Understanding the Shotcrete Pool Application

Shotcrete. Gunite. Gunite. Shotcrete. It's all the same right? Not so, according to Bill Drakeley, president of Drakeley Industries and Drakeley Pools, Bethlehem, Conn. "Gunite is a proprietary term. Or it used to be a proprietary term from the Cement Gun Company in the early 1900s. The gunite trade name now refers to the dry-mix process.

"It's still acceptable today, but legally, it's not correct. Some do gunite pools, but what they really mean is that they do concrete pools via the dry shotcrete process."

So what gives with the "intro to shotcrete" lesson, you may ask? AQUA spoke with Drakeley, the only certified American Concrete Institute examiner from the pool industry, to get an idea of what it'll take from the industry to deliver an even better product to clients across the country. And you won't be surprised about the answer: a better understanding of what the shotcrete process actually is and what it takes to deliver a quality product.

Wet Vs. Dry

"Shotcrete is an action; it's a verb; it's a process," emphasizes Drakeley. "It's not a physical product." Shotcrete is accomplished via the dry-mix or wet-mix process, and each are delivered using compressed air at a high velocity. So while both processes should deliver the same sheen, moisture content, and look when they hit the steel substrate, each has its own set of advantages.

During the dry-mix process, the material is delivered through a hose and hydration is added at the nozzle. The water-and-concrete mixture then gets blown onto the steel substrate to gain shape and texture. Because the water is added at the nozzle, the dry-mix process can be started or stopped at any point throughout shooting.

"The dry-mix process gives you control over the mixing and the consistency of the material at the nozzle, and lets one control that consistency to meet the conditions of the job," says Drakeley. He says that this process is best for intricate projects: moldings, bridge repair, infrastructure and jobs that have less volume and more detail.

The wet-mix process, on the other hand, is completed with a positive-displacement pump, and a ready-mix plant or delivery company delivers the material to the job in a wet condition. Unlike the dry-mix process, the wet concrete mix greets air at the nozzle before shooting.

Because the wet-mix process relies on a ready-mix plant, the variables are less, well, variable.

"Most of the batching is controlled at the plant," says Drakeley, "So you ask for a certain mix and you should be relatively sure, as long as you know your ready-mix company, that that's what you are going to get."

The importance of the velocity of the shot cannot be overstated. Density, encapsulation and permeability are all functions of how fast concrete is sprayed on to the forming and steel substrates. Compressor sizes and CFM (cubic feet per minute) all play huge roles in durability and strength. Drakeley recommends referring to ACI guidelines and sizing charts.

According to ACI standards, the wet-mix strength should vary from 4,000 psi to 8,000 psi, and the dry-mix strength should vary from 4,500 psi to 9,000 psi. While, technically, dry-mix shotcrete can put out a stronger product because of the water-to-cement ratios, it does have more variables that can go wrong in the process. That's why Drakeley prefers the wet process.

Higher Education

As with any profession, the more you understand about what it is you do, the better you'll be able to do it. With a variety of resources available to pool builders, ample opportunity exists to improve not only the quality of concrete pools built, but the way in which they are built, as well.

"ACI is the absolute law of the land and sets the minimum standards. Anyone who writes the specification on a shotcrete product in any industry - whether it is mining, tunneling, bridge repair, dock repair or pools - refers to the ACI. Period," Drakeley says. "That's where all the terminology and literature is written. That's where all the engineers and specifiers refer back to."

Drakeley recommends getting involved with the right organizations to learn as much as possible about all the variables involved with shotcrete. Join the American Shotcrete Association and the ACI. He also suggests taking advantage of the Genesis 3 Design School courses.

"We are trying to set the bar really, really high. The pool industry has a black eye, and for pretty good reasons. We have had little to no education on structural building and strengths that would correspond to end results specified by the ACI."

It's important to note the standards set by the ACI don't directly reference pool building, but rather the wet and dry shotcrete process. However, that doesn't leave room for anyone and everyone to interpret the process on their own.

"The ACI says nothing about pools," says Drakeley. "It says here's a process, here's how you apply it and here are the strengths you are supposed to get. So you take this technology and you apply it in your trade. But you can't change it just because you are doing it to a pool versus a bridge or a tunnel."

It's fair to note that an introductory shotcrete course doesn't exist in engineering school, and the majority of nozzlemen learn from on-the-job training. One guy learns as he holds the hose standing behind the nozzleman and then the next guy picks up the hose and the guy behind him learns, and the cycle continues.

"There should be a lot more guys who understand the process. Once you adopt the higher standards, your product becomes that much better," adds Drakeley.

Food For Thought

Improvement in any capacity is always a work in progress. And it's also an investment. Be it time or money, it'll take a commitment to make things better.

One of the most important changes (and an easy change to make) to improve the quality of concrete pools is to know who is shooting the pool.

"You want your company to understand the process, and you want to be sure to hire or sponsor a certified nozzleman," says Drakeley, noting that in his experience as an examiner, the knowledge crews have gained through certification has been invaluable.

"I hear, 'Man, I never knew that! I never knew the air velocity had to be so fast. I never knew the distance I stand from the wall where I'm shooting had to be within 2 to 6 feet. I never knew that my water to cement ratio had to be between 0.35 and 0.45. I never knew mix gradation.' Those changes and corrections are learned through certification. Little tidbits will make them that much better and put them that much above their competition."

Encouraging your nozzleman to get ACI certified is a simple step in moving toward a better end product, he adds.

Drakeley also suggests developing a personal relationship with the ready-mix plant or dry-mix supplier.

"Know where you are getting your aggregate from," says Drakeley. "Know all these people. Know the mix gradations. Get a sieve analysis and really know what it is you are putting into your pool. The biggest crime that is so easily fixed is not knowing what's going on.

"It just takes a little bit of reading, a little bit of knowledge, a little bit of questioning. You can get a hell of a lot smarter real quick if you can start studying some process problems."

A simple measure like curing can also mean the difference between a good and great shotcrete project. According to Drakeley, curing the pool prevents the mix water from leaving the concrete in an evaporated or liquid state, which ultimately weakens the final product.

When you cure for at least seven to 10 days - Drakeley notes a 28-day optimum curing time - the concrete gains strength, leaving the pool in a much better state.

"Once the concrete is up to the right strength, all the other issues - water tightness, surface bondablility, durability - are less of a concern," he says.

Drakeley also emphasizes safety rules during the shotcrete process. All too often, Drakeley says he sees crews wearing shorts and sneakers when they should be wearing proper eye protection, hard hats, ventilation masks, skin protection and work boots. He worries that people don't understand the ramifications of something - anything - going wrong on a job site.

"Safety is really important. It can reduce a lot of liability. We're in a litigious society. Lawyers aren't going away. You have to understand safety rules and you have to understand communication."

Drakeley is confident the tools and education available are just what's needed to take the shotcrete process to a more professional level.

"Bottom line is all involved (spec writers to installers) need to understand the process from beginning to end. Stop guessing at the intended strengths and proper techniques.

"Our trade is the highest-volume-user of the shotcrete process. We should not be the industry that has the most to learn about the process. The tools needed to raise our standards are now in place. We only need to act on them."