib. Then there was some delay (several days to a week) as we let the concrete cure.
Then we would spend about 6 hours (3 hours each) unpacking and moving the ribs out of the way. This included a lot of clean up.
And then the cycle could repeat.
After the ribs were out of the garage, I could polish them. The first half took about 4 hours per side to polish, so 8 hours per rib. But, thanks to an overly aggressive cup wheel and adverse weather conditions (hose freezing, etc), the second set took about twice that long. Lets put in an average of 12 hours per rib.
The flip and setup time wasn’t too bad, but lets write it down as an hour each there.
This brings the total hours per rib to about 28. I had 11 ribs to make, so about 308 hours total. If I had done those in 40 hour work weeks, it would have been nearly 8 weeks of constant work (yes, some of those hours or days were worked by other people like Sherri, Bonnie, Aaron, Dan, John and Mark). However, I already had a full time job, and also had other things to work on at the house, so I ended up spreading this part of the build over a whole year. The video on this article covers from July 2015 thru April 2016.
Needless to say, I am glad the ribs are all done.
After a long and difficult project, looking over the pics feels pretty good.
To illustrate how you get tangential arcs, important here because the load paths are smooth and in compression
3D model showing the ribs and curved steel I beams.
Model showing the kitchen, I included it here so you could see where the kitchen door slides into the rib.
Euclidean geometry can be built with nothing but a string and nails…
At the corners, it is best to overlap the rebar like this so that neither side can be pulled thru
Ryan and Carter helping with the hose
Moving those 5000lb ribs tended to wreck a portion of the wheels on each cart. Rather than toss out the whole carts, I would consolidate the good wheels together so some of the carts could be used again.
First rib lifted up… I tiled the image so the rib would look upright
At one end, the rebar is welded to a steel plate and this still plate gets welded to the steel compression ring when we set it up.
On cold days like this, I placed a heater in the middle and also covered with rigid insulation before leaving for the night.
Love this pic of Michael playing on Ribs 7 and 8 while we are finishing up prep for 9 and 10.
Work often runs me past sunset, especially in the winter.
Ribs 7 and 8, ready to pour.
We used a wet grinder to polish the concrete. It had various different cups and polishing pads.
Rebar was fun, the first time, but got old fast. In these joint areas, it got a little messy
#5 rebar along the spine with #3 rebar stirrups to tie it all together.
Some times I welded in extra bars just to keep everything oriented and spaced correctly.
Ribs, forms removed and ready to lift.
The boys playing with balancing against concrete paving stones we made
Polishing the ribs as a snow storm rolled in
Another snowy day… It was above freezing, so I still polished, I just had to get the ice out of the hose first.
Sherri and I were pretty happy after curving the last of the #5 rebar… Just a few more hours of rebar tying left on the last rib.
Sherri’s father (Mark) came out to help pour the last rib.
By this 11th rib, Sherri and I pretty much know what we are doing.
Sherri pulling down the last bits of concrete from the chute
Finishing up the last rib and knowing I wouldn’t need to reset it felt pretty good.
This bottom end of the rib was broken off when the strap slipped and it was dropped. You can see the yellow target that I pained to help guide the crane in. There was also a #5 rebar peg grouted in the middle of the square.
The crane company said I should label this pic as “Mike, the RTC oompa loompa”. All good guys to work with.
We used these bolts to attach the steel brackets to the ribs
The steel attached to the concrete
The view from below after the ribs were setup. This is our “open concept” living room…
Last post, we talked about finding problems with the architects drawings. Mostly, the issue was just that the drawing was not quite to scale. The dimensions shown were not actually the dimensions used. I had followed the dimensions as shown when building my virtual model, so by the time I got around to fitting the kitchen door, it didn’t work. Some things I could fix, such as the radius of certain rooms that had been mislabeled by a number of inches. Other things, such as the width of the arch shown on the kitchen wall elevation, were just plain wrong and I had to figure out how to deal with the misfit.
Note: In an earlier post and on a page, I talked about how I went with the architect who offered only 2D drawings because it saved me a lot of money on this difficult-to-3D-render home. At the time I understood that 3D is better for most rectilinear homes because it does help to find these problems, changes are properly propagated, etc. But for this complicated unconventional home, I reasoned that I would be paying a lot for the 3D cad skills when I really only needed the architects design skills and the final builders would only need 2D drawings. I never did get a lot of architectural input on my 2D design, and now that I am doing the 3D model myself, I am finding all the problems that a 3D architect would have found. Choose wisely, even if I am not really sure what the wise choice would have been yet.
