Shotcrete - Using or discarding scraped material from walls

[klanel]

On my pool, the material scraped from the walls, steps, etc went into the floor and was used there. So no matter what is considered acceptable practice in the shotcrete/pool building industry, this is what I have. I also have hairline cracks running the length and width of my unfinished shell along the bottom but have not seen any as of yet on the walls.

I have seen things on TV, read some on this forum and this seems to be a controversal subject with strong feelings one way or another. I would like to know if there is a written industry standard of practice for the re-use of shotcrete material or if it should be discarded.

So here are my thoughts based on my limited understanding of this subject. With Gunite, since the dry material is mixed with water as it is applied, the sand particles bounce off the surface as applied and should be discarded due to it being weak material. With Shotcrete, the material is premixed which should eliminate the inconsistency of the material as it is applied. The material that was re-used came from shaping the walls, steps, and fell into the floor where it was used. I've read some here about the strength from compaction during the application process and that the psi strength is increased as a result. If the floor was poured instead of shot and thereby lower psi strength, would it be sufficient strength for it's use?

Can anyone point to an industry written standard of practice that states that shotcrete which is scraped off should be discarded? I can see the logic with gunite but with shotcrete since it is pre-mixed wondering what the industry standards say. Does anyone have that information?

 

[cowboycasey]

I have not found anything.. I am also going with shotcrete and not gunite for that exact reason... I know if the shotcrete is 5500 psi already mixed once it hits the floor it is still 5500 psi... It should be stronger in the walls and where it is shot out..

I plan on digging an x or + sign in the middle of my pool from the deep end to the shallow end and from side to side.. and all the "scraped off" parts will go into it and then shotcrete on top of that... my thinking is a backbone for the pool that is thicker and stronger..

There is no way for shotcrete to lose the 5500 psi, it will not magically be 3000 psi because it got scraped off the wall... Carl from little tile has done thousands of pools.. He always recommends shotcrete.. when he does tile on a shotcrete pool the nail only goes in 1/4 inch... in a gunite pool he can usually nail it in 4 inches with 1 or 2 hits...

 

[JamesW]

Rebound is concrete that does not stick to the wall. Loose concrete should be discarded and not used in the pool.

Proper placement is at velocity to create the correct compaction.

Rebound is the material that ricochets off the application surface during the gunite operation. Rebound is typically made up of the larger and harder particles (which tend to ricochet) and does not contain adequate cement, water or density to develop significant strength. Rebound, therefore, should not be used in any application where strength, hardness or durability is needed.

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.

This Section of the International Building Code basically reinforces the fact that “rebound” cannot be used as a construction material. In fact, the code does not even permit “rebound” to be used as aggregate.

Use of Rebound in Swimming Pool Construction

https://www.shotcrete.org/media/Archive/2016Sum_PoolRecCorner.pdf

 

[JamesW]

Shotcrete and gunite are different from regular pourable concrete because it has to be able to support itself.

Due to the different formulation, it needs to be applied with force and velocity to get the full strength.

Using rebound is not an acceptable practice.

 

[cowboycasey]

James, so you are saying if we order 5500 psi shotcrete it comes to site at 3000 psi and then when shot onto the structure it becomes 5500 psi?

 

[cowboycasey]

I know it seems like I am hard headed but I promise I am not.. I have just not wrapped my head around how this happens.. And trying to research how it happens or what mixes are used I get nothing... I guess it is a big secret of what goes into shotcrete and how it is mixed...

Thank you very much for trying to help us out..

I know for a fact it is well know not to use rebound in the pool structure and the fact that rebound is almost always used in the structure/floor anyway...

 

[cowboycasey]

I could not see the article you linked but I am finding many more on that site... Learning in process

This is one of the statements from a pool company that is well respected and does the right thing..


Clearwater Construction Group, Inc., was called in to both finish
demolition of the previous work and build the new pool

Fig. 1: Shooting the negative-edge trough while the finishers quickly work with the silica-fume-enhanced concrete and remove
rebound and trimmings from the project

 

[JamesW]

WHY REBOUND MATTERS

Rebound is mainly larger aggregates with little or no encapsulating paste. It is porous and does not possess structural properties. Essentially, it is just a pile of rocks and sand.

https://shotcrete.org/wp-content/uploads/2020/05/2017Sum_ContractorsCorner.pdf

_____________________________________________________________________________________

Misconception: Some shotcrete contractors feel shotcrete rebound and overspray is able to be left in the floor and coves, or shoveled up into the benches because it will be covered up by a layer of “good” shotcrete and won’t affect the structural integrity or watertightness of the pool shell.

