Pool Professionals
Understanding Freeze-Thaw Damage
Understanding Freeze-Thaw
Damage to materials undergoing freeze-thaw is mainly due to repeated cycles of expansion and contraction as water within the material fluctuates between a liquid (water) and a solid (ice) in cold temperatures. The expansion pressure creates internal strain that can damage almost any material. Ultimately, in regions of the country where snow and ice are commonplace during the fall and winter months, the potential for freeze-thaw damage can be almost unavoidable.
In years when the winter weather is mild, there may be no visible damage. However, in years when the winter weather is more severe and conditions are conducive to freezing and thawing, damage is inevitable.
In these colder regions of the country, repeated cycles of freeze-thaw can damage almost any material including tile, stone, brick, coping, and concrete.
Freeze-Thaw Damage
For cementitious interior finishes of a swimming pool, this damage occurs in stages. Initially, a white efflorescence (laitance) will form at the surface, generally within the waterline transition zone, just above the waterline. Continued cycles of freeze-thaw will cause the surface of a smooth finish to weaken or become rough, and exposed aggregate finishes can begin to lose aggregate at the surface. With continued freeze-thaw cycles, the cement binder will deteriorate. Ultimately, thin layers of the upper surface will begin to crack, lift, spall, or peel away.
Freeze-Thaw Susceptibility
Horizontal surfaces (steps, benches, sun ledges) are the highest-risk areas if they are within the waterline transition zone, or if water is allowed to pond on horizontal surfaces above the water level. Vertical surfaces (walls, step risers) are only susceptible within the waterline transition zone. The above photos are examples of freeze-thaw (expansion and contraction) damage that can occur in this zone.
Freeze/Thaw Damage - Water Drained 2" Below Top Step
Freeze/Thaw Damage - Aggregate Loss at Waterline
Freeze/Thaw Damage - Water Drained 1" Below Bench
Freeze-Thaw Mechanism
Water is a good insulator. So, the interior finish is not subject to freezing and thawing while fully submersed. Even a few inches of standing water over the interior finish, will typically maintain the material above freezing (generally around 39°F), even when the ambient air temperature is below freezing. However, materials that span the 'waterline transition zone' are especially vulnerable. In this environment, a portion of the material is partially under the water, and a portion is partially out of the water. Here, water is absorbed and wicks upward through capillary action (see Figure 1) as much as several inches.
Within this waterline transition zone, water saturates the pore spaces within the matrix of the material. Meaning, the water fills the pore spaces, leaving little to no space available to relieve or offset freeze-thaw expansion and contraction. The water confined in these pore spaces can expand and contract by approximately 9% in volume during each freeze-thaw cycle. This internal strain or internal pressure, far exceeds the physical strength or ability of the cement matrix to resist, resulting in a breaking apart of the cement binder matrix. This area is subject to the harshest freeze-thaw conditions and eventual damage.
Diagram Courtesy of Jonathan Dongell
FIGURE 1
Freeze-Thaw Mitigation
Freeze-thaw damage is not a material or workmanship defect. However, there are ways to lessen or slow the rate of freeze-thaw deterioration. While most materials are susceptible to freeze-thaw damage at some point, some materials are less susceptible than others. Optionally, there are additives that can strengthen or densify the cement binder. There are also additives or coatings that can add a degree of water resistivity or water repellency to the material. Ultimately, when repeated cycles of expansion and contraction exceed the material's physical internal stress limits, damage will occur.
Other Important Considerations
- Rain and snow cause the pool level to fluctuate, causing new areas of the finish to be within the waterline transition zone. This can allow new areas of the interior finish surface to become susceptible to freeze-thaw damage.
- Protect the higher-risk areas of the interior finish, specifically horizontal surfaces, by covering them with a minimum of 4" of water.
Communicate with the Homeowner
It is imperative that the homeowner be informed of the potential, and possibly unavoidable risk of freeze/thaw damage. This should include informing the homeowner that the chosen method and procedure for the winterization of their pool involves careful consideration to mitigate such damage. In this way, the homeowner can make an informed decision to move forward with the winterization of their pool.