I am using SpaceClaim to build my virtual model. It doesn’t have a lot of the fancy textures or architectural features like widets for making sloped roofs or easily adding doors to walls. I also have Autodesk Revit, which does have those features. I used it for my earlier models (pre architect) and that tool was great for layout, etc, but I found it much harder for the complex geometry of my roof. SpaceClaim can handle the complex geometry. Spaceclaim is also great and modifying a geometry to fix a problem. It is a “direct modeler”, so you can grab any surface and “Pull” it to adjust it. Everything gets taken care of along the way. This made fixing the model pretty easy.
I needed to reduce the radius by 4 inches. Even on complex objects like this concrete over my quad-deck, it was easy to pull things into place…
Other tools, like Rhino3D, are popular with architects and could handle the geometry and has much better rending functionality (and my architect’s junior guy did model some aspects of my design in Rhino3D), but it doesn’t have the tools for easy modification and I didn’t have a licensed copy. I am told that Sketchup Pro, very popular with architects, could handle this. However, I tried the very popular free version, aka “Sketchup-Make” and it could not handle the ends of the vaults in the circular portion of the house (although it would be great for something more rectolinear).
Once the radius problems were fixed (and I added all the changes to my errata sheet), the door still didn’t fit because of the out of scale arch used in the architects elevation. I had to think about my options. I was not willing to re-scale the rib to match the architects mistake in one drawing. I decided that the rib at that location was “architectural”, not structural. This meant that I didn’t want to move it outward or upward and lose its architectural look/fit with the rest of the space. It also meant that I could cut the spandrel without needing to re-engineer the arch because that arch was not really bearing the load of the roof like the other arches were.
This left me with two main options.
1) I could cut the arch to fit the door. This would give me a full rectangle door to work with. I could get my “architectural” look back somewhat by coloring the door some how to continue the arch. Maybe I would add a veneer of granite, or stained glass or just stain or paint, shaped so that when the door was closed the concrete arch lines were continuous across it. Well, my wife did not like that idea. She is concerned about the structural aspects and she is probably concerned that it will be more work (the house is enough work as it is and she hates it when I add to my potential work load).
2) I could accept the fact that the arch crosses a big corner of the door. To test this idea, I have actually put duct tape across the corner of my office door in a way that matches the profile of the arch crossing the kitchen door. It has been there a while now and I have not minded it at all. Even if I brush my shoulders against the door frame, my head still does not hit the duct tape corner. If I go with this plan, I have multiple ways of proceeding…
2a) I could cut out a section (or just prevent concrete from forming in that middle space) so that I could fit a rectangular sliding door frame tucked into the arch. If I take out 4 inches of concrete from the middle of the spandrel, there will still be 4 inches on either side. Plenty of structure for an arch that is filled in on the underside with concrete anyway. The door would slide into the wall between the kitchen and basement stairs. I like this idea, but it will take some careful planning if I am to form the concrete rib with the void in exactly the right place… (planning the wall void is easier because I am building it right in place over a framework.)
2b) I could simply hang a sliding barn door (but a modern looking one with nice or hidden hardware) on the outside of this kitchen doorway. The door would be hanging in the mudroom and could slide the opposite way along the mudroom wall, so I wouldn’t need any voids in the wall between the kitchen and basement stairs. This is Sherri’s preference, at least partially because she thinks it will be easier to implement. I think the architect may also have suggested it at one point (because he didn’t know how I would get the mechanism inside the concrete wall). I don’t like the “fit” of it as much, but I will try to keep an open mind and think about it some more.
In the mean time, here is how things are looking in the kitchen (I modeled in some cabinets to make sure it all fit)… The three open blocks above the cupboards will be 8″ glass blocks and are there to let light from the main living space into the basement stair well.
And here is a wider view of the north side of the house (the original plan was to virtual-build just the section over the basement). Of course, this is just the initial concrete structure (plus door bucks). No earth cover, windows, etc. You can see that the mezzanine windows have been moved closer together to allow the dirt to cover the roof better. Inside, I added other details, including the spiral stairs, etc. Maybe I will include some of those pics in the gallery at the bottom.
Fusion welding HDPE Plastic Pipe
I got a section (about 6 ft) of 8 inch HDPE pipe from a contractor a while ago. It was old and cruddy and maybe had a bit of oil on it, but I took it so I could experiment with it. The expensive part about building earth tubes with HDPE plastic is that you have to hire someone to fusion weld them together… Or at least, you can’t buy a fusion welder from Home Depot. The fusion welder equipment is very expensive and only intended for professionals. I thought that maybe I could make my own fusion welder. The professional equipment specs I found on-line called for Teflon plates that could reach 450ºF (230°C), along with some jigs to help align the heater plate between two ends of pipe and then move the heater plate out of the way and press the pipes into alignment.
