Tag Archives: shotcrete

Shotcrete of the central circle


Posted on November 7, 2016 by

It was time to shotcrete the central circle of the house. This post is mostly a gallery of pics to tell the story, but first, the video…

The Video

The Gallery

Another round of shotcrete forms


Posted on October 15, 2016 by

There was just enough decent weather left in the season to prep and shotcrete the walls around the perimeter of the central circle.  We had to start with the formwork, and since these walls had simple curvature (rather than compound curvature), we decided to use OSB board screwed to vertical steel studs.  Of course, rebar was added in and tied.  I’ll get into details and lessons learned later in this post… And there is always a gallery of pictures at the end, but first, the timelapse video.

The Video

Extra bits

Steel Studs

These MarinoWare steel studs I have been experimenting with are a bit of a mixed success.  They are great for holding the formwork, rebar, electrical, etc. And I like to think that they provide some reinforcement for the concrete.  On the negative side, the shotcrete crew were not always able to properly encase them and possible voids in the walls along these studs probably reduced the wall strength and provided a path for water to channel.  I wouldn’t call them a total failure, but I have decided not to use them in this same way for the central tower.  Instead, I will brace that formwork from the outside.  I will continue to use these studs for the South wall because the design there has these outside the concrete, supporting the rigid insulation formwork.

Screwing into Steel Studs

Pre-drilling would just take too long and it would probably drive you crazy trying to keep the boards aligned to get the screws thru the pre-drilled holes.  Self-tapping drill point screws are what you need.  They should be long enough to get thru the form materials (3/8th inch OSB in my case) and still bite in nicely to the steel.  In places where you put an extra layer of OSB (such as to thicken the joints), you will need an extra 3/8ths of length.  Also, to save money, you want to use the shortest and smallest screws that will work.  Finally, the key thing is the head…  I recommend the Hex-Washer-Head because it will be held securely by your driver without any slippage or cam outs.  Do not buy Phillips head screws, you will seriously regret it.

Personally, I ended up mostly using TEKs #10 x 1 in. Zinc Plated Hex-Washer-Head Self Tapping Drill Point Screws that cost about 4 cents each.  If you can get them in bulk, you can probably reduce the price to half that.

Plumb and Braced

For some reason, the camera kept moving on to the next area before we would complete the important job of plumbing and bracing each section of wall.  However, that step was important and worth a small section here. The walls had some natural stability because of the curvature, but we still needed to brace them against the force and vibration of the shotcrete and they were not always naturally plumb.  On dirt, we could just brace with a 2×2 or 2×4, screwed to a block on the wall and then screwed to a stake.  However, for most of this job, we had to brace on the concrete deck and didn’t want to attach the bracing to the deck and risk damaging our radiant tubing…  The solution was to place a board on the deck, screw the diagonal brace to that, and also screw a horizontal brace and tie it back into the wall.  This worked pretty well.

In one location, the wall was out of plumb and took some serious pulling to try and force it plumb.  Some combination of the boards, studs, and rebar was fighting against us.  I ended up just using a strap and a come-along to pull it.  Part of that strap is still embedded in the concrete wall, but at least it is plumb.

Welding Rebar

I get a lot of negative comments from people saying that it is “against code” to weld rebar.  In reality, the building code has about 85 pages devoted to welding rebar.  The code talks about what types of Rebar you can weld (mine has a little “w” on it to indicate that it is weldable), diameters (most welding rules don’t kick in until #7), what types of welds you can do (butt joints are not acceptable), pre-heating, exceptions based on engineering approval, etc.  I was going to get into it a bit here, but it is probably better to just suggest you read up on it yourself.

While I agree that the heat treatment that comes from welding does influence the ductility and other material properties of the steel, in certain situations (some of my situations), it does make things much better.  I know what I am doing, everything is inspected, no need to worry about it.

The Gallery

Here is where we put some of the pics we took during this period…


Bedroom Shotcrete


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



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 visit was more than double the cost per yard of the basement job, 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.  If you are paying a daily rate and the volume can vary from 8 to 24 yards, it is difficult to budget.

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 than 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.

Update: Funny side update, but 6 months later the Shotcrete contractor texted me to say that the lift (which his crew operated) damaged his 900$ hose while it was pulling it around.  Later in the text conversation, he said he would be nice and not bill me for the damage.   ¯\_(ツ)_/¯


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 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.



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.


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