FAQs Archive - American Shotcrete Association

Q: We are having a pool installed and would like to know if shotcrete should be placed when severe weather is forecasted for late in the day or the next day? Is it possible to cover the area after application to protect it?

Shotcrete is a method for placing concrete. Concrete has a set time that provides hardness of the in-place concrete. Final set may take anywhere from an hour to several hours depending on the concrete mixture and the weather. Hot weather has a faster set time than cold weather with the same concrete mixture. If the pool is shot in the morning, it may be hard enough to tolerate a rainstorm in the late afternoon. The concrete should certainly be set by the next day unless there is a very cold overnight temperature or a problem with retarder in the concrete mixture. Tarps could be placed over the vertical walls to protect them from a sudden rainstorm. The floor may be harder to protect as the rainwater wouldn’t necessarily drain off the floor. If concrete is exposed to rain and the surface isn’t washed off in any way, the concrete should be good. Similarly, if you are expecting freezing temperatures overnight, the fresh concrete must be protected from freezing. This may require using vented heaters and insulated blankets. The goal of the cold weather protection is to keep the surface temperature of the fresh concrete above 50°F (10°C). More details on cold weather protection can be found in ACI PRC-306-16 Guide to Cold Weather Concreting available directly from ACI’s bookstore (concrete.org/store.aspx).

Q: Please direct me to the proper guidelines for the hydration of shotcrete.

By hydration, I’m assuming you are referring to curing of freshly placed concrete. A minimum of 7 days of continuous wet curing is recommended. Continuous means the exposed concrete surface is wet the entire duration. Not just “watered” twice a day as has sometimes been suggested by contractors. Use of a spray-on curing membrane is an option if for some reason water curing is not practical. If you are looking specifically at swimming pools, this ASA Pool and Recreational Shotcrete Committee Position Statement “Curing of Shotcrete for Swimming Pools” provides more details: shotcrete.org/wp-content/uploads/2022/01/218216_ SCMSummer21_PS7v2.pdf Also, for a more detailed look at curing, the American Concrete Institute (ACI) document, ACI PRC-308-16 Guide to External Curing of Concrete is an excellent reference. You can find this at ACI’s bookstore (concrete.org/store.aspx). Finally, a good reference on the basics of concrete, from materials to placement and curing, is the Portland Cement Association’s “Design and Control of Concrete Mixtures, 17th Edition.” You can find this on the PCA website bookstore.

Q: We are a shotcrete company in Southern California, and I have a technical question about an issue that has recently come up. Is there any official ASA or ACI guide/spec that recommends or lays out the use of a certified nozzleman on a job that he was not “job site” approved for? For instance, a nozzleman who did not shoot a test panel was overseen by the approved nozzleman for that job site. I understand that the approved nozzleman can oversee another ACI-certified nozzleman on the job so that he can rest and oversee production. Seeing as the approved man is overseeing the nozzleman, is it the same as him shooting the wall himself?

It appears you are talking about a nozzleman who has not been pre-qualified by shooting a mockup panel on a specific project. There is no official ASA or ACI document that addresses this issue. However, this situation is similar to what ACI guidance provides regarding ACI-certified Shotcrete Nozzlemen and Nozzlemen-in-training. ACI’s Certification Program Policy for Shotcrete Nozzleman and Nozzleman-In-Training Section 1.04 states: “ACI Shotcrete Nozzleman-In-Training (NIT) certification is available to applicants who do not possess 500 hours of work experience but do possess a minimum of 25 hours of hand nozzling work experience in the process being sought (vertical orientation only). The examiner of record may require an ACI-certified nozzleman to be present during the performance exam; however, the ACI-certified nozzleman’s interaction with the examinee is strictly limited to the safety of the shooting operation.”

We are having a pool installed and would like to know if shotcrete should be placed when severe weather is forecasted for late in the day or the next day? Is it possible to cover the area after application to protect it?

Shotcrete is a method for placing concrete. Concrete has a [...]

By Zach Dake|2023-10-30T14:21:17+00:00October 30th, 2023|Technical Questions|

Please direct me to the proper guidelines for the hydration of shotcrete.

By hydration, I’m assuming you are referring to curing of [...]

By Zach Dake|2023-10-30T14:23:01+00:00October 30th, 2023|Technical Questions|

We are a shotcrete company in Southern California, and I have a technical question about an issue that has recently come up. Is there any official ASA or ACI guide/spec that recommends or lays out the use of a certified nozzleman on a job that he was not “job site” approved for? For instance, a nozzleman who did not shoot a test panel was overseen by the approved nozzleman for that job site. I understand that the approved nozzleman can oversee another ACI-certified nozzleman on the job so that he can rest and oversee production. Seeing as the approved man is overseeing the nozzleman, is it the same as him shooting the wall himself?

It appears you are talking about a nozzleman who has [...]

By Zach Dake|2023-10-30T14:21:47+00:00October 30th, 2023|Technical Questions|

I have used ASA’s Position Statements from the Pool and Recreational Committee and find them very useful. Are there any design guides or books on shotcrete pool design that are available? I am a structural engineer and tend to design pools as retaining walls, but I believe some of my designs could be “value engineered” to reduce rebar in the case of walls with a vertical curve (base of the wall is curved and not straight) and possibly the use of a bond beam at the top.

The current International Swimming Pool and Spa Code (ISPSC) has [...]

