Lessons Learned: The Importance of Properly Hydrating New Concrete Pool Shells

When it comes to concrete curing, cutting corners today often leads to serious problems tomorrow. Observing proper curing practices is a hallmark of professional craftsmanship—and a commitment to delivering a quality product that homeowners can enjoy worry-free. As Scott Cohen explains, relying on homeowners to water their newly placed concrete can be a prescription for failure.

By Scott Cohen

One of the most critical steps in building a durable and structurally sound swimming pool is ensuring that freshly applied shotcrete or gunite is properly hydrated during the curing process. When shotcrete or gunite is sprayed into the pool shell, it begins a chemical reaction called hydration, where cement reacts with water to develop its strength and integrity.

If this process is interrupted—such as when the surface dries too quickly—the concrete’s strength is compromised, leading to a weaker structure and an increased risk of shrinkage cracks or other defects.

As is true of a majority of pool builders, when constructing a swimming pool shell using shotcrete (or gunite), we apply concrete pneumatically at high velocity onto a shaped form, either built forming or the soil itself. Shotcrete requires careful attention to curing because its exposed surface area can dry out quickly—especially in hot or dry climates.

Hydration is the essential chemical reaction between the cement in the shotcrete and water. This reaction forms calcium silicate hydrate (C-S-H) gel, which binds the aggregate together and gives the pool shell its strength and durability. The hydration process begins immediately after application, but it takes time—days or even weeks—to reach the designed structural strength.

Proper curing requires the shotcrete shell to remain moist. If the surface dries out too quickly, the process halts prematurely, resulting in weak, brittle concrete that is prone to shrinkage cracks, reduced durability, and potential leaks—problems that are extremely costly to repair once a pool is finished.

CRITICAL CURING

Curing is the process of maintaining proper moisture and temperature to support continuous hydration. For shotcrete pool shells, this typically means watering the shell consistently—usually for a minimum of seven days after application (per ACI 308 and American Shotcrete Association recommendations). This is done using gentle, low-pressure water sprays to avoid damaging the fresh surface.

Proper curing practices are vital to the success of the pool shell because they ensure the shell reaches the compressive strength required to resist soil pressures, support the pool’s finishes, and handle daily use. A well-cured shell resists cracking, prevents leaks, and ensures a long-lasting pool structure that homeowners can enjoy safely and confidently.

Without it, even the best mix designs can yield a pool that’s destined for early failure.

Industry standards, including ACI 308 guidelines, emphasize that concrete shells must be kept moist for at least seven days following application. In hot or dry climates, this often requires multiple waterings per day using oscillating sprinklers or a garden hose with a spray nozzle to keep the surface damp without causing erosion. Starting this process within hours of application is critical.

STANDARD PRACTICE

According to ACI 308.1-98, “Standard Specification for Curing Concrete,” Section 1.1.6.1 states: “When testing is not performed to determine the curing period, cure concrete for at least seven days provided that the concrete surface temperature is at least 10°C (50°F).

This requirement underscores the importance of maintaining adequate moisture and temperature conditions to ensure proper hydration and strength development in concrete structures, including shotcrete applications.

The ACI language closely aligns with what we observe in the field. I’ve personally seen firsthand the disastrous results when this step is neglected. In one case, a home flipper had a pool shot in a house that sat vacant. The contractor told the homeowner—per their contract—that someone needed to water the shell, but no one lived at the property.

The shell sat dry for a month before the coping crew showed up. When core samples were tested, they measured an average of only 1,250 psi (pictured above) —half of what was required by engineering specs and a fraction of the 4,500-psi recommended by the American Shotcrete Association.

The shell fell apart and had to be completely demolished.

Even the most well-meaning homeowners should not be put in a position where they are in effect responsible for the pool shell’s proper hydration. Many are prone to forget or get too busy to turn their attention to watering their newly placed concrete. They might delegate the watering task to a kid or other family member who may not follow through.

Relying on a homeowner to water a pool shell is not only unreliable but also dangerous. Construction sites are full of tripping hazards: forming stakes, uneven terrain, and loose materials. I recently worked on an expert witness case where a senior homeowner slipped into an empty pool shell while trying to water it, sustaining severe head and back injuries. That is an outcome that we all should work to avoid.

YOUR RESPONSIBILITY

That’s why professional contractors today use automated watering systems—temporary sprinkler systems with timers—to ensure consistent hydration. These systems use low precipitation rate rotor-stream sprinkler heads that distribute water slowly, allowing it to soak in rather than run off. The setup is inexpensive: we connect the system to an actuator valve, which later connects to the pool’s autofill line, and a simple Home Depot sprinkler timer runs the schedule.

Temporary sprinkler systems like these ensure the pool gets properly hydrated. We use “low precipitation rate rotor-stream sprinkler heads” that put the water down slowly to ensure a chance to soak in rather than just run off.  They put water  down at 1/4 the rate of a regular spray sprinkler head.  We reuse the heads on multiple projects and PVC is cheap. The system is attached to an actuator valve that we use later to connect to the autofill line.  We connect a $35 Home Depot sprinkler timer to run the sprinkler program. It’s cheap insurance to ensure the home-owner stays away from the dangerous construction site and our pools are properly hydrated. We typically run these makeshift systems for 14 days.

This approach is cheap insurance: it costs maybe a couple hundred dollars in hose and sprinkler heads, which can be reused. More important it keeps homeowners safely away from the construction site while ensuring the concrete develops its designed strength. Our standard practice is to run these systems for at least 14 days, setting the stage for a strong, crack-resistant pool that lasts for decades.

If you don’t follow this simple advice, and leave wetting the pool to the homeowner, the results could be far worse than you might care to imagine. That’s why I advise all pool builders to set up their own sprinkler systems, and rest easy at night.  

Scott Cohen is a nationally respected expert witness in pool-construction defect cases. He is a “garden artisan” and president of Green Scene Landscaping & Pools, a watershape design and construction firm based in Chatsworth, CA. A widely published author and popular speaker, Cohen is known for his gardens that combine outdoor living with inspired artistic details.