Information about design and construction of earth sheltered homes and a journal of my own progress

Bedroom Shotcrete

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Posted on August 18, 2016 by

After a lot of setup work, we were finally ready to have the shotcrete sprayed over our bedroom wing.  The Quonset garage was actually done at the same time, but that is another post.

In this post, we have some pics to illustrate the process and challenges, along with some tips for anyone planning to do something similar.

But first, The video.

The Video


Challenges

Financial

Try to get a quote that includes some expectation of how far they will get.

With each shotcrete visit, the cost (when I divide out per yard) has gone up dramatically.  I don’t have the calculation right in front of me, but this second visit was something like 48% more per yard than the first one, and that first one totally blew the original estimate out of the water.  So, as you can imagine, we have now (writing after the 3rd shotcrete session) blown thru all the money we allocated for shotcrete in the budget.  It is not so much that the shotcrete company underestimated the daily costs, but they dramatically overstated the amount they could get done in a day.  During this run, there was one day when they only got 8 yards (because the compressor broke), other days got 16 or 24. They said they would pro-rate, but in the end, the costs were all the same per day.  It is very easy for something you expected to cost 3 days to go up to 5 or more if you can’t be sure if you will get 8 yards or 24.  We will need to come to a better agreement that factors in some of my expectations when we do the next shotcrete.  We want to be fair (it is hard specialized work), but we don’t want to go bankrupt either.

We will need to come to a better agreement that factors in some of my expectations when we do the next shotcrete.  We want to be fair (it is hard specialized work), but we don’t want to go bankrupt either. Next time, I am going to try and work in some better expectations of how far they should get for the money.  It probably seems basic, but I am sure it is easier said then done.

A few more guys is a good thing.

Each day costs thousands of dollars for the base crew and concrete… The extra finishing guys are just a few hundred dollars each.  That is a bargain when you realize how much energy they save the base crew (so those guys get more concrete up) and how much smoother they make the walls (so you can save on waterproofing).

The lift will save you more than it costs.

The lift rental was over a thousand dollars.  This got worse because the shotcrete took a weekend and several days more than expected.  But actually, they guys at Wolverine Rental were pretty cool about it and made me a good deal that factored in days when I wasn’t actually using the rental, even though it was sitting on my lot.

The crew would have liked a nicer lift with a more powerful engine and tracks, I can’t even comment on how much more that would have cost because I simply couldn’t find one at any of the rental places in my area.  I suppose it would have saved a little time (they got stuck a few times), but probably not enough to justify the additional cost (I am guessing it would be a lot more).

The crew also complained about not having a second lift for the finishing crew.  Looking back, this may have paid for its self…  I’ll have to do a more careful financial analysis when I get a chance, but probably.  Looking forward toward my remaining shotcrete jobs, I don’t think there will be enough lift work to justify two, but maybe…

After saying all the negative stuff first for some reason… I will also say that I am pretty sure the lift really saved us more than it cost.  Certainly, it was necessary for reaching the awkward hard-to-reach places with that shotcrete hose.  But it also helped in the other areas where they could have used scaffolding or some “cheaper” method.  It helped move the hose around and allowed the guys to conserve their energy so they could get further in a day and do a better job.  I can do a quick mental calculation and say that it easily saved me much more than it cost.

Chunks

One of the mix trucks must have had some old cement set inside it after a previous delivery.  When it brought us our concrete, those chunks broke off and jammed up the concrete pump causing us to lose nearly an hour while the Michigan shotcrete guys tried to get the chunks out again.  Because we delayed the cement truck past the time they expected it to stay on site, the concrete company charged us an extra fee (75$), which was nothing compared to the value of the delay to us…  Any longer and we would have had to dump the remaining concrete. At the time, Sherri took this picture of the chunks so we could argue against the silly charge, but in the end, the 75$ wasn’t enough to bother fighting.

The chunks of concrete that came out of the mix truck and jammed the concrete pump. Sherri’s shoes for scale.

 

Equipment

The concrete comes from the supplier in a mix truck.  The factory adds the dry ingredients into the truck according to the recipe from the shotcrete guys.  It is basically a 7 sack mix with sand and peastone and Fly Ash (carbon nanospheres that help it flow) and various other admixtures determined by the weather and other conditions.  Along the way, water is added and the barrel of the truck turns to mix it up while on the way (which is why it is called “transit mix”).  They need to get the concrete out of the mix truck and on the wall within 90 minutes of starting to mix it.