Fact: When shotcrete impacts a surface, the material that bounces off is called “rebound.”

This rebound material is mostly aggregate (sand and rock) and much less paste than in the shotcrete mixture design.

Thus, rebound material is substantially weaker and more porous than the shotcrete mixture supplied.

When rebound is incorporated in any final shotcreted section, one is introducing a layer of weakness within the concrete section.

The section will then not act monolithically, as the designer intended; plus, it gives a weak plane in the shell that will encourage cracking and delaminations when the pool is exposed to wetting/drying and seasonal temperature variations.

Rebound and overspray must always be removed and not incorporated in any of the structural pool shell.

https://shotcrete.org/wp-content/uploads/2020/03/2016FallShotcreteEMag.pdf

_____________________________________________________________________________________

For strength and watertightness of the concrete pool shell, it is essential that all concrete is well placed with high velocity from the nozzle, providing thorough compaction of the material. This is equally important for both wetmix and dry-mix.

https://shotcrete.org/wp-content/uploads/2020/01/2018Sum_Guarino-Oakes.pdf

 

[JamesW]

James, so you are saying if we order 5500 psi shotcrete it comes to site at 3000 psi and then when shot onto the structure it becomes 5500 psi?

No. The psi of concrete is the design specification that the concrete is expected to achieve under ideal conditions, including the workmanship of the contractor doing the placement.

There are many ways that a contractor can diminish the quality of the final product.

Pourable concrete is fundamentally different in that there is enough paste and cement to fully coat all of the aggregate unless it is done incorrectly.

Shotcrete and gunite are much more stiff and plastic to allow the material to avoid sagging when used in walls.

The lack of fluidity makes placement much more complicated and you have much less leeway.

 

[AQUA~HOLICS]

Klanel, I only know of one expert in this field who can answer your question with a definitive answer, Ron Lacher of Pool Engineering Inc. Hope this helps, Ray.

 

[JamesW]

Klanel, I only know of one expert in this field who can answer your question with a definitive answer, Ron Lacher of Pool Engineering Inc. Hope this helps, Ray.

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

 

[klanel]

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

What I was inquiring about was not rebound but the material scraped away from shaping. In the code cited, it defines rebound as the material that bounces off during the application of gunite but later mentions shotcrete like they are interchangeable.

From what is posted so far, what makes the most sense to me is the shotcrete material being applied with less fluidity than say a poured product and being less forgiving in obtaining maximum strength. What I noticed, by the time they started finishing the bottom area, it looked to me as if the material was starting to dry. So while it was material that was shaved off and not 'rebound', I do think it would have been structurally stronger for them to have removed some of that material and applied fresher material. I would also say that the area where they last filled in was where the cracks seemed to originate from, But by that time the sun was getting higher and the day was getting hotter. If they had started my pool several months earlier when they said they would, it would have been cooler weather and less affected by the heat. If I were going to offer anyone suggestions about when to start pool construction, in my climate I'd suggest the Fall when the concrete would not be exposed to as much heat and also allow them plenty of time to get it done in time for the next season.

 

[JamesW]

What I was inquiring about was not rebound but the material scraped away from shaping.

That is called trimmings or shavings and it is also to be discarded.

All concrete must be placed with velocity for proper workmanship.

However, neither method is advised. It is especially difficult to properly remove ALL the shavings, rebound, and excess concrete from under the reinforcing steel in the floor/wall cove. For strength and watertightness of the concrete pool shell, it is essential that all concrete is well placed with high velocity from the nozzle, providing thorough compaction of the material. This is equally important for both wetmix and dry-mix.

https://shotcrete.org/wp-content/uploads/2020/04/2018SummerShotcreteEMag.pdf

https://shotcrete.org/wp-content/uploads/2020/03/2016WinterShotcreteEMag.pdf

 

[klanel]

Thanks for posting the trade info about the need for shotcrete trimmings to be discarded. Since the cracks occur just in the bottom of my shell, it makes sense to me that the likely reason may be due to not following the standards of best practices. I wonder how many residential builders discard the material?