By Jessica Carvalho|2023-06-07T16:35:16+00:00June 7th, 2023|Technical Questions|

What would be the right concrete mixture for a swimming pool? I found out that a few concrete plants have different mixes, so I wanted to know what would be the correct one.

There is no special requirement for shotcrete placement in pools. [...]

By Jessica Carvalho|2023-06-07T16:34:18+00:00June 7th, 2023|Technical Questions|

A new gunite pool of ours was sprayed in an irregular fashion by a non-certified worker in the Bahamas. For the most part, the pool looks good, but one wall was measured at 3 in. The rest of the pool is 6 in. The rebar in the thin area was encapsulated which was good, but a couple of linear cracks in the wall formed even after ample wetting during the initial cure period. Six months have gone by. Our plan now is to pressure wash the cracked areas and add an additional 6 to 8 in. of gunite thickness which may not look too bad since the pool is a natural lagoon style pool. Staples with gunite over the top would be the other option. What is your opinion?

Properly prepared surfaces along with proper shotcrete materials, equipment and [...]

By Jessica Carvalho|2023-06-07T16:33:47+00:00June 7th, 2023|Technical Questions|

ACI 318-14 (Building Code Requirements for Structural Concrete) requires post-installed expansion anchors to meet the testing criteria of ACI 355.2-07 (Qualification of Post-installed Mechanical Anchors in Concrete). ACI 355.2 specifies certain anchor testing and evaluation requirements to verify suitable anchor performance and to determine other aspects (such as failure mode) to use in conjunction with Chapter 17 of ACI 318 when designing the post-installed expansion anchors. Anchor testing is required largely to be performed by an independent agency and normally is conducted in normal weight and/or light weight concrete that meet pertinent ACI, ASTM and other requirements. Some post-installed expansion anchor manufacturers (like Hilti) have not had their anchors tested per ACI 355.2 in shotcrete type concrete, only tested in normal weight and light weight concrete. As such, these anchor manufacturers typically do not publish/offer any permitted load ratings, installation torques or other design and installation requirements for their expansion bolts when used in shotcrete. Instead, they recommend site testing to determine anchor performance or that the responsible design engineer can make an engineering judgment on anchor acceptability, as appropriate, if site testing is not performed. Do you have knowledge of any expansion bolt manufacturers that have tested their products is typical shotcrete? If yes to #1 above, do you know if the testing was done per ACI 355.2 requirements?

Shotcrete is a placement method for concrete. With proper equipment [...]

By Jessica Carvalho|2023-06-07T16:32:55+00:00June 7th, 2023|Technical Questions|

We have a wet-mix shotcrete steel fiber overhead application progressing in our state. The question is about the use of a steel trowel finish, as opposed to say a magnesium or wood float finish. In the ASA Shotcrete Inspector seminar, it was stated that a steel trowel is less durable, reduces freeze-thaw resistance and shows cracking more proximately. As this particular application is overhead and, in a tunnel, there is not as much of a concern with water infiltration and the associated freeze-thaw exposure. We usually don’t allow steel trowels for flat work, due to deicing salts, but that concern wouldn’t apply here. My superintendent has asked me to reach out to you to see if you might have any further detailed advice on this type of application. Construction is wanting a smooth finish and looks do matter here as it is a high-profile project. If the DOT were to allow the steel trowel for finishing, what would be your concerns or suggestions to this approach?

Freeze-thaw deterioration is dependent on the concrete being saturated in [...]

By Jessica Carvalho|2023-06-07T16:31:59+00:00June 7th, 2023|Technical Questions|

I have a question regarding shotcrete pools. Does the ASA have a position on how to detail reinforcement at bulky elements that are shot interior to the main pool shell? This would typically involve stairs or large stoops. I notice a lot of contractors shoot these as unreinforced bulk elements, but this practice appears to promote cracking at the face of the pool shell. I’m only asking because I saw a few of these this past summer.

Shotcrete is a placement method for concrete. Thus, any concrete [...]

By Jessica Carvalho|2023-06-07T16:31:15+00:00June 7th, 2023|Technical Questions|

I was taught in engineering courses that conventional concrete should not be counted on to carry tensile stress. For steel reinforced concrete, the reinforcing bar is designed to carry all tensile loads. Although concrete obviously has some tensile strength, it is too low and prone to cracking failure to consider it in design. In fact, I believe you can assume it is cracked from the shrinkage during curing. Is gunite treated the same way? I have a pool that is developing a crack through an elevated wall/beam and down into the plaster to the bottom floor at the sun shelf. I witnessed the plumbers cutting some rebar in the beam to allow for PVC plumbing to water sheer (up at top of beam, just under the tile topping) and I worry this is the root cause along with settlement that put the top of the beam in tension. The rebar down low should be intact and I hope the crack width may stay minor down in the plaster. On top of the tiles beam where the maximum tensile stress would have been, the crack is fairly wide. The crack movement opened up a gap in the grout line between tiles of about 0.08 to 0.10 in. (2-2.5 mm). I think it was a real sin for them to have cut the rebar. If it is necessary to reinforce the tensile side to halt future movement, I would think cutting a slot or two in the gunite across the crack (say 12 in. [300 mm] each side. Up high just under the water sheer) and epoxy a rebar in the slots.

Shotcrete, both dry-mix (gunite) and wet-mix are a placement method [...]

By Jessica Carvalho|2023-06-07T16:30:31+00:00June 7th, 2023|Technical Questions|

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