Once on site, the concrete truck dumps the wet concrete into the hopper of the concrete pump.  This pump uses a 110 HP Cummins diesel engine to power two large 6-inch diameter pistons that can handle pumping concrete with aggregates at a rate of up to 50 cubic yards per hour.  Of course, they never got above 8 yards per hour, but that is more a function of the complexity of my design.

Meanwhile, the compressor sends high-pressure air thru the smaller hose (1-inch diameter) to the nozzle where it meets the pumped concrete and blasts it thru the nozzle and out into the air…

All of this comes together in the nozzle.  The most obvious control is a mixing valve for adjusting the ratio of air to concrete right behind the nozzle.  The concrete pump is controlled by a remote that was usually carried by another guy (who could go over and check the pump when something goes wrong).  The nozzle man indirectly controls the flow rate of the concrete pump by communicating with hand signals (and occasional verbal abuse) to the guy with the remote.  The nozzle is also pretty heavy and the video clips show how the nozzle man has to work with his whole body to control it, often while standing in very precarious positions.  The end of the nozzle is a rubber tip that can be switched out depending on the task or concrete properties.  The nozzleman can control the fan of the shotcrete spray by pinching this tip with his fingers.

From there, it is all muscle and skill as the nozzleman builds up the concrete in the right places, properly encasing the rebar and all the other annoying things I have in my walls.

These are the main pieces of equipment that make shotcrete possible.

 

Big job

Shotcrete requires a lot of setup to get rolling.  Once the guys get rolling, they can only put up so much concrete in any one place before it starts to schlep off.   Instead of doing the bedrooms and Quonset separately as two smaller jobs, we combined them.  I am pretty sure this saved us money and gave the shotcrete guys somewhere to go when the one-half of the project needed some time to cure.

Gallery

I guess I’ll tell the other stories here in the gallery.

 

Rebar and Shotcrete over the Quonset Hut

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Posted on July 15, 2016 by

Part of my earth sheltered home design included burying a Quonset hut.  Actually, this was the easiest and cheapest part of our build and if we had done the whole house this way, we might have been done the first year.

This post is for phase 3 of building our earth sheltered Quonset hut.  In phase one, we put up the steel Quonset (with a little help from our friends).  Phase 2 was getting up the Fox Blocks ICF endwalls.  That part was interesting, but took more time (and several posts).  We chose ICFs for the endwalls because they made the transition from wall to parapet for retaining the earth very straight forward.  Wood end walls would probably have been cheaper and faster, but not nearly as durable..

I have seen Quonset huts buried directly (with just a pond liner for waterproofing), but I wanted to put in a layer of reinforced concrete first. Phase 3 is to cover the quonset in reinforced concrete before we can bury it in phase 4.  (actually, I guess there was a waterproofing step before we can bury it also).  In this way, it is really the reinforced concrete that supports the earth load.  The Quonset hub becomes fancy form-work and an interior finish.

Here is the video about phase 3.

The Video:

Information:

Hot:  The day we put the insulation and plastic on the roof was officially the hottest day on record for our area…  The reflective Quonset hut was making it feel twice as hot and the heated metal was melting the duct tape that we used to tape down the Styrofoam. I edited the water breaks out of the timelapse, but they were about every 15 minutes.  Fortunately, we got clouds and cooler weather for the second half.

Crew:  Just  a quick note that I appreciated all the help I got on this portion of the build, both amateur and professional.

Rating:  My Quonset is rated for 75 lbs per square ft.  I am sure the engineers were thinking snow load and not concrete, and there is probably a significant safety factor built into that.  Either way, I am sure I significantly exceeded the official rating by about 50 lbs/sqft. Fortunately, my shotcrete was also sitting on the footings, shotcrete above that was really sitting on the shotcrete below, etc. The stiffening shotcrete really formed an arch from footing to footing and is probably not loading the Quonset hut significantly. Maybe if all the concrete were wet (no internal stiffness) at the same time, we might have had a problem, but instead, the shotcrete was curing and carrying load as it was applied.  Spreading the shoot over the 3 days probably helped.

