Swimming pools were first built using shotcrete in the 1930s. Shotcrete is especially suited for pools to accommodate its many curves, penetrations, and difficult details. Formwork is minimized and when shot in stable ground can be largely eliminated. The flexibility of placement that shotcrete affords allows every pool owner to have a uniquely shaped pool. Shotcrete pools are watertight, durable, attractive, and economical.

The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.

User agreement: The answers provided to submitted questions are intended for guidance in planning and executing shotcrete applications. This information is intended only for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations, and who will accept responsibility for the application of the material it contains. The American Shotcrete Association provides this information based on the best knowledge available to them and disclaims any and all responsibility for the information provided. The American Shotcrete Association will not be liable for any loss or damage arising therefrom.

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    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 no provisions for design of concrete pool shells. Many structural engineers use ACI 350 Code Requirements for Environmental Engineering Concrete Structures for pool structures, especially commercial pools. ACI 350 is based on ACI 318 Building Code Requirements for Structural Concrete but has modifications to provide a design for concrete structures that are normally exposed to water, and thus need more crack control for watertightness. ACI 350 also addresses requirements for durability for concrete exposed to liquid so that structures will be expected to be serviceable for at least 50 to 100 yrs. Some engineers feel ACI 350 is overkill for pools and may use ACI 318, or just use their past experience. Generally, use of ACI 350 will require a higher percentage of reinforcing steel, have closer steel spacing and somewhat reduced tension in bars to control cracking. ACI 350’s concrete cover provisions may also be somewhat higher than ACI 318, to provide more corrosion protection of reinforcing. ACI Committee 506 is developing a guide document for construction of shotcrete pools, but does not directly address design. ACI has recently authorized a new technical committee to develop a Code for Design of Pools and Watershapes. However, staffing the committee and then developing a consensus standardized document will take several years.

    Regarding the cove of the floor-wall joint, if you have a cove or the bond beam you can use the additional “d” distance for your vertical steel from external loads on the walls though the moment has to be carried in the thinner sections of the floor and wall adjacent to the cove. If you consider the bond beam is a stiffening element for the top of a straight wall acting as a panel between the ends in a rectangular plan pool you may be able to reduce some of the vertical bending stresses. However, that may be hard to quantify for a freeform pool without a more advanced analysis.

    Regarding reference books you may find David Billington’s Thin Shell Concrete Shells useful for analysis and design of concrete shells. It is an old book (1982 for 2nd Edition) and may be hard to find but may be helpful. Hopefully ACI’s new Code Committee for Pools will set the standard of practice in the pool industry and be specifically referenced by ISPSC.

    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. Basically, you are building a watertight structural concrete shell. Concrete should be a minimum 28-day compressive strength of 4000 psi (28 MPa). Most shotcrete uses a 3/8 in. (10 mm) maximum coarse aggregate size due to our small diameter delivery lines. The coarse aggregate (rock) being about half the weight of the sand content. We typically need a cement-rich paste so minimum cementitious content (cement, fly ash, slag) of 700 lb/yd3. We also need a low w/cm ratio for the ability to stack in the wall so maximum of 0.45, with most wet-mix concrete 0.42 or less. You may find our ASA Position Statements for our Pool and Recreational Committee helpful in providing more detail. Our current statements include:

    • Compressive Strength Values of Pool Shotcrete
    • Shotcrete Terminology
    • Sustainability of Shotcrete in the Pool Industry
    • Watertight Shotcrete for Swimming Pools
    • Monolithic Shotcrete for Swimming Pools (No Cold Joints)
    • Forming and Substrates in Pool Shotcrete
    • Curing of Shotcrete for Swimming Pools

    You can find the position statements freely available at shotcrete.org/products-services-information/resources/.

    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 placement techniques will produce a construction joint that acts monolithically and not be a “cold” joint. Shotcrete placed onto an existing concrete surface will provide an excellent bond IF the following conditions are met:

    1. Make sure the surface is roughened and clean.
      1. The amplitude the of roughness should be +/- 1/16th in. (1.6 mm) or more.
      2. If the surface was not roughened when it was shot be sure to have the contractor roughen it.
      3. A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
    2. Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
    3. Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
      1. The dry-mix shotcrete (Gunite) should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
      2. Be sure the shotcrete contractor is using an air compressor able to produce at least 385 CFM (11 m3/min) of air flow at 120 psi (0.83 MPa).
      3. Use of an ACI-certified shotcrete nozzleman is recommended.
    4. No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.

    This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

    The pool wall thickness is a part of the pool engineer’s design. Adding 6 to 8 in. to the existing 3 in. (75 mm) would be making the overall thickness 9 to 11 in. (225 to 275 mm). This would require more reinforcing steel, so you should verify with the pool designer the additional reinforcing steel requirements.

    You may also find our ASA Position Statements on Pool Construction helpful as a reference for proper pool shotcrete construction. They are freely available at: https://shotcrete.org/products-services-information/resources/

    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 structure using shotcrete placement should be designed using appropriate concrete design codes and standards. ASA does not have a published position on reinforcement of these types of pool elements though we are in the midst of developing one. As most concrete experiences drying shrinkage and associated cracking, most designers would include some level of reinforcement in these types of sections to control tensile stresses from shrinkage and temperature changes creating volume change in the concrete. ACI 350 (Code Requirements for Environmental Engineering Concrete Structures) is the ACI Code that deals with concrete liquid-containing structures with provisions specifically for providing liquid tightness and durability in continuously wet environments. The ACI 350 Code requires up to 0.5% reinforcement for shrinkage and temperature stresses. ACI 318 is the Concrete Code for Structures and requires 0.18% minimum reinforcement for shrinkage and temperature. Designers may choose to use the lower ACI 318 value since they consider the benches and steps not part of the water retaining pool shell. Other designers would consider the higher ACI 350 values as they are interested in better crack control. Overall, having a substantially unreinforced thickness of concrete would lead to more cracking that would be problematic in the pool.

