Shotcrete sun shelf - problem?

[senewpool]

The concrete in my shotcrete pool was done two days ago. My pool has an 8 x 18 sun shelf that is going to have approx 9'' of water in it. The concrete crew shot the walls, including the small sun shelf walls, and put the trimmings in the bottom of the pool and then shot the floor over the trimmings with the exception of the sun shelf where the floor was just poured. They were using the same hose that was used to shoot everything else but I guess the compressor was turned off b/c the concrete was coming out of the hose with no pressure. Now the area between the poured concrete and the shot concrete (which also has some trimmings mixed in) around the edges of the pool shelf appears to be very weak. I can push out lose sandy feeling concrete with my finger. When I water this section the water is absorbed into the joint. We are going with a pebble tec finish. Will the pebble tec seal up this area? Is this going to cause problems with ground water being able to come up or leaks in the pool shell? Is this going to allow water to get at the rebar and cause rust? I've been watering the shell every couple of hours since the concrete setup.

The PB came out at the end of the pour and said it "looks good". He has been in business a long time and has a good reputation. The shell comes with a 30 year warranty. Am I worrying about nothing or should I press this issue?

Thanks for any advice. This is all new to me.

 

[ajw22]

What you call trimmings is called rebound. Rebound should be discarded and never used in the pool, although many crews use rebound around steps and to fill in areas. Rebound has a low cement to sand ratio and is weak in strength.

The 2006 International Building Code states in section 1913.6:

1913.6 Rebound. Any rebound or accumulated loose aggregate shall be removed from the surfaces to be covered prior to placing the initial or any succeeding layers of shotcrete. Rebound shall not be reused as aggregate.

You should point out the weak areas of the shell to the builder and he should remove any crumbling areas and fill it with hydraulic cement.

Use of Rebound in Swimming Pool Construction - Pool Engineering

Rebound is the material that ricochets off the application surface during the gunite operation. Rebound is typically made up of the larger and harder

www.pooleng.com

Rebound – A Cost Cutting Construction Method That Can Ruin Your Pool – Pool Contractor

www.poolcontractor.com

Horizontal cracks on gunite pool & spa steps- aka Rebound Cracks (with photos & video)

Rebound cracks are predominately horizontal cracks that occur 2-3” below the top of steps and love seats.

mikethepoolman.com

 

[senewpool]

Thanks for your reply. I'm a little confused on the terminology that I've read on the internet. The excess material I'm referring to was the extra shotcrete on the walls that was scraped off to make them a uniform thickness. I thought rebound was more an issue for gunite but since my pool was done with shotcrete the concrete was already mixed in the truck to a 5,000 psi formula. I questioned the guy on the concrete crew about it and he said the compression from the "shot" concrete will make it cure out to 7,000 psi but the extra concrete they were scraping off the walls would still be 5,000 psi concrete that "exceeds the standard" so it wasn't an issue. However, the crumbly looking bad concrete on the sun shelf must be the scraped off rebound/trimmings and it looks pretty bad. Based on what I'm reading and the links provided I'm concerned the integrity of the pool. In any event, the rest of the pool shell looks like I expected it to and there are no signs of cracks except on the sun shelf where the poured concrete meets the crappy looking stuff around the edges. I didn't watch the whole job but I'm guessing the other trimmings/rebound that were knocked back into the pool were shot over so at least the surface of the rest of the shell looks good but I'm concerned about if there are problems beneath the surface. The place on the sun shelf must be the only place where this inferior material didn't get shot over since they just poured the concrete in the center of the shelf. Also, the sun shelf center was done last so the concrete around the edges had probably already started to harden some. Other than pointing out the crumbling areas on the shelf that need obvious attention is there anything else you suggest? Would it be a good idea to pay an engineer to inspect it?

 

[ajw22]

Good luck finding an engineer who will take on the liability of giving you a professional opinion.

You will be told they shouldn’t have used rebound and you have some crumbling areas. You know that.

You need to work out with the builder what the remediation will be. If the builder gets difficult then you may need to find a “consultant” who is a concrete structure expert who can advise you on the remediation alternatives.

