Basically, the basement of our earth sheltered home was filled with approximately 11 cubic yards of concrete slag that needed to be broken up and removed so we could prep for pouring the basement floor.
It was something we have known we needed to do since last year, but were putting it off for obvious reasons.
Here is the video:
Why did this mess happen?
All this concrete was wasted shotcrete that wasn’t on the walls and should not have been on the floor either.
As you may recall, I had used steel studs to frame the basement and then placed metal lath on the inside to “catch” the shotcrete. I had been told (by the shotcrete guys) that the lath would be enough to prevent much of the shotcrete (peastone) from blowing thru. I was told to expect a thin layer of concrete on the inside, thin enough that it would break up into small fragments just by walking on it and that it would actually save me from needing to add as much pea stone later.
Watching the shotcrete being applied, it did appear that not much passed thru when it was applied at a downward angle onto the previous shotcrete. They did do it this way for the first couple levels, and actually raised a scaffold jack platform twice as they went. But then they got a bit tired and started shooting horizontally and even at an upward angle. This allowed much more shotcrete to pass thru.
The effect was cumulative with blow thru coming from so many different angles, each adding its own layer of concrete. The round central room was especially bad for this with at least 3 layers of 2 inch thick concrete across the floor.
And once the crew was working on the inner walls, there was also “rebound”, shotcrete that doesn’t stick to the wall, and “trimmings”, concrete that is cut off the wall because too much was applied in the first place.
All this concrete (that I paid for) ended up on the floor, but not in a good, “wow, you got bonus concrete floor along with your shotcrete” kind of way. On average, I would say we had about 3 or 4 inches across most of the floor (in several layers), and up to 8 or more inches near the walls, especially in the corners. It was uneven and lumpy and even had boot prints in it. The whole feeling was somewhat “war torn” and more than a little depressing.
When I setup the main level, I plan to back the metal lath with fiberglass screen. The metal lath will still provide the strength to catch the shotcrete, but the fiberglass screen will prevent any material from passing through.
Gallery
Thought I would try to put some extra pics in here…
David working on the concrete slag floor, he did a great job and filled up 33 5 gallon buckets of rubble.
David helping to tear out the old mess
And the Story.
I like to include the text of the video, along with some extra info that doesn’t fit in a narration, so that the content is google searchable.
For this job, I had hired some teens, rented a jack hammer and taken the day off work to make the long weekend even longer.
The big question was, “How would I get this slag out of the basement?” The final solution that I came up with was a Bagster dumpster that I got from Home Depot for 30$.
The plan was to load it up and use my trusty skid steer in to lift it up and out of the basement.
It took a bit of trial and error to figure out the best way to lift the bag and to empty it, but fortunately, we had lots of tries to get it right. You can see how we did it in the video.
The bagster is supposed to be for only a single use, but it held up very well, load after heavy load, for a number of days. The only tear was caused by dragging it up the rough wall in the first lift.
This first day, we were mostly focused on the edges where the thickest concrete was because I didn’t want to rent that 75 lb jack hammer for a second day. The heavy jackhammer was actually very effective on the thick concrete, but kept getting stuck in the thinner stuff. For that, the 11 lb breaker was much more effective. My Dewalt hammer drill also got a work out. At the start of the day, I couldn’t get the teens to touch the power tools, but by the end of the day, they were much more comfortable with me and the tools and were taking turns on the jack hammer.
On Saturday, my parents were in town, even though I warned them that we would be taking on the worst job of the build so far. I also hired Zack again, he was one of the teens from the day before.
My father got to cutting a slot in the footings (doorway) for the radon tube while the rest of us got cracking on the concrete slag. Our radon tube was made of a 4 inch corrugated drain pipe, wrapped in landscapers fabric to keep dirt out. It just gives radon an easy way to escape so it won’t build up under the basement floor.
Then my father and I worked on the floor drains while the others just kept right on cracking up that concrete. In order to get the slope correct from the floor drain in the central room all the way to the outer wall, we had to cut open the tops of the footings.
We had planned for holes in the footings to run these pipes, and I had even come prepared with 4″ PVC to use to form them. However, the guys doing our footings told me they brought their own 4″ corrugated drain pipe, which they nailed in place very quickly. The problem was that the flexible pipe “floated” up in the middle when the footings were poured. Instead of being a straight sloped hole thru the concrete, they bowed to the point that we couldn’t even get the 2″ pipes thru. I guess they were not used to the footings being so wide. Narrower footings probably wouldn’t have as much deformation due to “floating”. You may recall this same issue cost me time and money during several other stages of the build. Hopefully this was the last of it.
