By Kent Kimes, Kimes Engineering
Significant changes are coming to the way swimming pools and watershape structures are designed and constructed. One of the most notable shifts will be the introduction of a dedicated code, ACI 322, Concrete Pool and Watershape Code, which will establish reinforcement requirements that exceed those dictated solely by structural loading.
In Florida, for example, a typical 6-inch pool shell less than 5 to 6 feet deep can currently be engineered using #3 bars at 12 inches on center each way (OCEW). That level of reinforcement satisfies the structural loads for the vast majority of the more than 20,000 pools built annually in the state. This design is appropriate for standard in-ground concrete pools without unusual geometry or above-grade wall conditions. Deeper pools or shells with walls exposed above ground naturally require a different structural design, which your engineer incorporates into project-specific specifications. However, there are currently no design codes written specifically for swimming pools or recreational watershapes.
Engineers today rely on two general-purpose American Concrete Institute standards:
- ACI 318 – Structural Concrete
- ACI 350 – Environmental Engineering Concrete Structures
Neither was written expressly for pools. Recognizing this gap, the ACI has been developing ACI 322, the first code tailored specifically to swimming pools and watershape structures. The committee has been working for three years and is nearing completion of a draft suitable for an internal ballot.
Key Technical Principles Guiding ACI 322
Design Life Expectation: Pools and watershapes should be designed for a 50–100 year service life.
Inherent Watertightness: The concrete shell itself, not plasters, membranes, or applied waterproofing, must provide inherent watertightness.
The Critical Role of Shrinkage & Temperature (S&T) Steel
Committee discussions have focused heavily on the importance of shrinkage and temperature reinforcement, which affects durability and watertight performance over the long term. A peer-reviewed study, also used to develop S&T requirements in ACI 350, forms the technical basis for this conclusion.
For comparison, Current Florida Practice: A standard 6-inch shell with #3 @ 12″ OCEW provides 0.15% steel, which is common and generally sufficient for structural demands.
ACI 318: Provides a minimum S&T ratio of 0.18%, but only for structural slabs, not slabs-on-ground, making its relevance to pool shells unclear.
- In a 6” shell 0.18% means #3 @ 10” OCEW
- In a 8” shell 0.18% means #3 @ 7” OCEW, or #4 @ 12” OCEW
ACI 350: Written for watertight structures (e.g., tanks, reservoirs), requiring 0.25%–1.0% S&T steel depending on geometry and restraint conditions.
- In a 6″ shell: 0.25%, 0.50% ≈ #4 @ 6″ OCEW, 1.00% ≈ #5 @ 5.2″ OCEW
Likely ACI 322 Requirement: Minimum 0.25% S&T Steel
The current proposal expected to pass the committee ballot is a minimum S&T reinforcement ratio of 0.25%. This level is believed necessary to achieve long-term watertightness and durability.
For a 6-inch shell, this corresponds to approximately #3 @ 7″ OCEW, or #4 @ 12″ OCEW.
Thicker shells required for structural reasons may need even higher S&T reinforcement than dictated by load analysis alone.
Other Anticipated ACI 322 Provisions: Maximum bar spacing: 12 inches, Bundled bars prohibited, Minimum concrete strength: 4000 psi, Epoxy-coated and fiber-reinforced rebar prohibited, Defined concrete coverage around spa jet piping, Designs must be completed by a licensed design professional.
Timeline and Adoption
A public draft of ACI 322 is expected in about a year. After ACI finalizes and publishes the code, the Florida Building Commission is expected to adopt ACI 322 as a referenced standard in the next edition of the Florida Building Code.
