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Watertight – adjective. 1) constructed or fitted so tightly as to be impervious to water. (

Water-tightness is a necessary characteristic that every swimming pool should possess. Swimming pools shells that are not designed to be watertight vessels can be an owner’s worst nightmare. Several steps can be followed to ensure your swimming pool is “impervious to water”. Whether it’s adhering to the applicable standards, addressing all of the possible loading conditions, or testing for water-tightness throughout construction, all of these steps can save you an enormous amount of money and headache in the long run.

Starting early on in the design phase, it is crucial to ensure that the concrete pool shell design will adhere to all of the applicable standards. The majority of the United States uses the International Building Code (IBC) as the model code for the state. Several states adopt and modify the code, then issue their own state building code. A select few use the National Fire Protection Association NFPA 5000 code. Because both model codes adopt, through reference, American Concrete Institute ACI 318 “Building Code Requirements for Structural Concrete”, the ACI 318 code is often used as the standard building code for swimming pool design, relative to life-safety and load-carrying requirements. The ACI 318 code (1.1.4) states: “For special structures, such as …tanks…, provisions of this code shall govern.” However, the commentary (R1.1.4) states, “Some special structures involve unique design and construction problems not associated with this code…Detailed recommendations for design and construction of some special structures are given in the following ACI publications.”

ACI 318 goes on to reference ACI 350, “Environmental Engineering Concrete Structures.” ACI 350 was developed primarily to address special structures, such as pools, that do not fit into the general building category. ACI 350 addresses the durability and serviceability issues associated with liquid-containing structures, i.e. swimming pools. The primary serviceability issue with regard to swimming pools is water-tightness. When designing swimming pools, it is recommended that attention be given to both ACI 318 for strength and materials, and ACI 350 for serviceability and durability considerations. Using ACI 318 alone for pool design will result in the pool structure not meeting any water-tightness criteria.

While in the design process, it is important for the structural engineer on the project to study all of the different loading conditions that could affect the pool shell. These loading conditions, if not properly accounted for, could potentially have detrimental effects on the concrete shell. The following are the four most common cases where adverse loading conditions may arise:

  • Case 1: Construction, backfilling operations
  • Case 2: In-service, empty pool shell with soil pressure only
  • Case 3: In-service, pool shell full of water
  • Case 4: In-service, empty pool shell with buoyant or heave pressures

In order to ensure a watertight vessel, each of these cases need to be thoroughly studied and properly dealt with throughout the design and construction process.

During the construction phase of a project, water tightness tests shall be performed. Water tightness testing is specified using ACI 350.1-01, Tightness Testing of Environmental Engineering Concrete Structures. The type of testing used for swimming pools is “Hydrostatic Test for Open or Covered Tanks.” This is referred to as HST. The first test specified is HST-VIO. VIO is visual inspection only. This test is a general look at joints, piping, the back of walls, etc. to determine if water can actually be seen leaking from the structure. The second test specified is HST-100. This is a quantitative test actually measuring the water loss. The number 100 represents a 0.100% loss of water during testing. Measurements should be taken and recorded every 12 hours for 3 days. The allowable leakage rate for an unlined pool structure should not exceed 0.1 percent of the total water volume in a 24-hour period. (Example: 0.001 x 200,000 gallon pool = 200 gallons per 24 hour period.) This excludes the addition of evaporation loss.

The water tightness test applies to all pools, spas, surge tanks, and gutter systems. The test should be completed before the application of the pool finish, i.e. paint, plaster, or tile. The concrete structure should be allowed to set for a minimum of 28 days to allow for curing before the water tightness test is performed. Pools that do not pass the water tightness test, shall be made watertight and retested prior to the application of the pool finish.

If followed correctly, these few simple steps and guidelines can help reduce the risk of your pool shell losing water. Every pool is unique is their own way and may require additional attention. Having some basic knowledge, however, is key to minimizing the risk and prolonging the life of your pool.

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