Then we came back out again on the holiday Monday, just my wife and kids. Sherri and I cracked things up with the 11 lb “breaker” and the boys scrambled to collect the pieces into the buckets. When the buckets filled up, one of us would dump the bucket in the bagster. The boys were motivated by being paid 1$ per 5 gallon bucket. They worked for several hours before wearing out.
With the big chunks finally removed, we raked the smaller bits and then brought in some pea stone, which is required by code in my area.
I came back on another afternoon with Zack and my friend Aaron to get the second half of the pea stone down and rake it all level. At one point in the video, you can see Aaron intentionally took a pea stone shower, just to see what it would feel like. I don’t think he will do that again.
The final product was a a peastone under-floor that meets building code. The black pipes are to channel radon out of the home and the white pipes are plumbing or drains. The inspector approved the work and we were able to rake the pea stone level and move on to the next step.
Next step is to get the vapor barrier, insulation and radiant floor tubes down here so we can pour the basement floor.
The core of my unusual earth sheltered design includes 10 precast concrete ribs. I did try to get these done professionally and asked 4 concrete casting companies for quotes, only two got back to me and the average was about $80,000, plus shipping. I asked what they planned to make the forms out of, and both companies said, “plywood”.
Since each of the 10 ribs only includes a few hundred dollars worth of materials and the forms are also relatively cheap to make, I decided to try it myself.
It was definitely more work than I expected, but I was able to keep the cost well below even my estimate by using a lot of the junk wood left over from forming the footings. I think the cost for both forms was less than $300, and then I put in about $200 worth of rebar and 220$ worth of concrete into each. That is less than one 10th the cost of having it done professionally.
The rebar cage was inspected before the pour (you can see the inspectors feet at one point in the video), and passed.
I think the first two ribs turned out well, and there were already some lessons learned, but I will make a few more before I add “Lessons Learned” to my Precast Concrete page.
In the mean time,
Here is the video;
And here is the story;
I like to include this text because it makes it “searchable”. I also try to provide more info than I can provide talking thru the video.
Design
If we get rid of all this peripheral stuff, at the core of this design are radial vaults around a central tower. I need the load from the earth above the vaults to be carried to the footings. I also wanted to keep the vaults small while allowing an open concept living space between them. My solution was to support the vaults with these Euclidean-egg shaped concrete ribs. I chose this shape, with its continuous curve, because of its compressive strength and drawability. The inside end of these ribs will sit on this steel compression ring that will help distribute the load to the tower and steel posts and ultimately into the footings. The egg is tilted so that there is lots of head room near the tower where doorways need to be placed. On the outer edge, an integral column carries the loads directly into large 4’x4′ footing pads.
Eventually, I will span these ribs with steel arches that will form the shape of the vaults and hold up the metal lath that will catch the shotcrete, but I am getting ahead of myself…
First, I need to cast these ribs.
I worked out the forming details in 3D, and it was well worth doing even though I didn’t stick to the plan exactly. The original plan was to layout the wooden forms on a plywood base in the garage (this didn’t quite work out as planned). I used shelf brackets and thin plywood to build the sides of the forms. In the orginal plan, I imagined using 1/4 inch plywood, but In the end, I used multiple layers of 7mm lauan underlayment and a final layer of white board.
The specific rebar layout was designed by my engineer. I really think it is probably “over designed” and working with that #5 rebar is a pain in the butt, but considering how important the strength of these are, I wasn’t going to short it… In fact, based on my own calculations, I actually added additional rebar in the the one place where I felt the engineer had not gone overboard enough… I put an extra two 20ft pieces of #4 rebar along the central spine and belly of the arch. I also integrated 3/8ths inch steel bearing plates at the high end of the arch.
The concrete volume is about 35 cubic feet, which, at about 150lbs per cubic foot, puts the weight of the arch at around 5000 lbs. Maybe the crane operator can tell me the exact weight later.
Form Construction
For practice, I also made a quarter scale model a couple years ago, and practiced drawing out the full scale arch on my driveway.
