Portal: Pool

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/.

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/

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.

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.

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.