The builder says he will fix the cracks but I think that means just applying a pebble finish over them. I can't help but think about things like drywall or tile in a home where cracks transfer up and through the top layers without the use of reinforcement material such as fiberglass. It seems that in the case of a concrete pool that epoxy applied under pressure to the cracks would be the proper way to treat the cracks to prevent cracking of the finish. I know it's been discussed before but has anyone had the hairline cracks pebble finished over to develop cracks?

 

[senewpool]

Thanks for posting the trade info about the need for shotcrete trimmings to be discarded. Since the cracks occur just in the bottom of my shell, it makes sense to me that the likely reason may be due to not following the standards of best practices. I wonder how many residential builders discard the material?

The builder says he will fix the cracks but I think that means just applying a pebble finish over them. I can't help but think about things like drywall or tile in a home where cracks transfer up and through the top layers without the use of reinforcement material such as fiberglass. It seems that in the case of a concrete pool that epoxy applied under pressure to the cracks would be the proper way to treat the cracks to prevent cracking of the finish. I know it's been discussed before but has anyone had the hairline cracks pebble finished over to develop cracks?

The shotcrete crew did the same thing when they built my pool. The only cracks I have are on the sun shelf where, I believe, a cold joint formed between shot concrete and trimmings and the middle of the shelf that was poured last instead of being shot. The builder is chipping out this section and filling it with hydraulic cement before plaster. I’m pleasantly surprised that I don’t have any cracks in the rest of the pool, especially where the floor meets the walls since this is where the trimmings ended up, and it has been 5 weeks.

 

[klanel]

The shotcrete crew did the same thing when they built my pool. The only cracks I have are on the sun shelf where, I believe, a cold joint formed between shot concrete and trimmings and the middle of the shelf that was poured last instead of being shot. The builder is chipping out this section and filling it with hydraulic cement before plaster. I’m pleasantly surprised that I don’t have any cracks in the rest of the pool, especially where the floor meets the walls since this is where the trimmings ended up, and it has been 5 weeks.

It's probably standard (or substandard) practice with residential pool construction but that doesn't make it right. At least they are taking care of yours. What area in GA are you in?

 

[JoyfulNoise]

Surface imperfections are expected in the shotcrete process (gunite is technically a different process but the terms get thrown around interchangeably). Hairline cracks at the surface are unlikely to extend into the depth of the shotcrete shell. There is ample enough rebar used to create the tensile strength needed for cracks to not propagate. The structure is likely fine and the plaster surface coating will smooth out all visual issues. It is the plaster material layer that is the hydraulic sealing material anyway.

Yes, using rebound or trimmings is very poor practice and you should make your pool builder aware that this is what the shotcrete subcontractor was doing. I would inform the PB in writing (e-mail) and ask that they fully inspect the surface to ensure that the shell is properly constructed. This has two purposes - it creates a paper-trail for the future should there be an issue with the shell cracking in the future (pretty rare) and it puts the onus on the pool builder to inspect the work.

 

[darkrider]

As others have pointed out, the 5500psi rating for shotcrete is only when it is applied and cured under the correct conditions. While pre-mixed shotcrete has less rebound in general, rebound is still rebound. I certainly wouldn't want them mixing it back into the floor! Here is a good read on proper shotcrete application: You are being redirected....

 

[.ben]

Has anyone successfully incorporated language in their contract along the lines of "Gunite/shotcrete rebound and trimmings/shavings to be discarded and not used at any location"?

 

[JoyfulNoise]

What this is is a failure on the part of the PB not being involved enough in the job site. It's very rare to find integrated pool builders (PB's that own every aspect of the job) as many PB's simply are glorified project managers that maintain a schedule, hire subs and transfer payments. A shotcrete sub-contractor is going to hire mostly unskilled labor plus one or two guys that know how to correctly operate the concrete pump and hose. You might be lucky if the shotcrete business owner is there watching his guys but you're likely just getting a foreman and crew of mostly inexperienced day-laborers. So this is why it is critical for a PB to be onsite so that he can insist on details like not using rebound or trimmings. Even though we can sit around all day and discuss the "why's" about not using trimmings, the sub on the ground is just going to do whatever needs to be done to get 2-3 jobs knocked out in a day. So they are not going to really care unless there is the guy writing the check (ie, the PB) there to yell at them.

Sorry this happened to you, but I really do think it will be all right. It's always sad to see substandard work done especially when you're laying out so much money but the only thing you can do now is hold the PB accountable and get it all in writing.