But this year, it was time to try the real thing. I laid out cheap particle board sheets, screwed them together, coated them with waterproofing and began to construct my curves. This is really a scaled up version of the techniques that old-time graphic designers used to create fonts with tangential curves (before they had computers). The key to tangency is that when two arcs meet, the radial lines where the arcs meet must pass thru the center of both.
Then it was time to cut boards to fit the profile of the arch. This involved tracing them on the floor, then cutting them with a band saw and sanding them. The curves from the band-saw were are not perfect, but I sanded off the main problems and the successive layers should smooth things out.
Then I realized that my particle-wood-base idea was not going to work… humidity and temperature changes during the day just caused too much warping and movement. The waterproofing had been a waste of money and may actually have contributed to the problem because the edges were absorbing moisture and swelling differently to the rest of the boards and this led to curling. Hopefully I can reuse these waterproof boards somewhere else later.
Before removing them, I drilled thru to mark the key centers of the arcs on the floor, then I came back and drilled larger/deeper holes that could hold a nail and work as pivot points so I could quickly reform my egg-arch shapes without redoing the whole Euclidean process.
Then it was time to build up the sides of the forms… I used shelving brackets as planned. Each is supposed to be able to hold 250 lbs. I spaced them less than a ft apart. Even if they had to carry the full weight of the concrete laterally, they should have strength left over. I had to back them with some plywood blocks (not considered in the original design) so my other layers could attach to something.
I needed to make sure that the forms could be easily taken apart. I didn’t want any shifting at the joints, so I used a series of 4 inch offsets with the layers of thin plywood… Where these “separable” layers overlapped, I had to be careful not to glue or screw. Instead, I relied on notched pieces of 2×4 to keep the overlap closed tightly until the lateral load of the concrete was in place to keep it tight. Then I would remove the pegs. (In the video, these were sticking up from the form with yellow tips)
Since I didn’t have the wood base anymore, I eventually needed to attach these form segments directly to the concrete slab with tapcon screws.
The forms for the spandrels (those cut out holes in the ribs) were a bit trickier. I made the series of parts for them with a very carefully planned process of angled passes thru the table saw and router… Then I nailed them all together with my air gun. I had included a fairly significant draft angle so I could get them out of the concrete at the end. This made skinning them with the last white layer a bit more difficult and I had to trim to fit.
I had a similar problem with the tighter curve portion of the rib… I guess my form walls were not perfectly vertical and I had some trouble with the 3rd layer and had to trim it off. After that I used smaller pieces so the angular error didn’t add up.
Eventually, I attached the remaining form pieces to the slab and caulked all the joints… It used up three tubes of silicone.
Then my friend Dan showed up to help out… He took on the job of creating the hangers across the form. These will be used to hang the rebar skeleton so it is positioned in the middle of the concrete. There was also some thought that they might help hold the form together against the pressure of the wet concrete.
Meanwhile, I got started on the Rebar… I had ordered a pallet of pre-bent “Stirrups” and placed just the amount I would need on that pallet in the middle. These were custom ordered and delivered for not much more than the cost of straight rebar. Most places quoted me the price per lb, rather than per bend, so once you pick your supplier, it is pretty easy to estimate costs.
The majority of the rebar was the very thick “#5” rebar. It is not easy to work with, and even more difficult to curve precisely. Dan came up with this idea of clamping the pieces directly to the hangers and forming them in place… It worked very well.
The#3 rebar stirrups need to be threaded on to the long rebar peices ahead of time for easy placement.
Tying rebar is not very fun (time-lapse did not capture many smiles, and this smiling pic above was early in the process. I forget what Dan had just said). You need to bend over a lot and (because I kept not wearing gloves) my hands were pretty cut up by the end of the process… I was glad to be working in doors during all that rain though.
As we neared concrete day, another friend, Aaron, came out to help with the second rib. We used the same process to bend the long pieces and were quite a bit more efficient with this second rib, but it still took me half a day to get all the rebar in and tied.
Pouring the Concrete
Finally, Concrete day arrived. We poured the basement floor and ICF blocks first (another video), and then it was time for the ribs.
Sherri sprayed everything down with form release agent so the concrete wouldn’t stick to the floor or forms. Actually, we used a bug sprayer, but it wasn’t able to spray finely enough (it should be more like a mist) so we wasted a lot of the release agent trying to get coverage. More is not better. The heavier application of release agent was running off the whiteboard instead of coating it. I plan to buy a proper sprayer for next time.
Then the concrete pipe was brought in and things got crazy for a while.
