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
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
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
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
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
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
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,”
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
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.”