 

[cowboycasey]

5,000 PSI shotcrete will dry at 5,000 PSI shotcrete.. the rebound is still rebound and should not be used in the shelf... it will make it a weaker shotcrete, how much who knows, I do not think it is as bad as gunite though...

 

[sktn77a]

I do not think it is as bad as gunite though...

??? How so ???

 

[Jm13004]

Go to pooleng.com. They have some very good articles on shotcrete/gunite. Both are good "IF" applied correctly.

 

[cowboycasey]

??? How so ???

so from what I have read and seen is gunite can be 2000 PSI or 3000 PSI or 5000 PSI, it is all about the water and powder that the operator sets when they are shooting the gunite (it is mixed at the end).. Shotcrete is mixed at the plant and comes out exactly what is set at the plant, if you buy 5500 PSI shotcrete it is 5500 PSI when it is shot into the pool..

As far as rebound I am not even sure if it is better but the rebound of the shotcrete should still be 5500 PSI even in the rebound, it is just not nematically applied..

I am now out of my depth on gunite/shotcrete

 

[ajw22]

As far as rebound I am not even sure if it is better but the rebound of the shotcrete should still be 5500 PSI even in the rebound, it is just not nematically applied..

Shotcretes strength comes from being strongly accelerated by high air pressure in the nozzle, the concrete strikes the receiving surface with such force that it is compacted. This results in consolidation of all the concrete rather than the sometimes haphazard consolidation that can occur with cast-in-place concrete. Fully consolidated concrete provides greater strength and lower permeability than poorly consolidated concrete. The compacted, low-permeability concrete created by shotcreting is the ideal concrete for structures intended to hold water.

Rebound is not compacted and is poorly consolidated concrete that does not have the same strength and permeability.

 

[cowboycasey]

so 5500 PSI shotcrete gets stronger when it is applied, interesting.. I am always learning..

Thank you for the info

 

[ajw22]

so 5500 PSI shotcrete gets stronger when it is applied, interesting.. I am always learning..

Thank you for the info

The concrete coming out of the truck is not 5500 psi strong until it is properly applied and compacted with the shotcrete process.

Rebound has failed the compaction process.

 

[cowboycasey]

Ok, now I am really confused...

If I order 3000 psi concrete for my sidewalk it is 3000 psi and it is poured in... once it dries it should dry at 3000 psi or more depending on how long you keep it damp

if I order 5000 psi shotcrete would it not be the same.. if I poured it in instead of air blown in it would still be 5000 psi strength...

I know blowing it in makes it stronger, so do they start out with a lower 3000 psi strength mix and when blown in it gets to the stronger 5000 psi??

Or am I completely missing it and need to research more

 

[senewpool]

That is my confusion as well. If it is a 5,000 psi mix in the truck wouldn't it be 5,000 psi if poured. The lead guy on the concrete crew said the 5,000 psi will cure to 7,000 where it is shot b/c of the compression from shooting it and that scraping the extra off the walls into the pool floor was fine b/c it will still be 5,000 psi and exceed the minimum strength needed of 4,000 psi which made sense at the time. My PB said he has had samples of other pools built with the same mix and same concrete crew that always hammer test to at least 7,000 or 8,000 psi which supports what the concrete sub leader told me. For the stuff that was scraped off the walls it wasn't really "rebound" in the sense that it didn't bounce off anything, they just shot the wall a little wider than it needed to be and scraped the extra off within a few minutes after the guy with the nozzle moved to another section. It looked like the pressure from shooting the floor over the scraped off stuff got it all stuck together good... I don't know the terminology. The first thing they did was shoot the walls on my sunshelf and the last thing they did 5 hours later, after shooting the whole rest of the pool, was pour the concrete on the sunshelf floor (I don't know why this section wasn't shot like everything else) so I think the scraped off shotcrete around the base of the sun shelf walls had already started to setup and wasn't vibrated or otherwise compacted so I basically have a cold joint of poured concrete against trimmings that weren't compacted which is my problem looking area.