The pipe was very heavy and stiff and generally difficult to move. The pump truck guy took pity on me and stepped in to show me how it was done… Instead of trying to lift it, he was leaning into it. It reminded me of my windsurfing days as a kid. I still never got the hang of moving the concrete pipe effectively, but we got the job done. Next time, I am hopeful that the pump truck operator can park his truck in a slightly different location so that his articulating arm can better access the forms.
We used a concrete vibrator to consolidate everything and it really helped. I had surveyed people on facebook about this and most told me not to worry about it, but these are so critical to the design, but structurally and as visual elements, that I didn’t want to skip on this part. As soon as the vibrator entered the concrete, it liquefied (like sand in an earthquake) and flowed into all the crevasses. Actually, when the vibrator touched the rebar cage, its vibrations were transferred and affected the concrete several feet away. I am glad I didn’t just try to hack it with a sawzall or something like that. I don’t think it would have had the momentum to move all that form or rebar.
The concrete was poured very quickly. I was exhausted and we took a 5 minute break for drinks, etc.
After the concrete setup for a bit, I cut the hanger wires and removed them (along with the hanger boards) and then we troweled over the surface so you don’t see any holes. My wife, Sherri, actually liked the finishing part of the job. We used buckets of water to clean the concrete off the hangers and tools. And, of-course, it was all a race against the setting concrete.
Eventually the crew that had been finishing the basement came in and help with the final edging, etc. This pic is immediately after the chaos, before the cleanup. The wires were hanging the rebar cage. This all cleaned up just fine.
Next Step… we need to take the forms off, and move the ribs out of the garage so we can reset the forms and pour 4 more times.
The skeletal steel frame of my earth sheltered house is critical to its success, and getting from the plans to hard steel on site took a lot of effort and is therefore worth a post…
I’ll probably end up putting notes about the more nitty gritty details and lessons learned into the technotes and design tips section of this site… When I am done learning.
In the meantime,
here is the video.
Steel frame basics
So, the structural design of my house is essentially Shotcrete sprayed over a steel frame skeleton. You may not see this often, but you can find many examples on line.
Check out some examples on the Formworks site (their facebook page is excellent), or the site of their brother company, PBS. I would love to know the falling out story behind that split, but it is clear that the brothers did their early work together because a number of homes are shown on both sites. After the split, the Formworks designs get pretty interesting, while the PBS designs stay pretty simple in terms of geometry.
Formworks uses 3×4 I-beams to span vaults of up to 50ft across. They weld Z-brackets on the side of the IBeams to make it easier to add the rebar later. Special brackets are bolted to the slab foundation and then the I-Beams are slipped into place and bolted to the brackets. The rebar is dropped into the z-brackets and they are hammered down to lock tight. They also have a variety of specialty hardware for bolting the IBeams together, attaching floors, etc. It is a pretty good system, aimed at helping the “do-it yourself” earth sheltered home builder be successful, and refined over many years of actually building these sorts of homes…
So, why would I try to improve on that?
Well, for starters, I didn’t really like the large flat front parapet wall design that many of these homes end up with… I guess it looks pretty good if the home is mission style, but I was wanting something that looked a little more integrated with the earth. I wanted the hard edges a bit more broken up. It is just personal taste, but I wanted something more like a single story “Hobbit home” than a 30 ft tall “Lonely Mountain” edifice. This meant that I was going to be keeping my heights and spans low. I also couldn’t resist the idea of mixing and matching in a whole bunch of different arch forms. The resulting jumble of small arches required a new approach.
My wife and I have talked a few times about how stringing together 4 Quonset huts would have saved a lot of time and money. I would be done and writing my earth sheltered book by now if we had gone that route. I actually had some pretty good ideas for how the exterior could be softened (we were going to go “modern wave”), but we couldn’t agree on a good way to finish the interior and we had already put so much thought into the other plan… But maybe a simpler plan would be a better idea for you?
My overly complicated design includes 3 groin vaults, 3 apses, 10 radial vaults (of 3 basically different sizes), a portion of a “toroidal” vault and one simple vault over the mud room. In some cases, those steel arches are sitting on precast concrete ribs or spanning shotcrete walls. It was a lot to plan out.
Square or Round tube?
