50+ cracks in day-old Shotcrete.

bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Can you verify that the people on your job have all of that experience?

How do you know that it's the same people?

Maybe all of the good people left and they recently hired all new inexperienced people.
The foreman who ran the nozzle was very experienced. I'm unsure of the other laborers. None of them seemed new. But that's of course hard to really know.
There are many factors that go into getting a good shotcrete or Gunite shell and all have to go right to get the best result.

What was the rated PSI of the mix?

Maybe the mix got too hot from sitting in 102 degree weather for too long and the driver added a bunch of water to try to save the mix.

Maybe a lot of rebound was used and there was not good compaction.

Maybe the thickness is less than necessary.

What was the thickness supposed to be?

A shotcrete/gunite expert would probably need to do multiple core drill samples through cracked and non-cracked spots is to see if the cracks go all of the way through, to check the thickness and to do compression testing on the core samples from the non-cracked spots.
I can't recall the psi, but the foreman was proud of their applications. It was in the high thousands, like 5-8k unless I'm mistaken.
The engineering sheet shows the floor to be 6" total. 3" under and over the rebar.
It didn't really 'sit', or at least it didn't seem to. The trucks all came on time and they were able to keep moving pretty steadily. But I also don't know what 'sitting' would constitute. Whether 5 minutes is too long or if we're talking hours.
 

JamesW

TFP Expert
Mar 2, 2011
31,511
It didn't really 'sit', or at least it didn't seem to.
What about the trip from the plant to the job?

Maybe the truck got stuck in traffic for an hour?

Maybe ask the concrete supplier for a load document describing the load, mix and if they had to add water.

Note that some concrete suppliers keep good records and some don't.

Also, since you are not the customer who ordered and paid for the concrete, they might not release the information to you directly.
 

bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
What about the trip from the plant to the job?

Maybe the truck got stuck in traffic for an hour?

Maybe ask the concrete supplier for a load document describing the load, mix and if they has to add water.

Note that some concrete suppliers keep good records and some don't.

Also, since you are not the customer who ordered and paid for the concrete, they might not release the information to you directly.
I'm not sure that's a variable in this case. It was a ready/site mix truck. Square bodied. The concrete was mixed real-time as it came out of the shoot wet, into the Shotcrete crews pump trailer.
 

bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
For reference I did not have any cracks in my gunite.

View attachment 425568
That's a great feeling I'm sure! I noticed you're in a different climate than I am. Frankly, I'm suspicious the hot weather had a lot to do with it. The Shotcrete was light grey within hours of being applied. This is also probably why our builder brushed it off - he sees it all year long. 🤷‍♂️
 

JamesW

TFP Expert
Mar 2, 2011
31,511
Maybe ask the American Shotcrete Association for an opinion, or a referral to a local expert for review.


 
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AQUA~HOLICS

In The Industry
Jul 19, 2021
689
Southern California
The Shotcrete surface did not stay hydrated. Creating surface cracks and some very deep looking from the photos. Temperature over 100 along with winds pulled the moisture out due to not being continuously hydrated. This was why I asked what color the Shotcrete was once hydration process started, once the Shotcrete turns lighter in color on the surface it’s dried and no longer hydrated, it can be rehydrated but it’s not to get to that point to begin with. Continuous hydration is the key to a stronger shell, it slows the curing process down by absorbing the heat created during this process, some companies will lay rolls of carpet upside down on the floor to hold the moisture on the surface of the shell.
Only suggestion I have is to fill the bottom of the pool with water and see if you have excessive water loss, this will help explain the severity of the cracks and your concerns to the builder.
 

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sktn77a

Gold Supporter
May 16, 2010
2,453
Chapel Hill, NC
Those do look like surface shrinkage cracks but they are wider than I have seen before. I would get a written assurance that, should any issues arise because of this, the builder will warranty any repairs for the life of the pool (do you have a lifetime warranty on the gunite shell?)
 

jp4LSU

Well-known member
Jun 22, 2021
91
DFW, TX
Pool Size
16500
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I didn't have cracks in the initial gunite spray. However, they did come back re-contour the floor several inches and there were areas where the gunite seemed a little thin at the end of the spray. It was hot in DFW at the time and little cracks showed up even though I was watering regularly.
IMO, those cracks seem pretty wide. I would definitely get a structural guarantee and maybe a 3rd party to inspect it.
We have friends that had a crack show up stretching the length of the pool. We have clay soil here and it was determined that the soil had shifted and that pylons should 've been used. So they were in a litigation mess getting engineers inspecting it and making recommendations and negotiating with the PB. Its been a long mess for them.
I would call another gunite company and have them look at it.
 

MinerJason

Bronze Supporter
Jan 29, 2018
287
Tucson, AZ
This photo shows that the floor was made up of a lot of material that came off the walls while shaping. Am I correct in understanding this is rebound? And should have been removed?
Rebound is the stuff that bounces off during spraying and doesn't stick. Slightly different from material carved off during shaping, but they're equally bad, and yes should be removed to the extent that it's practical.
They did shoot the floor with the nozzle as well. But it would have been the pressurized material on top of what you see here.

I'm actually not sure how a crew could effectively remove all that extra cement; a good deal is carved off of the walls when shaping and would be really hard to shovel out from under the rebar.
The rebound under the rebar on the floor is very difficult to remove, but should be a small enough quantity and at the very base of the layer so isn't usually a big deal. The floor adjacent to the walls should have been shot before carving material off the walls, so that it would be easy to shovel the carved material off of the floor into buckets and removed. Large volumes of material shot onto the walls, then carved off the walls, then placed loosely on the floor, will create a much weaker area, which will also be more prone to shrinkage cracks.
You would really need a gunite expert to do an on-site evaluation to get a good answer.

