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.
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 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.
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.