Originally, I had planned a mixture of both square and round tube (based on tangency to the ceiling below), but I ended up switching to all square tube because it is stronger in its primary load direction (because there is more material where the peak stress occurs) and costs less… But I later discovered that it is harder to roll-form (without deforming or collapsing). Paradoxically, some places insisted on the steel being thicker (0.1875 instead of 0.12, or 50% heavier) so it would be easier to roll. The company I ended up with just took their time and the deformation is barely visible.
Radial Vaults:
The radial vaults spanning the curved ribs were the most tricky to plan because I needed to adjust the radii of the steel arches for the location and curvature of the precast concrete ribs they were sitting on. I ended up deciding to have a level interior and used a simple formula (Rarch=2πrθ/360) for the arch radii at each location, but used the 3D model to calculate the length of the spacers between the arch section and the curved concrete ribs.
Elliptical Arches
Again, in order to avoid the “flat south façade” look that too many earth sheltered homes end up with, I wanted the dirt to spill down around the bedroom windows. I decided to “miter” the corners. It seemed like I should use properly shaped steel to get that miter shape right and that required ordering “ellipse” shaped frames. This turned out to be quite a bit more difficult than the regular arches.
They basically made them by creating pieces with simple radii and then welding them together. Then they tweaked them a bit (more of an art than a science) to get them to match a full sized template that they had asked me to make. The end result cost about 3 times what I would have paid for a simple arch, so I hope the shape they give the bedrooms is worth it in the end.
To make the template, I used the two foci and a string method (shown in the video). The coolest thing I did there was make a little car out of Lego that had a place to hold the Sharpie and two pulley wheels to hold the string as it went around. This made drawing a smooth curve much easier.
When it came to quoting me for the arches, it became clear that most companies were quoting me per “roll”, regardless of the length of the roll or even the size of the steel. Since I had a number of half arches for forming my apses and the corners of the bedrooms, it was clear that I could save some money by ordering those as full arches and then cutting them in half later. I made all the drawings this way… For instance, that ellipse piece will be cut in half as a corner piece before it is installed. It is also easier to weld the legs on straight if I do it before the pieces are cut in half.
Some of the companies were strictly rolling and wanted me to do everything else myself. The one that I ended up with including the cutting in the base quote, but would charge me 50% more to weld the pieces together. Since I would still have other welding to do anyway, and because I kind of like doing it, I decided to do all of it myself (or with friends and family).
For the next few weeks, I will be focused on getting these arches welded together, and the bedroom ones in place. I am also still working on the precast rib forms and trying to get the quad deck guys to come out…
This first day, we were mostly focused on the edges where the thickest concrete was because I didn’t want to rent that 75 lb jack hammer for a second day. The heavy jackhammer was actually very effective on the thick concrete, but kept getting stuck in the thinner stuff. For that, the 11 lb breaker was much more effective. My Dewalt hammer drill also got a work out. At the start of the day, I couldn’t get the teens to touch the power tools, but by the end of the day, they were much more comfortable with me and the tools and were taking turns on the jack hammer.
The final product was a a peastone under-floor that meets building code. The black pipes are to channel radon out of the home and the white pipes are plumbing or drains. The inspector approved the work and we were able to rake the pea stone level and move on to the next step.
The core of my unusual earth sheltered design includes 10 precast concrete ribs. I did try to get these done professionally and asked 4 concrete casting companies for quotes, only two got back to me and the average was about $80,000, plus shipping. I asked what they planned to make the forms out of, and both companies said, “plywood”.
Formworks uses 3×4 I-beams to span vaults of up to 50ft across. They weld Z-brackets on the side of the IBeams to make it easier to add the rebar later. Special brackets are bolted to the slab foundation and then the I-Beams are slipped into place and bolted to the brackets. The rebar is dropped into the z-brackets and they are hammered down to lock tight. They also have a variety of specialty hardware for bolting the IBeams together, attaching floors, etc. It is a pretty good system, aimed at helping the “do-it yourself” earth sheltered home builder be successful, and refined over many years of actually building these sorts of homes…
Well, for starters, I didn’t really like the large flat front parapet wall design that many of these homes end up with… I guess it looks pretty good if the home is 
The radial vaults spanning the curved ribs were the most tricky to plan because I needed to adjust the radii of the steel arches for the location and curvature of the precast concrete ribs they were sitting on. I ended up deciding to have a level interior and used a simple formula (Rarch=2πrθ/360) for the arch radii at each location, but used the 3D model to calculate the length of the spacers between the arch section and the curved concrete ribs.
