The year 2010 marked a century since the process of pneumatically applying concrete was developed. But there appears to be a debate brewing about this very fundamental part of building pools and spas.

Two camps disagree over the minimum strength of concrete, measured in pounds per square inch (or psi), at which pools should be shot. Many engineers and builders based in areas not subject to freeze/thaw conditions consider 2,500 psi to be acceptable in normal situations. And they’ll remind you that, in fact, the norm was even lower just a few years ago, at 2,000 psi.

But others have begun advocating a position held by the American Concrete Institute and the American Shotcrete Institute that states concrete for all structures exposed to water should be shot at a minimum of 4,000 psi. The Genesis 3 Design Group, for one, has taken the formal position that this strength level should be upheld on all shotcrete and gunite shells.

Both groups point to codes and standards to back their arguments. And both are convinced that no further discussion is needed, so water-tight are their positions. Here are their points of view.

Some consensus

There are some areas on which both parties agree.

Everyone allows that under harsh or unusual conditions, such as freeze/thaw cycles, bad soil, sloping properties and vanishing-edge walls, more than 2,500 psi is needed. Most codes, for instance, require that pools and spas in high-sulfate areas be shot with 4,500-psi concrete. Cold-weather regions are removed from this argument altogether. Commercial installations also often need higher strengths to meet code.

Additionally, there is general agreement that, when guniters and shotcreters do their jobs correctly, the end product will likely test higher than 2,500 psi anyway. This is because the high-velocity application itself compresses and densifies the material, resulting in higher cement ratios than those found in conventional concrete. In the case of wet-mix shotcrete, experts say it should end up close to 4,000 psi. However, there’s some disagreement regarding dry-mix shotcrete, or gunite. Some say that this process also will yield 4,000-psi strength, while others claim it can go as low as 3,000 psi with proper application.

The argument for 2,500 psi

One of the biggest points of contention is exactly which codes and standards apply to swimming pools.

In California, building officials typically mandate 2,500 psi shotcrete for pools. Some experts believe this requirement comes from codes regarding retaining walls and foundations. “Pools are designed as if they are a retaining wall,” says Ron Lacher, president of Pool Engineering in Anaheim, Calif. “They are designed to retain earth much the same as a basement wall. The same type of engineering is involved.”

The code specifies that retaining walls, basements and foundations that are not exposed to weather should be shot at 2,500 psi. When there is exposure to weather, the code requires different strengths for different conditions — 2,500 psi where the exposure is negligible (meaning the weather is milder), and 3,000 psi in cases of moderate or severe exposure.

“Could we say that a swimming pool is exposed to the weather? That is debatable, because it’s got the water-resistant coating on one side and it’s in-ground,” Lacher says.

Many building codes state that concrete must adhere to the parameters outlined in ACI-318, a code from the American Concrete Institute. That language stipulates that concrete strength reach at least 4,000 psi in containers needing low permeability because of exposure to water — and provides a key argument by those who advocate 4,000 psi and higher. However, this generally doesn’t apply to residential pools, Lacher says, because they receive a special exemption in several cases, most notably the I-Codes (International Building Code and International Residential Code).

Additionally, those who state that 2,500-psi concrete is sufficient as a starting point  suggest that the ACI code shouldn’t apply, because of the presence of plaster and waterproofing compounds in pools and spas. These factors, they say, mean that low permeability is not needed from the concrete.

Lacher also says that the low-permeability stipulation was not meant to prevent leaks, but rather to protect the concrete from chemicals. “Durability has to do with chemical attack,” Lacher says. “Water tightness is really not the issue.”

In California, the 2,500-psi specification is a recent change. The code used to stipulate 2,000 psi, and many professionals argue that there are hundreds of thousands of pools doing just fine with 2,000-psi concrete. Therefore, they say, it stands to reason that 2,500 psi would be sufficient. “There isn’t any argument in the building official arena about what the minimum concrete strength is,” Lacher says.

Furthermore, proponents say, the code is simply a guideline, while the engineer of record is there to provide true, situation-specific expertise. “Your strength requirement is determined by your designer,” explains Neil Anderson, president of Neil O. Anderson and Associates in Lodi, Calif. “As engineers, you can do anything if you can justify it. The codes are written as minimum standards and guidelines, and they’re written on a conservative format. But they’re listed as a guide: ‘In the lack of having engineering, here’s the standard.’”