They might be able to tell by a visual inspection or they might need to do some sort of testing.
I do QA/QC of shotcrete as part of my job (though nothing to do with pools). A visual inspection is not going to tell them much of anything.
View attachment 425569
Note that the person doing a compression test seems to be directly exposed to the sample being tested, which is super stupid since it will explode when it fails.
Lol. You can see the lexan door to protect the operator has been replaced with plywood, and is wide open instead of being closed. And to top it off he's not wearing any eye protection either. While concrete samples rarely "explode" during UCS testing, it's still very dumb. Small chips do occasionally fly off, and could easily hit the operator in the eye.
The foreman who ran the nozzle was very experienced. I'm unsure of the other laborers. None of them seemed new. But that's of course hard to really know.

I can't recall the psi, but the foreman was proud of their applications. It was in the high thousands, like 5-8k unless I'm mistaken.
The engineering sheet shows the floor to be 6" total. 3" under and over the rebar.
It didn't really 'sit', or at least it didn't seem to. The trucks all came on time and they were able to keep moving pretty steadily. But I also don't know what 'sitting' would constitute. Whether 5 minutes is too long or if we're talking hours.
Depends on the concrete mix and the ambient conditions, but in general you want it to be placed within 90 minutes of the first time water touches the dry cement. In the hot dry conditions you've described, it would ideally be less than that, maybe 60 minutes. There are some plasticizers and other admixes that can be used to extend this time slightly.
I'm not sure that's a variable in this case. It was a ready/site mix truck. Square bodied. The concrete was mixed real-time as it came out of the shoot wet, into the Shotcrete crews pump trailer.
The mix is always a huge variable, regardless of where it happens. Asking for the recipe/mix and records would still be a very good idea, especially since they used an admix they're not accustomed to using (flyash). In particular you will want to note the water to cement ratio, and also note any additional water that's added later, like in the shotcrete hopper or during shaping/troweling. A common mistake I've seen is adding a little extra water when it's really hot and dry out, thinking that it'll counteract the evaporation and rapid curing. Unfortunately adding more water only ends up reducing the end strength and increasing the quantity and severity of plastic shrinkage cracks. The proper solution to hot dry conditions is adding a plasticizer to increase the working time. I'm a bit confused why they prefer a mix with no fly ash though, especially given the location. Fly ash used in appropriate quantities slows down the cure time, which is exactly what you want when it's super hot and dry out, and it also has the benefit of increasing the strength of the final product.

In the end, I wouldn't lose too much sleep over it. The plaster is what makes a pool waterproof, not the concrete shell. Yes, it sucks that the quality of the shell doesn't appear to be ideal or up to par, but it's unlikely to cause any serious issues given the soil/bedrock situation you've described, and I'm guessing that the builder is providing some sort of warranty on the shell construction in the case that issues do come up. I would document everything as thoroughly as possible, collecting as much data and information to back up the claims that the shell quality may have been compromised, just in case something bad happens down the road. You may even try to use this to negotiate a stronger or longer warranty from your builder.

Keep watering the shell religiously for the next 10-14 days, get your builder to fill the wider cracks with hydraulic cement to help ease your mind about it, and don't worry about it too much. Worry more about your upcoming pool start up and what kind of pool toys and deck furniture to buy. Just my $0.02.
 
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JamesW

TFP Expert
Mar 2, 2011
31,511

Note that the testing is done at defined times after the initial placement.

Most likely, a core sample will need to be tested for strength if you are not confident in the product.
 
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bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Pool builder took a look at it today. He indicated he was relieved after walking the pool, and that it looked better in person than the images showed. Said it wasn't anything he hadn't seen before this time of year.

He said if they were larger, they could inject them with epoxy or some other mixture (wasn't hydraulic cement, he said they would only do that if he thought it went all the way through), but that he hesitates to do that because he always worries about layering, and the bonding between the plaster and the patch material. In other words, if the cracks are superficial of an inch or two, then more harm than good could come from patching/filling.

He reiterated some of the things mentioned by many of you on this thread: the plaster is the waterproofer, that our soil report indicates very stable support. He also suggested the cracks would arguably give the plaster a stronger bond and that they sometimes have to grind ridges into the plaster if it's too smooth. He also mentioned he would be more concerned if the cracks connected together for long stretches and/or if there were areas that were scalping/delaminating. None of which he saw. Several of them look much like the 'tearing' that occurs if you were to bend a fudge bar, which is why I was virtually certain they were superficial shrinkage cracks because of the wind and heat.

Currently the plan is to leave it as-is. He also reminded me he doesn't want to come back to fix it later, and he wouldn't move forward if he was at all concerned.
 
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bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Wanted to follow up with an update due to some interest via DM. Figured others would benefit as well.

Plaster was completed 8/1/22. Crew did a decent job; we do have some areas to smooth out. Thus far no cracks have telegraphed through the plaster, though of course we still have a couple weeks of active curing left and then many years of mild maturation that will prove the issue.

We are mostly placated for now and it seems we are in the clear. I'll see if I can upload a photo of the completed pool for those curious.
 
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bradgray

In The Industry
Jun 16, 2021
32
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Completed plastering. We went with blue quartz and standard white plaster. The quarts is hard to see in the photo; photo was taken at midday. With the pool full, it's a really calming lagoon blue. I don't have any photos of the pool full without half-dressed wife or kids getting in it. So I'll need to snag one separately at some point.
 

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