Portal: Pool

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.

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/