Shooting a Gunite Roof

This one is the long awaited shooting of the gunite roof. As per usual, we’ll start with the time lapse video and end with a gallery. If I am up for it, I’ll post some lessons learned in between.

The video

Lessons Learned

Gunite vs Shotcrete: This topic was already covered, but I’ll link to it here. I’ll also add that it is all in the mix. If your gunite is just a basic sand and portland mix, it isn’t going to be as strong as a shotcrete mix with some fancy additives, heavier aggregate, etc.

Lath and Screen vs Hardboard: For the shotcrete, the lath and screen worked well. None of the shotcrete passed thru the screen because it uses a pea stone aggregate. However, the gunite used sand as the aggrigate and did pass thru the screen. The hardboard was great in some ways, but the weather really beat it up and gave it a rippled surface.

Shooting and finishing: Both are important. My shotcrete crew could have done just as good a job as the gunite crew, but they didn’t. For shooting, you want someone who understands what you need to get done and is just putting up what you need. If they are paid by the cubic yard, they may put up more than you need, etc. You definitly want enough finishing people to finish the surfaces adequately. If your crew is strong on shooting, but doesn’t have enough man power to finish, you will be sad.

Cleanup… If the crew treats cleanup as an afterthought, you will be sad. There will be mess. Make sure the crew has enough man power to deal with it. Beyond that, you probably need to keep your eyes open for things the busy crew may miss. For instance, I wish I had better covered some of the near by boulders to prevent them from getting gunite on them and I really wished I had better cleaned off the polished concrete ribs before the gunite set. That mistake cost me many many hours.

Do it yourself? My rule of thumb is that if a job takes tens of thousands of dollars worth of specialized equipment, you probably shouldn’t be doing it yourself. Add the experience, skill and stamina required and I would double down that you probably shouldn’t be doing your own gunite or shotcrete. That said, I do know of some who have and kudos to them.

Gallery:

XPS gunite formwork for the south wall

In this segment, we formed the south wall for gunite.  The video, details and pictures can be found below.

The Video


Details

Order of the layers

We had formed previous shotcrete walls with steel studs and lath and steel studs with XPS insulation.  We liked the idea of combining the insulation in the form because it saved us the step of attaching it, however, the steel studs caused “void shadows” in the shotcrete and water could travel thru those voids, etc.

For this south wall, we decided to use the steel studs to form the XPS, but we would put the shotcrete outside the insulation, on the opposite side from the steel studs.  We could then use just a single layer of XPS and then backfill between the Studs later to increase our R value.

If this were a load bearing wall that needed to incorporate rebar in the shotcrete, we may have come up with a different idea.

Curved Bucks

To make the curved bucks, I used CAD software to figure out the size and angles of the little wood blocks, making sure that the longest side was something easily measurable on a straight edge.  In this case, I used a 5 inch length (4.999 is close enough to 5) and an angle.  I setup the miter saw with a stop block so I wouldn’t have to measure each one.  Then I would just need to flip the wood board for each cut so the blocks came out with the right trapezoid shape to assemble into the circle.

Gallery

A picture is often the best way to tell a story, so here are a bunch of them.

Preparing the mezzanine and tower forms for shotcrete

Our earth sheltered home has some interesting design elements, and at the center of all that is the tower.  This post is about how we setup the formwork for the tower and the mezzanine next to it.  I’ll tell most of the story in the descriptions in the gallery, but first, the Video…

The Video


The Gallery

Most of the story is in the descriptions

Electrical Main Breaker

“Nearly all men can stand adversity, but if you want to test a man’s character, give him power.”  – Abraham Lincoln.

“Never underestimate the power of stupidity”  – Robert Heinlein

OK, so maybe those quotes don’t have much to do with hooking up my main breaker, but I wanted power, even if it tested my character, and I was not too stupid to get it ;^)

In my region, the power company has set things up to favor a proper service entrance setup rather than temporary “construction” power.  The temporary setups have more paperwork and lots of rules about the setup, such as you can’t have outlets more than 10 ft from the transformer.  They also make it more expensive than hooking up the actual house main power.  On the phone with the power company, and I said, “With all this hassle and expense, why would anyone want to get a construction hookup?” and they guy on the other end just said, “Exactly.”

Of course, I had no wall to mount the electric meter or breaker on, so I would need to continue with my generator for a while longer.  I appreciated having my generator, but I didn’t like the noise and I really don’t want to make things noisy for my neighbors.  I also don’t like paying for gasoline.  Getting electricity on site just makes the whole building process easier.

Once, when I stayed past sunset the night before a pour day, a neighbor drove by and told me to, “Go home.”  All I could do was reply, “I’m working on it.”  I shut the generator and lights down within 15 minutes and finished up the column forms by flash light.

