Beneath the water and under the plaster and tile finish lays the fundamental element in most swimming pools: concrete. It has been used to reinforce a pool’s structural shell for over 2,500 years; the Ancient Romans were the first to use it in their baths. But what is it about concrete that has kept it in use throughout history and into the modern day?
Concrete is made up of three basic components: water, aggregates and Portland cement. Water hydrates the cement and provides the exothermic chemical reaction which begins the hardening process. The amount of water in a concrete mix can vary, but in general, the lower the water/cement ratio, the stronger the resulting material. Contrary to popular belief, concrete and cement are not the same thing; cement is actually just a component of concrete. The cement and water form a paste that coats the aggregate and sand in the mix. That paste hardens and binds the aggregates together. The aggregates vary based on the mix design, but usually include small, medium, and large aggregates such as sand, gravel, and crushed stone.
Other than stone, the aggregate category also includes additives which alter the properties of the concrete. Some additives accelerate the concrete setting time, while others slow it down according to the designer’s needs. Two common additives are Fly Ash and ground granulated blast furnace (also known as “Slag.”) These common pozzolans react with calcium hydroxide to improve the workability of concrete, improve surface finish, and reduce the heat generated by setting concrete.
The American Concrete Institute (ACI) has extensive documentation on the use of both Fly Ash and Slag in concrete mixes. As these additives tend to be less expensive than cement (and may be used as cementitious replacements up to 50%), they can be tantalizing options with significant budgetary implications. Fly Ash and Slag may even be used interchangeably if one is not available.
It is important to note that while up to 50% of cementitious material may be substituted with Slag (per ACI standards), a rule of thumb in pool structural design is to cap the replacement at 15%, as the long-term performance of the concrete may be affected.
From purely a structural performance perspective, with an eye to serviceability and durability, the ACI does not have any issues the use of pozzolans. However they will affect the concrete, something contractors need to be aware of prior to construction. Additionally it is critical that the concrete batch plant is aware of the application of the concrete prior to determining the mix design.
How does all of this affect swimming pools? The use of pozzolans in swimming pool concrete has been used for many years. Regardless of the pool’s intended finish (tile, plaster, or paint), it is important to follow the manufacturer’s recommendations for finish application; such as ensuring the concrete is clean and free of debris.
As always, please contact a structural engineer for any structural design solution. For more information on swimming pool concrete or the application of a pool shell finish, please refer to following online resources:
Portland Cement Association – http://www.cement.org/index.asp
National Plaster Council – http://www.npconline.org/
The Tile Council of North America – http://www.tcnatile.com/
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