I started by taking an old toaster apart. I was going to run the elements between two Pyrex glass plates that I found in the cupboard. I figured they were garage sale plates and not part of a set (there were just 3 of them). Boy was I wrong… Those were part of a special 3 plate cake holder thingy that my wife loves. Good thing I checked first.
I decided that her fold-able electric grill would be better because it already has two nice Teflon surfaces and dials for adjusting the temperature. I would just need to break it in half so it would fold outward instead of inward, and probably disable what ever safety switches its designers had included to prevent me from using it that way. Of course, I would also need to buy her a new grill (I already got her a new toaster), but that would be a lot cheaper than hiring a guy with a professional fusion machine, so win-win.
I put rings sliced from my pipe on the grill and the edge softened right away. I then lifted them off and pressed them together… Instant fusion weld… Actually, I guess I heated them too much (too soft) and pressed them together too hard, because I got a bit of a bead inside.
Later, I sliced up my samples, including a cross cut so I could see that the fusion weld was as strong as the rest of the pipe. For scale, the pipe shown in this image is 1/4 inch thick (twice as thick as the pipe I plan to use eventually), so the bead is about 1/12 of an inch. (sorry the pic isn’t very good, my camera doesn’t do macro well, but you can see the bump where the soft plastic at the join pushed into the pipe). Well, that was easy. I am sure I can handle that.
Of course, I wanted to see what else I could use to fuse the plastic… I have a small benzomatic torch. I thought maybe it would burn the plastic, but, even with the direct flame to the plastic, it only burned for a second (some surface residue) and then it just softened the HDPE nicely. The problem was the heat was not even enough… So for an additional experiment, I used the benzomatic to heat a piece of metal and put the plastic against the other side… That distributed the heat well to soften the plastic evenly and wouldn’t require any electricity. For one attempt after the metal was probably too hot, the HDPE plastic did stick to the metal a little, but a piece of my wife’s parchment paper fixed that problem (just like fusing perler beads). I later hooked up my benzomatic hotknife attachment and found I could cut the HDPE pretty well with that.
In general, I found that the HDPE plastic softened easily, once soft, it was a bit tacky to the touch, but would instantly fuse with other HDPE plastic. I found that the joints seemed as solid as the rest of the pipe. I also found that the heavy plastic also kept its heat well (high Specific Heat Capacity), so I had quite a bit of time to get the two pieces together.
The only hard part was aligning the two pipes perfectly. I imagine that would be even more difficult with 20ft long sections of pipe, but I am sure I could build a simple jig to make that alignment much easier.
I started back up the process of getting quotes last week. I probably called a dozen companies. Only one has got back to me with a quote (so far). A couple others just had follow up questions. And in one case, I am still waiting for a call from the “lady in the office who knows the email and such.” I need the email address to send in the plans.
The one quote that did come in this week was for the footings. It was about 1/4 the price of the last footings quote I got and this guy seemed much more interested in the project and much more pleasant to work with.
Previously, excavators had all told me that they would get down to the depth at the top of the footings and would let the whoever did the footings excavate from there. The other foundation people I spoke to agreed with this and included several thousand dollars of additional excavation in their quotes. However, this latest foundation guy said that it was very difficult to dig a precisely curved trench with their equipment and my sandy site probably wouldn’t be well suited to trench footings anyway. It would be much easier for the excavator to level out the area to the bottomof the trench depth (an extra foot) and then the foundation guy could lay out the curved forms (just thin plywood staked in place) in an open flat space in much less time and much more precisely.
This foundations guy is actually a full service concrete company that also has Shotcrete equipment. It looks like his experience is mostly limited to smaller jobs like turning “michgian basements” into real basements. I still prefer my other shotcrete guy, if I can ever manage to arrange a meeting with him. The foundations guy also said he would do flatwork and gave me reasonable rates for that.
I know I have mentioned this tiny house design site before, but I saw another post that I want to share… They have a few small underground homes and even more green roof homes and I recently stumbled on to another one (posted mid 2012) here. Man that look so easy to build compared to mine ;^)
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…
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…
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.
Rebar skeleton, mostly #5 rebar tied together with #3 rebar loops.
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.
Closeup on the high end spandrel. You can see the PVC holes that will help tie the rib into the central tower.
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.
Table to calculate costs of the forms and rib material