50+ cracks in day-old Shotcrete.

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Greetings again TFP!

My post today is filled with excitement and concern. I've done service and repair for pools for many, many years, and as of last week - the pool guy is finally getting a pool!

I will concede, we are very good at servicing and repairing the systems of our several hundred service clients, but I am NOT a builder. I have minimal experience here to fall back on with regard to the specialized areas within pool building. We do startups every week, but we've never built one.

I am using a reputable builder, one that's been in the business for decades. I've also cared for his clients pools for years with zero problems. It's why we chose him for our personal pool. We've been grateful to have someone we trust do our pool.

Fast forward to yesterday (Friday, 6/17/22), and we had the shell shot. I'm going to call it Shotcrete, because my understanding tells me that's what it is; the mix arrived in a compartmented truck (ready-mix/site-mix), and the concrete truck dumped the wet mix directly into a pump trailer, that only mixed with air.

Back to reputational background - the crew doing the Shotcrete has also been around for decades. They are actually one of only two Shotcrete options in our area, with the other option being his old partner. In short, this Shotcrete company has done nearly every pool in SoUtah and have a good rep. I chatted with the foreman before they shot it, and it was clear he was knowledgable and proud of their PSI numbers and track record. I was feeling very confident and grateful this entire build and even through that point.

That was, until the Shotcrete had sat for a few hours. And gnarly cracks began to occur before I even had a chance to wet it.

They started early, but we hit 100 degrees by noon. To make it worse, we had 25mph southern winds beating on us. This said, that's the norm for SoUtah (we are generally mirror Vegas weather within a few degrees) so this was not their first time in these circumstances. I sent pictures of the cracks to my builder, and he was not concerned in the least. So naturally, I went online to make sure I understood why I was over-worried. Except, the result was that I grew more concerned.

> One shoulder-angel tells me they are plastic-shrinkage cracks because of the brutal heat and wind curing the outside at a much faster rate. If so, I have a sense of relief that these aren't structural and shouldn't telegraph through the plaster.

> The other shoulder-angel tells me, it's because they said they had to use a mix with fly-ash due to powder shortages (which is not their preference, they prefer pure powder) and because they used the rebound. I did not see them pack stairs or walls with it, but it was definitely carved off and shoveled below the floor rebar, and then they shot on top of it. I'm worried this may mean the cracks are more than cosmetic, and it'll leak.

The images show pretty well, that 99% of the cracks are in the floor and the transition to the wall. The walls themselves are virtually crack-free.

I've attached 3 images below are of the Shotcrete a few hours after the crew left (the dry-looking images) and then 2 of what it looks like after 24 hours of intermittent wetting (the obviously wet ones).

Please let me know if I'm falling for ignorant fear on this. I just can't make heads or tails of the advice online for other folks who have posted pictures looking for advice. I'm trying to verify if I have to act more alarmed than my builder seems to be.
 

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ajw22

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Jul 21, 2013
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Northern NJ
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Do you have any pictures of the rebar before the shotcrete was shot?

Was all the rebar adequately spaced away from the dirt?

Structurally the question is if the encapsulation of the rebar is adequate and you have enough shell thickness.

Do any of the cracks look like they go through the entire thickness of the shell?
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Do you have any pictures of the rebar before the shotcrete was shot?

Was all the rebar adequately spaced away from the dirt?

Structurally the question is if the encapsulation of the rebar is adequate and you have enough shell thickness.

Do any of the cracks look like they go through the entire thickness of the shell?
It's really difficult to tell how deep the cracks go. This is why I'm concerned. I will say that we've filled the bottom of the pool with water from wetting it, and the cracks there do not seem to be draining that water.

Attached are images of the rebar. I can't believe it, but I didn't take any until the day we did shotcrete.

They did space all of the rebar with the concrete spacers. Floors and walls equally.
 

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bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Allen asked very good questions.

I would keep watering it. Did you set up a sprinkler to help you water it?
We did for a bit yesterday, but because we have a dam between our spa and pool side and its still been 25mph winds, it failed to do it evenly. So we've been soaking it every few hours. Except while we sleep.
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
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?

I just have a hard time understanding how a crew that's done this for 15 years would have a good reputation and so few failures if they habitually made these sorts of mistakes. So I don't know whether they are mistakes, or aren't.
 