Crenelations:  The Quonset crenelations (groves) were about 7.5 inches deep.  Across the top sections, we decided to fill with polystyrene strips 4 inches thick, 10 inches wide and 24 ft long.  Some quick math, and I can tell you that we placed more than 7 cubic ft of Styrofoam in each groove, which means we reduced the load on the top of the roof by about 1000 lbs per groove.  There were 20 grooves, and I filled 16 with Styrofoam, so that is a 16000 lb reduction.   I left 4 of the crenelations without Styrofoam (and added extra rebar) so they would have stronger hoop beams across.  We filled the concrete to a depth at least 4 inches above the crenelations, so the concrete in the crenelations was nearly 12 inches thick.

Plastic: The plastic liner was primarily to keep concrete from directly contacting the Quonset, and secondarily to provide an extra waterproofing layer.  I wasn’t fanatical about it.  The plastic is tough, but not impervious to puncture.  I have heard of people using pond liners with a heavy felt underlayment, or even grinding down all the bolt ends that could puncture the plastic.  Pond liner and underlayment can easily cost more than 50 cents per square ft (and the Quonset surface is well over 2000 sqft).  Our 6 mil plastic sheet cost less than 1/10th of that, so when it got a few little holes and tears I didn’t worry about it too much.  I had thought that the plastic would make it more difficult to walk around on top of the Quonset (because it could slip), but actually, it improved the walking conditions quite a lot.  I sagged the plastic to allow it to properly fall into each crenelation when the rebar and concrete were added.

Openings: The Quonset hut is a nice strong shape to work with (mine was an “S-Type”, the “Q” type would have been even better, but less practical without the straight stem walls).  Loads are transferred around it like pressure on an egg.  As you know, any crack in an egg weakens the overall structure tremendously.  I planned to have 3 openings in my Quonset shell, two skylights and a side door (to the mudroom). The most stressful time for the Quonset would be when it was covered with 46 yards of wet concrete (186,300 lbs) and two tons of rebar.  You do not want to have holes in the Quonset at that time.  Instead, I just created bucks to keep the concrete out of these three areas.  Later, I can come back and cut the Quonset steel with a grinder to make the openings.

Rebar: Each crenelation got 4 pieces of #4 rebar.  One vertical piece was “inside” each crenelation, centered and about 4 inches from the inner surface.  The other verticals were at a layer about an inch past the outer surface of the Quonset hut, spaced 8 inches apart.  The horizontal rebar was mostly tied to the outside of the vertical rebar and spaced no more than 12 inches apart.  Some horizontal pieces were placed first, against the surface.  These first horizontal pieces made it easier to place the outer layer of vertical rebar without things falling inside the crenelations.  They were kept off the Quonset hut by placing a few 1.25 inch rebar chairs (according to code).  We wanted most of the horizontal rebar further out where it could help hold more shotcrete up.

The other important rebar code section to worry about is overlapping the pieces.  We had a 40 ft long Quonset hut with 20ft long pieces of rebar.  I just placed them end to end and then came back later and tied 4ft long segments along that seam.  That was more than enough to overlap both sides by 40 diameters (code).

Electrical and Plumbing: I didn’t include it in the video, but we also ran plumbing and electrical before adding the shotcrete.  These were then inspected.  Part part of this was running white vent tubes up the side of the Quonset so they would come up the skylight curbs.

Balance: The Quonset hut shape holds even pressure very well.  It will actually get stronger when I put the balanced earth load on top…  But while applying dynamic load (such as the shotcrete) it is important to apply it evenly to both sides so the Quonset hut isn’t pushed flat.  Our shotcrete was poured over 3 days.  The first day got most of the short vertical walls done (our Quonset hut is an “S type”).  This stabilized the base and got us ready for the second day where they added most of the shotcrete, including filling some crenulations all the way across.  In the video, you can see them working one side and then the other, back and forth.  This required moving the heavy hose, which is tiring.  They made their lives a little bit easier by toeing the hose with the lift whenever they could. Then the crew left for the weekend (not ideal) and came back on Monday to finalize the top, add the shotcrete for the skylight curbs and do a final coat for smoothing.  All this time was partially because they also shot the bedroom at the same time (upcoming post), but planning to shoot things in a balanced and paced way is a good idea even if you could shoot faster.