    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 for concrete. Wet-mix uses premixed concrete while dry-mix simply adds water to the concrete materials at the nozzle. Both dry-mix and wet-mix with proper materials, equipment, and placement with produce quality concrete sections. The embedded reinforcement in the pool shell is designed to carry tensile loads. This may be bending stresses from structural loadings (settlement or water/backfill), or volume changes from drying shrinkage and temperature changes. Cutting a reinforcing bar would certainly negate its ability to carry loads in the vicinity of the cut and reduce the load carrying capacity until the development length allows the reinforcing bar to start carrying it full load.

    The layout of your cracked section isn’t clear from your description. An 8 to 10 mil (2 to 2.5 mm) crack is sizable in a water-containing structure. Fixing the existing crack with a reinforcing bar epoxied in place across the crack may be effective. However, that solution would only carry any additional load on the section (structural or volume change), as the existing loads have already created cracks. Thus, you should also address filling the crack as part of the solution. This may be with epoxy injection or swellable polyurethane grouts. You should consult with the pool design engineer for their recommendation on the best method for repair.

    We are reinforcing an exterior pool wall with shotcrete, and wanted to know what preparations need to take place for the shotcrete to adhere correctly, and what the minimum thickness needs to be? We also need to level out the floor up to 5 in. (125 mm) that will gradually go to zero to meet other side. Can shotcrete be used in this application?

    When shooting onto existing concrete sections the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. This will produce a construction joint that acts monolithically and not be a “cold” joint. Shotcrete placed onto an existing concrete surface will provide an excellent bond IF the following conditions are met:

    1. Make sure the surface is roughened and clean.
      1. The amplitude of roughness should be +/- 1/16th in. (1.6 mm) or more.
      2. If the surface was not roughened when it was chipped out, be sure to have the contractor roughen it.
      3. A high-pressure water blaster (5000 psi [35 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
    2. Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
    3. Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
      1. The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
    4. Be sure the shotcrete contractor is using an air compressor able to produce at least 185 ft3/min (5.2 m3/min) for wet-mix and 385 ft3/min (11 m3/min) for dry-mix (gunite) of air flow at 120 psi (0.8 MPa).
    5. Use of an ACI-certified shotcrete nozzleman is recommended.
    6. No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
    7. A minimum thickness of no less than ½ in. (13 mm) is recommended.

    This article on the excellent bond between shotcrete provides more detail: shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf:

    Regarding the additional floor thickness, though it may be shotcreted by an experienced nozzleman, it is difficult to properly shoot horizontal surfaces and control rebound and overspray from the shotcrete placement. We suggest that casting and vibrating for consolidation the horizontal sections is preferred to shotcreting. You should consider a bonding agent since the concrete is cast against the existing concrete floor without any impact velocity. We would also not recommend tapering down to 0 in. thickness. The feather edge will tend to be an area that may easily spall over time. Thus, we recommend cutting an ½ to ¾ in. (13 to 19 mm) deep shoulder so the concrete can have some thickness at its thinnest locations.

    I’m in the market for a saltwater pool and was wondering what the ASA recommended concrete compressive strength should be. I’ve read California mandates a 2500 psi (17 MPa) minimum, but some construction companies use 5000 psi (35 MPa). What is the ASA standard for a saltwater pool?

    Our ASA Pool Position Statement on Compressive (Strength) Values of Pool Shotcrete states pool and shotcrete contractors have a responsibility to provide a pool structure that not only meets certain design specifications, but also meets basic durability values expected with shotcrete applications. The American Shotcrete Association’s (ASA) Pool and Recreational Shotcrete Committee and ASA Board of Direction have reaffirmed a 4000-psi minimum for in-place compressive strength pool concrete.

    As a saltwater pool has a higher chloride content than fresh water you may want to consider using the requirements of ACI 350 (Code Requirements for Environmental Engineering Concrete Structures) where Table 4.2.2 – Requirements for Special Exposure Conditions has this requirement:

    “For corrosion protection of reinforcement in concrete exposed to chlorides in tanks containing brackish water and concrete exposed to deicing chemicals, seawater, or spray from seawater – maximum water-cementitious. Materials ratio, by weight is 0.40 with a minimum 28-day compressive strength of 5000 psi.”

    In summary the minimum 28-day compressive strength should be no less than 4000 psi, but for enhanced durability you may consider 5000 psi as required by ACI 350. You may find more guidance in our ASA Pool Position Statement “Watertight Shotcrete for Swimming Pools” at www.Shotcrete.org/Resources.

     

    Where Form and Pour Stops

    Though our primary business, Revolution Gunite, specializes in dry-mix process shotcrete, we also own a design firm called Waterforge, Inc. In this firm, we develop architectural conceptual plans and engineered construction drawings, mostly for swimming pools and other
    watershapes, including foundations and complicated site conditions.