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ajw22

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Jul 21, 2013
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I can see they had chairs under some of the rebar. But int he pics in some of the areas dirt is against the rebar. So we don;t know the rebar setup when the shotcrete was shot.

What is your PB's plans for the cracks? Will he go around and fill them with hydraulic cement?
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
I can see they had chairs under some of the rebar. But int he pics in some of the areas dirt is against the rebar. So we don;t know the rebar setup when the shotcrete was shot.

What is your PB's plans for the cracks? Will he go around and fill them with hydraulic cement?
There has not been any suggestion of any correction. I have not yet pushed the issue to that point.

I do expect he'll have a chance to see it tomorrow or Tues. And I can ask further questions. I'd like to be armed with information so I can make educated requests based on a reasonable alarm status.
 

ajw22

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Jul 21, 2013
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They should be shooting the shotcrete over and around the rebar on the floor to encapsulate it.

If they simply spread rebound on the floor to cover it then it is not proper shotcrete done under pressure.


The wet process includes first thoroughly mixing and introducing all ingredients, including water, into the shotcrete equipment. Then, wet material is pumped to the nozzle where compressed air is introduced to provide high velocity placing and consolidating the material onto the receiving surface.

20220617_100735-jpg.425522
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
They should be shooting the shotcrete over and around the rebar on the floor to encapsulate it.

If they simply spread rebound on the floor to cover it then it is not proper shotcrete done under pressure.


The wet process includes first thoroughly mixing and introducing all ingredients, including water, into the shotcrete equipment. Then, wet material is pumped to the nozzle where compressed air is introduced to provide high velocity placing and consolidating the material onto the receiving surface.

20220617_100735-jpg.425522

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.

So is this rebound the likely reason our floor is cracking, but the walls are not?

When you inspect the cracks, very few look like normal concrete cracks. They are obviously "wet tears" as the concrete was drying out in the sun. You'll notice the shearing on that first close up image with my thumb.
 

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ajw22

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Jul 21, 2013
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The good crews put rebound into buckets and carry it off. It is hard work so the lazy ones spread it around.

It sounds like the cracked concrete did not have good compaction from the shotcrete process.
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
I think at this point you are stuck with going forward. Patch the cracks with hydraulic cement.

No way to undo the shotcrete without starting over from the hole in the ground.
If it's indeed the rebound or just surface shrinkage, what do you feel the likelihood is that the cracks go all the way through? And will cause a future leak by telegraphing through the plaster?
 

ajw22

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Jul 21, 2013
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If it's indeed the rebound or just surface shrinkage, what do you feel the likelihood is that the cracks go all the way through? And will cause a future leak by telegraphing through the plaster?

I doubt it. It is the rebar that provides the structural stability to concrete.

You are not in a state where the ground moves regularly.
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
Gnarly cracks began before I had a chance to wet it, was the Shotcrete lighter in color when the hydration process started?
I'm not sure I fully understand the question. The shotcrete color was as shown in the first set of photos. Then when we wet it, it of course darkens. The shotcrete is not especially splotchy or discolored, at least not on the floor where the concerns lie.
 

JamesW

TFP Expert
Mar 2, 2011
30,746
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.

We can speculate, but that's about all we can do.
 

bradgray

In The Industry
Jun 16, 2021
25
St. George, UT
Pool Size
13500
Surface
Plaster
Chlorine
Liquid Chlorine
I doubt it. It is the rebar that provides the structural stability to concrete.

You are not in a state where the ground moves regularly.
Souther Utah does have a blue clay that is intensely problematic, but not in my immediate area. My official soil report was "7-9 feet of sand, silty gravelly with medium cobble underlain by basalt bedrock". We had maybe 4 feet of sand and then we hit pure cobble, and then large rocks surrounded by the cobble. In fact, the excavator indicated if we had done even a few feet deeper, they'd have to bring out the jackhammer attachment. I do feel confident the pool is not going to shift. The only worry I have is that these cracks are more than surface.

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.

We can speculate, but that's about all we can do.
I'm not sure one exists in our area. Our builder and the Shotcrete team are the experts here. I again, I don't mean to doubt either the builder or the Shotcrete crew's experience. The Shotcrete crew was very proud of their reputation, and they've done thousands of pools. In fact, he mentioned our pool was small for them - they usually do huge commercial pools and lazy rivers.

I just can't help but ask questions if I don't understand something when the result is different than I expected.
 

JamesW

TFP Expert
Mar 2, 2011
30,746
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.
 

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