The Compressor:  The shotcrete is moved to the wall in two ways.  The mix truck dumps the concrete into a hopper where it gets pumped (by a very expensive concrete pump) thru the hose to the nozzle.  In the nozzle, the “nozzle man” injects compressed air to blast the concrete at the wall.  Part of the magic of shotcrete (the strength, lack of cold joints, etc.) comes from the way it impacts/compacts the wall particle-by-particle.  Unfortunately, the compressor broke down soon after starting on the Quonset hut and somewhat spoiled the day (but somehow didn’t reduce the cost).  Ironically, this same compressor had broken down when they did the basement shotcrete, which was the last time the crew had come out.  They already had a concrete truck on site with 8 yards of concrete in it, so we decided to pump it out.  I knew that this wasn’t quite ideal in terms of speed of application or strength, but it did work and I didn’t have to worry about cold joints, since it was the first bit to be applied.  The next day, they rented a nice new compressor for only 60$ / day and it ran flawlessly.

Hard work:  Pretty much everything about shooting shotcrete is hard work.  Moving the hose, aiming it all day, keeping everything flowing, finishing the surface, repairing any issues with the equipment, cleaning up, loading up, moving scaffolding, etc.  It is all hard work and I appreciate the effort of the guys from Michigan Shotcrete.

 

Gallery:

These are pictures taken from my cellphone or time-lapse screenshots…  Enjoy.

Rebar and lath for the bedrooms

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Posted on May 6, 2016 by

Last year, we got started on the steel structure.  This year (2016), we got all the rebar and lath up in preparation for shotcrete.  First, the video…  Then some info, but mostly a larger picture gallery than usual.

The Video

Details

Statistics

This process took from 2016-05-05 to 2016-07-26, so nearly 12 weeks of the calendar.  Of course we also worked on other things during that time (such as the garage which will be a separate video). Specific to this bedroom wing, we worked (at least for a couple hours) on 26 different days.  The time-lapse camera (which I ran pretty faithfully) recorded 77,653 images.  At one every 5 seconds, that means it was running for 388k seconds, or 107 hours.  If we divided that into 8 hour days, it comes to about 13.5 days.  About half the time, I was there by myself, 1/4 of the time with Sherri, and the last quarter Sherri and I had other help (Hunter, John, Bonnie, Joe & Jessica (my parents), Dan, Ethan and the plumbers).

If I had turned all 77,653 images into video at 29.97 frames per second, it would have been a little over 43 minutes of video.  I edited that down to under 10 minutes (less than 1/4).  In some cases, I edited out scenes, in others (such as that last interior wall), I just ran the speed of the video up to x900.  You are welcome ;^)

Rebar Chairs

We added rebar chairs to stiffen up the assembly and prevent “bounce”.

It is important to leave some space between the rebar and the lath for the concrete to completely encase the rebar.  To achieve this, we made sure to tie the lath on loosely (leave room for a couple fingers).  this works pretty well for the roof because the weight of the concrete will push the lath down and away from the rebar, but no further than the wire ties.  However, in the walls, the concrete can “bounce” the lath and then fall off the wall.  After seeing my setup, the shotcrete guy asked me to stiffen up the walls by adding rebar chairs where the lath was bouncy…  I had these chairs left over from the quad deck floor and they worked perfectly.

Welding

Welding was great because it really stiffens up the assembly so you can climb it without fear… and it actually doesn’t take much longer than tying.  In many cases, I just tied enough to keep the bars in place and pull any wide intersections close enough to weld.  Then I would just weld the rest of the connections much faster than I could have tied them.

The downside to welding is that the heat can actually change the properties of the steel and make it more brittle if you try to bend against the weld…  However, in my case, the welds are really just there to keep the steel in place long enough to pour the concrete.  After that, it is really the concrete that keeps the steel together (and vice versa).  My welds are intentionally shallow, just enough to tack the pieces together without significantly weakening the rebar.

You may find some places have building codes against welding rebar, but if you read them more carefully, they are really talking about cleaning that surface crud off the steel.  You get that sort of thing with arc welding, but not with the MIG welder that I use.  But in any case, there are no such rules for residential construction where I am building.

Curving Rebar

When you curve rebar, it is always trickier to curve the first and last couple feet.  But the middle curves pretty easily.  So, I usually curve the full 20 ft long pieces and then cut the nice continuous curve into as many pieces as I can get.   If the piece has a 5 ft straight wall before the curve, then I just start curving the rebar 5 ft from the end.  I usually start by “over curving” the steel a little bit and then straighten it out to get the final radius that I want.

Gallery

Here is a gallery of pics.  Some are just as people started or moved the go pro time lapse camera. Others are just candid pics that went by too fast in the timelapse.  There are also occasional cell phone pics in there also.  Thanks to everyone who came out to help.

 

 

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