By Jeff Freeman
For the past several years, a number of people in the swimming pool industry have debated whether saltwater has a detrimental effect on concrete and, in some cases, on stone decks.
As one who has worked extensively with saltwater chlorination systems, I've studied this issue in depth in an effort to understand the pertinent factors and position myself to advise pool builders and consumers about appropriate preventive measures.
What I've found is disturbing: All too often, I encounter self-interested, so-called "authorities" in the industry who are badly mischaracterizing the situation and, I believe, are deliberately misinforming people in a cynical effort to support their own market agendas.
Let's start with the key false assumption: Many make the argument that when water containing dissolved salt splashes onto a deck, the concrete absorbs the water, and the accumulation of salt crystals in the concrete matrix later expand and cause damage. This idea is based solely on circumstantial observation that decks surrounding pools using saltwater systems are demonstrating some sort of failure, whether it be cracking, delamination, efflorescence or alkali-silica reactivity (ASR).
It's my position — and concrete engineers and people with advanced degrees agree — that this line of argument is absolutely false!
Standing the Test of Time
First, consider the historic fact that the Romans and other ancient civilizations used seawater to mix concrete and built structures that have stood for millennia. Second, consider the number of massive concrete applications that come in regular contact with seawater — including seawalls, the piles supporting piers, foundational structures for bridges and the support structures of oil-drilling platforms. These often stand for decades or longer under constant exposure to saltwater with no visible or measurable structural damage caused by the intrusion of dissolved salt.
With these reported deck failures, I believe that those who have installed the decks are simply seeking a way out of the responsibility — or that they just don't know what is really happening and have found a convenient scapegoat. Again, their position crumbles when you look at recent history.
For years, in fact, a vast majority of swimming pools were sanitized using sodium hypochlorite (bleach). Everyone who knows anything about basic pool-water chemistry knows that the use of bleach results in a buildup of dissolved salts.
Although it's an over-simplification, the processes of ridding water of pathogens and of oxidizing organic compounds converts sodium hypochlorite to sodium chloride — that is, salt. If you track the buildup of salt in pools treated with bleach, you'll see that salt accumulates rapidly and in many cases far surpasses salt levels found in pools using saltwater chlorination, which happen to operate at about one-tenth the salinity of seawater.
This salty, bleach-treated water splashed on decks across the country for years with no repercussions. But now all of a sudden when that same compound is present in a pool using saltwater chlorination, it's a problem? The argument just doesn't hold up.
Also, consider that since the 1960s, people salted the tops of freshly poured decks with rock salt to create popular pitted surfaces. The salt creates spaces in the concrete surfaces and is subsequently rinsed away with no further effects.
It's the Moisture, Not the Salt
Another way to look at this issue is to consider that in some areas, the soil beneath the concrete has an extremely high salt content, often hundreds of times higher than the levels found in salt-chlorinated pool water. In those extreme conditions of constant exposure to moisture in salty soils, high levels of salt can indeed wick into the concrete.
Even in these situations, however, it's not the salt in the soil that's the problem, but rather the moisture. If the moisture can't escape the concrete as the result of use of a topical coating, then you indeed run into problems.
The fix is simple: Decks should be installed with a moisture barrier beneath them, even something as simple as a layer of plastic sheeting. Moisture won't be able to touch the concrete, and this set of potential problems will be eliminated.
Some would add that creating high-density concrete is a big help as well, the thought being that concrete with greater compression strength not only withstands greater structural stresses but is also less permeable. Additives such as fly ash and lithium nitrate are other avenues for decreasing permeability — and have commonly been used in building the concrete hulls of ocean-going vessels for that reason.
I would never argue against higher-density concrete, but as water (whether salty or not) can still penetrate the concrete and can eventually damage structural steel and cause other harm, I would always recommend the use of a moisture barrier in building decks.
The other part of this discussion has involved failures of stone decks surrounding pools, again with the blame being placed on the water's salt content. In this case, it's a simple matter of people not taking the material they're working with into full account.
Stone is a natural material and comes with varying levels of density. Some of it, such as many types of sedimentary rock, will be highly susceptible to water damage. Even the most resistant of species will eventually fall prey to the effects of water, which acts as the "universal solvent."
We all know that water cut the Grand Canyon, eroding and dissolving ancient igneous rock. In the context of stone used around pools, in decking or on the waterline, it should be treated as a raw building material and should be waterproofed to keep moisture from penetrating the surface.
Again, this has absolutely nothing to do with saltwater used in pools for the purpose of chlorine generation; rather, it is simply a basic recommendation that every builder should follow.
Covering Their Tracks
Through the years, some have suggested that this vendetta against saltwater pools has been conducted at the behest of chlorine manufacturers stung by saltwater chlorination's progress in the marketplace.
As tempting as it might be to indulge in conspiracy theories, I've never seen any direct evidence of such a campaign. Frankly, given that chlorine used to sanitize pools is such a small percentage of these large chemical companies' business, I doubt they would be highly motivated to debunk saltwater chlorination systems.
No, I believe it's much simpler than that: Saltwater chlorination is still relatively new to the market. Given a lack of understanding of the basic chemistry and material issues, some have seized a bogus opportunity to cover their own errant tracks. The instant we apply the most basic levels of science and common sense, those arguments immediately wash away.
Jeff Freeman is founder and president of Fluid Logics, a water-treatment and hydraulics consulting/manufacturing firm in Upland, Calif. He has designed hydraulic systems for a variety of complex commercial and residential projects and is a widely recognized industry authority on water chemistry.