Shotcrete Science (w/ Charles Hanskat, P.E.)

The Science Behind Your Pool's Foundation: Understanding Shotcrete Technology

When most pool owners think about their swimming pool, they focus on crystal-clear water, perfect chemical balance, and maybe that new LED lighting system. But underneath it all lies a marvel of engineering that most people never consider: the concrete shell that holds everything together. In a fascinating episode of the Rule Your Pool podcast, host Eric Knight sat down with Charles Hanskat, P.E., Executive Director of the American Shotcrete Association, to uncover the science behind shotcrete—the high-velocity concrete placement method that forms the backbone of most concrete swimming pools.

As the Rule Your Pool podcast expands beyond its traditional focus on pool chemistry, this deep dive into construction fundamentals couldn't come at a better time. After all, as Knight points out from his years of experience, many pool problems stem not from water chemistry issues, but from the concrete itself.

Demystifying the Gunite vs. Shotcrete Debate

One of the most common questions in the pool industry centers around the difference between gunite and shotcrete. The confusion is understandable, but Hanskat clears it up with some fascinating history:

Shotcrete is a placement method for concrete. The pool industry uses the term gunite for what we in our technical documents call dry mix shotcrete. It comes from the original shotcrete, which was taking dry material, conveying it with air through a hose and adding water at the nozzle. And it was developed in 1907.

— Charles Hanskat, P.E., Rule Your Pool Podcast

The term "gunite" actually comes from a 1914 patent held by the Cement Gun Company, who trademarked everything that came out of their concrete "gun." What the industry calls gunite is technically "dry mix shotcrete," while the newer method developed in the 1950s and 60s is called "wet mix shotcrete."

The key differences are:

• Dry Mix (Gunite): Dry concrete materials are conveyed through a hose using compressed air, with water added at the nozzle to create concrete at the point of application
• Wet Mix Shotcrete: Pre-mixed concrete is delivered by ready-mix truck, pumped through hydraulic cylinders, and air is added at the nozzle for velocity

The Critical Role of High-Velocity Impact

What makes shotcrete special isn't just how it's mixed—it's how fast it hits the target. Hanskat reveals a surprising fact that illustrates the power behind every cubic foot of shotcrete:

If you've ever seen it going in place, you'll notice that it's coming out of that nozzle very fast. And honestly, it's about 60 to 80 miles per hour. So if you think about you're driving your car down a freeway, 70 miles an hour, you hit a concrete column. That is the impact that we get with every cubic foot of concrete that's being shot created.

— Charles Hanskat, P.E., Rule Your Pool Podcast

This high velocity serves multiple critical functions:

• Compaction: The impact forces out air bubbles and consolidates the concrete for maximum density
• Encasement: Ensures complete coverage around reinforcing steel and plumbing
• Adhesion: Creates strong bonds with existing surfaces and reinforcement
• Vertical Placement: Allows concrete to stick to vertical and even overhead surfaces

Quality Control: Wet Mix vs. Dry Mix Performance

Pool builders often wonder which method produces better results. According to Hanskat, both can deliver equivalent quality when executed properly, but they require different skill levels:

Both of them are equivalent when done with the proper materials, equipment, and the expertise of the shotcrete. The expertise of the shotcrete for dry mix is a little bit higher than required for wet mix. With dry mix, or gunite, the shotcrete is actually controlling the water into the concrete right at the nozzle.

— Charles Hanskat, P.E., Rule Your Pool Podcast

The dry mix method requires more skill because the nozzleman controls the water-to-cement ratio in real-time, essentially creating the concrete at the point of application. With wet mix, the concrete is already properly proportioned when it arrives on site.

For wet mix shotcrete, quality control focuses heavily on the ready-mix supplier maintaining proper water-to-cement ratios. Pool applications typically require lower ratios (around 0.42) compared to driveways or sidewalks because the concrete must stick to vertical surfaces without sagging.

The Engineering Behind Pool Concrete

Understanding why shotcrete works requires grasping some basic concrete science. Concrete excels in compression but performs poorly in tension, which is why reinforcing steel (rebar) is essential for pool walls that must resist water pressure and soil loads.

The deformed surface of rebar creates mechanical bonds with concrete, but only if the concrete fully encases each bar. This is where the nozzleman's expertise becomes crucial—they must understand how to direct high-velocity concrete around complex reinforcement layouts and plumbing penetrations.

One of the most encouraging aspects of properly placed shotcrete is its longevity:

Concrete typically has plenty of cement and part of that cement, that's what gives it strength. There's generally unhydrated cement that is in the concrete. And so if it's exposed to water, it will get stronger and stronger.

— Charles Hanskat, P.E., Rule Your Pool Podcast

This means that properly constructed pool shells actually improve over time, with unhydrated cement particles continuing to strengthen the structure through ongoing chemical reactions with water.

Practical Implications for Pool Builders and Owners

For pool builders, understanding shotcrete science translates into better decision-making around contractors, specifications, and quality control. Key considerations include:

Contractor Selection: Look for companies with proven track records in your chosen method (wet or dry mix). Dry mix requires more skilled nozzlemen, while wet mix depends on reliable concrete suppliers.

Equipment Quality: The size and maintenance of air compressors, pump types, and delivery systems all affect the critical 60-80 mph velocity needed for proper compaction.

Environmental Factors: Weather conditions, ambient temperature, and humidity all affect concrete placement and curing.

For pool owners, this knowledge helps explain why concrete problems often manifest years after construction. Issues like delamination, cracking, or premature deterioration frequently trace back to improper shotcrete placement—inadequate velocity, poor consolidation, or incorrect water-cement ratios.

The Future of Pool Construction

As Hanskat notes, shotcrete technology has over a century of development behind it, with applications ranging from 20-million-gallon water tanks to subway tunnels. This proven technology continues evolving with new materials, equipment improvements, and enhanced training programs.

The American Shotcrete Association's certification programs for nozzlemen and inspectors represent efforts to standardize and improve industry practices. For pool builders, these certifications offer valuable ways to verify contractor qualifications and ensure proper installation techniques.

Understanding the science behind shotcrete reveals why concrete pools, when properly constructed, can easily last 50 to 100 years. The Roman Pantheon still stands as testament to concrete's durability, and modern shotcrete techniques build upon millennia of concrete knowledge while adding the benefits of high-velocity placement and precise material control.

Whether you're building pools or simply want to understand what makes your backyard oasis possible, the science of shotcrete represents a fascinating intersection of chemistry, physics, and engineering—all working together to create structures that will outlast generations of swimmers.

Episode Chapters

• 00:00 Introduction and Guest Background
• 05:30 Defining Shotcrete vs. Gunite
• 12:45 The Importance of High Velocity
• 18:20 Wet Mix vs. Dry Mix Comparison
• 25:10 Water-Cement Ratio Control
• 32:40 Concrete Strength and Longevity