 

This post is just the story of how I got that power hooked up. If you know anything about electrical, my mistakes will be obvious to you, no need to point them out now. Anyway, I learned a few things along the way to getting it right and ended up still saving a lot of money.

The Video


Why?

Why do it myself?

The quotes to get my electrical done were not affordable.  I tried to get other quotes, but most electricians never got back to me or told me they were not interested in bidding for such an unusual home. The few that did get back to me wanted more than $64,000.  I asked one of them about the crazy price and he said it was about triple his normal rate per square foot because he still needed to figure out how to do it.  Some of this was because I have electric backup heat and electric hot water (on demand), which increased the overall power requirements, but most of it was just because the house was not like anything they had ever done and they had to estimate high to cover their risk (understandable).

I have already wired the basement, and that only cost about $400.  I was already pretty familiar with that sort of basic wiring, and you can find tons of information in books and online, so it was no problem. Taking care of the service entrance and main breaker was the next step.  However, it was something I was unfamiliar with and most books on wiring don’t cover beyond a basic hookup.

Why those two big switches?

My house is long and the service entrance was at the far corner of the garage (closest point to the transformer).  If I ran all the circuits from that back corner of the garage, it would be a huge task to wire the house, with many of the runs going well over 150 ft.  It would be a wire-pull nightmare. Also, the cost of all that wire, especially the bigger stuff for the on demand hot water heaters, well pump, etc. would really start to add up.  The concrete walls and unusual shape would also make it tricky to run circuits between the various sections.

So, I decided to put a breaker in each major section of the house.  I figured one in the in the laundry room, for the bedroom wing, and one in the mechanical room (under the middle of the house), to cover the rest of it.  However, the total AMPs required between these breaker panels was also going to be well over 200, so I couldn’t just have a main breaker and a sub breaker.

Code requires that the power from the meter socket should not travel very far before it passes thru a breaker or disconnect.  If I was not going to have the Main Breaker right on the other side of the wall from the meter, I would need a fusible disconnect switch to turn off the power in case of a short.   You can get reasonably priced switches for 30 amp or even 100 amp circuits, but they get big and expensive for 200 amp circuits.  My 200 AMP fusible disconnects are pretty much considered “commercial” equipment and I had to special order them.

There was also the issue of scheduling.  I had the back wall of the garage up well before I could put up either of the other panels.  The wall of the laundry room won’t even exist until June 2016 (or so). Having the switches lets us hook up the main power now, but just leave the switch off until I have the panel installed.  This is why the disconnect on the left is not connected to anything.

2Switches&Breaker

The breaker panel to the right of those big switches was originally a temporary idea so I could have power during the build.  I planned to move that one to the Laundry room eventually.  However, I have since decided to replace it with a smaller panel and just wire the garage circuits from there instead of from the mechanical room.  This will save me time and money and lets me get the garage circuits going sooner.

My mistakes

Building codes are important, but the code is not always clear and there is a learning curve when applying book knowledge to the real world.

electrical-cautionI had passed my basement wiring inspection easily, but the service entrance was more mysterious.  What info I could find in books or online really only covered the basic situation where the meter and main breaker are just inches apart.  From reading the code, I knew I would need to add switches, but I couldn’t find out much about how to install them, etc.  I hoped those expensive switches would at least come with some instructions, but they didn’t.  I figured I could figure it out well enough.

Some of the mistakes were things that I had known earlier in my life, but temporarily forgot or just messed up in the rush to beat the ground freeze.

Oh well, mistakes were made and that is what in the inspection process is for.  I will try not to mess up like that again, but, for now, I am moving on, still happy that I saved many thousands of dollars over getting a pro to do it.  I also learned a bit more than I would have otherwise.

Turning on the power

On the first day, two trucks and 5 guys from the power company showed up.  I had booked the day off work to be there and was anxious for them to get started…  They didn’t.  Only one guy even got out of the truck to talk to us. We asked what the delay was and he said they were a bit confused about where to run the cable.  Basically, they wanted to run from a slightly closer (180 ft distance box), even though it would require digging across the driveway, which they did not have the equipment for anyway.  I had already got the power company to agree to a 15 ft longer path from another transformer that would stay far away from the drive way or any thing else and only cut thru soft sand. Anyway, they didn’t want to start until the project manager at the power company approved what I told them he had said. He wasn’t in that day, so they would just wait.

I thought that was crazy to just sit around all day, but they told me not to worry because “This has been spec’ed as a two day job.”  He basically made it clear that I didn’t need to worry about how long they took because the cost was fixed.  They stayed, sitting in their trucks, all day, but did literally nothing towards hooking up my power. I was pretty annoyed that I would need to take a second day off work, but at least the weather was good and I was able to work on other things.