Engineers who design pools with 2,500-psi concrete also point out that they are sparing their builder customers from a special inspection that’s required anytime the concrete is specified at 3,000 psi or higher. If 2,500 psi is sufficient, they say, this would amount to needless hassle and cost.

“If I’m doing a tunnel lining or a highway overpass, I can’t have flaws, period,” Anderson explains. “Therefore 4,000-psi minimum, preconstruction test panels with core tests, certification of all nozzle operators, full-time special inspection — the whole gamut is going to be done, and for good reason. But pools don’t need to be to that standard to be perfectly fine and functional.”

To specify that the shotcrete or gunite have a strength of at least 4,000 psi would mean adjusting the mix so that it contains more cement than its 2,500-psi counterpart. But some believe the mix, while important, is not as crucial as the application technique. “You can have 5,000-psi shotcrete and still have a pool that leaks like a sieve because of poor shotcrete application,” Lacher says.

While it’s true that properly applied shotcrete or gunite often reaches higher strengths than 2,500 psi anyway, some engineers prefer to specify the lower strength, thereby allowing a little room for imperfections in workmanship. “We specify 2,500 psi because that’s all that’s needed from a structural standpoint,” Anderson says. “We also specify it that way because we know that it’s not a perfect world and they’re not going to always get perfect workmanship. But it’s not an inferior product.”

Considering that pool walls must be at least 6 inches thick, to allow for 3-inch coverage on either side of the rebar, 2,500 psi is sufficient, Anderson states. “To get those minimum requirements of reinforcement coverage, 2,500 psi is all you need for a design,” he explains.

The argument for 4,000 psi

Those who believe that shotcrete must have a strength of at least 4,000 psi are just as adamant in their stance.

“There’s nothing that would support 2,500 psi,” says Charles Hanskat, a managing partner at Concrete Engineering Group in Northbrook, Ill., who currently serves on the ASA board and is a past ACI board member.

Both ACI and ASA advocate at least 4,000 psi in containers needing low permeability because of exposure to water. ACI stipulates this minimum in its own standard, while ASA endorses it. More importantly, members of this camp point out, the I-Codes reference the ACI code. In addition, ACI-350, a standard for environmental structures, lists the same minimum.

“[A pool] is not just a retaining wall, because you have water on the one side, and so it needs to be a liquid-containing structure,” Hanskat says.

The higher the pounds per square inch, the more compressed the concrete, and more compression means less permeability. This group doesn’t accept the argument that plastering a pool mitigates the need for low permeability. “Plaster is not totally impermeable,” Hanskat explains. “You have to have a concrete shell that’s water-tight to begin with, before you put the plaster on.”

They also bring up the issue of groundwater, stating that a lower-psi shotcrete provides inadequate protection for the back of the shell, which isn’t plastered.

This group allows that, yes, depending on the project, an engineer can design with less than 4,000 psi and still attain structural stability. But, they contend, that’s not the only reason to specify 4,000 psi.

“Mathematically I can show that you only need 2,000-psi on certain pools,” says David Peterson, president of San Diego-based Watershape Consulting and director of engineering and code compliance for Genesis 3. “But 4,000-psi is required for durability — things like permeability, crack control, shrinkage and how tough the structure is. It doesn’t necessarily have to do with structural requirements.”

In addition, a 4,000-psi mix will minimize rebound, the bane of guniting. This happens when small pieces of aggregate aren’t coated with enough cement paste, and fly out of the mix as it is being shot onto the shell. This rejected material can leave sand pockets behind, which lowers the quality of the finished product. Adding more cement to the mix means a reduction in rebound, these professionals say.

And moreover, it’s simply not enough to rely on the workmanship of the applicators, they add. “You’re not going to say, ‘OK, I’m going to design 2,500 psi and, oh yeah, it usually hits 4,000, but if it doesn’t, so what?’” Hanskat says. “If you’re going to have 4,000 psi, you’re going to have 4,000 psi — that’s the starting point.”

This is especially important, they add,  considering that standards allow for a 10-percent discrepancy between the stated and actual strength. So a pool designed with 2,500-psi concrete could actually end up as a 2,225-psi product.

If anything, Peterson says, the higher stipulation can keep everybody in check. “The thing I like about the 4,000-psi [requirement] is that, to get there, you have to do a lot of things right,” he says. “If the spec was only 2,500 psi, you could blow off a lot of these rules and shoot concrete any way you want — you could be really sloppy about it and probably get your 2,500 psi.”