PowerCompanyCrew

The next day, they were back before 9:00 AM.  They sat in their trucks for the first hour or so.  We went down to ask them if everything was OK, but they just said they would get to us soon. Eventually, a few of them came up and looked at the meter cabinet while a couple others got the trenching machine off the trailer.

During about a half hour time frame, they hand dug the first couple feet by the wall/foundation and then used the excavator arm to dig a short trench, maybe 20 ft long.  This was done by 10:22 AM.

Trench1

They said something about not having any 3 inch pipe with them and said they would need to wait while someone brought some.  This is the standard pipe that they always use, so I thought it was funny that they hadn’t brought any with them.  However, they only needed a few feet of it and I had several pieces laying around. I offered them a piece, but they said they would just wait for the delivery.  They waited in their trucks until 11:40.

Trencher

The next step was to use the trencher to lay the cable.  One guy drove the machine and another fed the wire into it by hand.  The wire was pulled down thru a plow that split the earth to lay the wire and then let the earth close up behind it.  It basically moved at walking speed, so the cable was laid within 15 minutes. The cable had been laid on the ground next to the path before hand, so the whole process looked similar to pulling up a zipper. Then they took a 20 minute break before the one guy came back up with the machine and filled in the trench at the top at 12:21.  Again, that was just a few minutes of work.

Trencher_Plow

They put the machine back on the trailer and were quiet again until 1:54 when some other guys came out to hook up the meter socket (meanwhile the trencher guys stayed sitting in their truck).

AllenHere I should note that, other than something to cut the cable, the only tool these guys need to attach the cable to the meter socket is a 3/8ths inch Allan key.  This is the same sized Allan key they need for every single service entrance job that they do.  Guess which tool they didn’t have?  I loaned them mine.

Roughly 20 minutes later, they were done.  hooking up the 3 wires.  After packing up, one guy came to tell me that I could start to use the power. That first attempt video was shot right after they left at 2:22 PM. That didn’t work. Obviously, I was pretty annoyed with the crew by that point. I called the power company and was told someone else was planning to come and finish the setup.

New_Meter_Open

After my family had gone home for dinner, a sixth guy showed up around 6:20.  He did something to start my meter (I didn’t see exactly what he did, but it was quick.)  It seemed to me that perhaps I had misunderstood the first crew and the power was not supposed to be on until this guy came and did his part? I asked him about it, but he didn’t seem to understand my question.  He quickly sealed the meter box and told me I could start using the power, and then he was on his way.  I filmed that second video, with working electricity at 6:26.

So, let’s do the math.  They were at my site for most of two days, but my time-stamped time-lapse pictures show that parts of the crew worked on my hookup for less than an hour and a half.  I guessed three hours in the video, but I see that was much too generous.  If they were doing something in that truck, I can’t imagine what.  Keep in mind that this is a private company, not even government work.  But they do have a monopoly in the region, so maybe that explains it.

Costs

The 2/0 cable was about $1.50 per foot and I needed 3 cables, each 15 ft long, so 65$.  There was also the 45$ worth from my first try that was wasted.  The ground wire was about 10$, plus 25$ for the two grounding rods and acorn clamps.  The main panel (200 AMP QO) was about 200$ and the 20 amp breaker I installed for the construction circuit was $7.  Those big switches (200 amp fusible disconnects) were very expensive at over 330$ each, and each took 30$ worth of big fuses.  There was maybe 20$ worth of 2 inch conduit. Then there were a few small things like Tapcon screws to mount the boxes, etc.  I had them already laying around, but lets say 5$ for all that.

So the hardware total is, $372 for the service entrance and main breaker, and $720 for the switches, so $1092 total.

The price to get the power hooked up was reasonable…  820$ up front covered the meter box and all the hookup, including the 195 ft of cable from the transformer. Since it took 5 guys a couple days to get it done, I am guessing it cost the company more than that to do the work, but they plan to get the rest from me over a lifetime of electric bills.

It was going to cost an additional 3$/ft if the ground froze (nearly 900$ more).  So, after getting the back wall completed in october, I was in a rush to get the service entrance installed and inspected and the hookup scheduled.  That rush may have contributed to the mistakes.  It was only due to the merciful “Al Gore” winter that let me delay into the middle of November without the ground freezing.

 

So total to get the service entrance and switches up and running is ~$1910.  A big chunk of my electrical budget, but only a tiny fraction of what the professionals would have charged.  Of course, there were also a few evenings worth of time to get it done and then rip it out and do it again properly.

 

 

 

Precast Concrete Ribs

Tower_3D_2The 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.

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...

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 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.

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.

Rib_01_ConstructionBut 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)

G0018073_Overlap_Region

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.

Preformed_Styrups_small

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.

2015-06-10_HappyRebar

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.

FormedConcrete

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.