For many pool owners, the first sign of a problem is small, thin cracks that seem cosmetic or easy to dismiss. But over time, those cracks can grow, spread, and begin to affect the performance of the pool itself.
In some cases, the cause isn’t external at all, but a slow-moving chemical reaction happening inside the concrete. This is called alkali-silica reaction, or ASR, which is a chemical process that occurs inside the concrete, including shotcrete or gunite, and can lead to progressive cracking and long-term irreversible damage.
ASR is not just surface cracking. It is an internal reaction that can damage concrete from within.
What Is ASR?
ASR is the result of a specific combination of constituents and conditions coming together within the concrete.
Pool concrete is made up of several components: fine and coarse aggregates, such as sand and gravel, cement, and water. Chemical admixtures and supplementary cementitious materials may also be included.
Under the right circumstances, a reaction can occur between reactive aggregates, which contain reactive forms of silica, and alkalis in the pore solution, which primarily come from the cement.
When moisture is present, this reaction produces an expansive gel. As that gel absorbs water and expands, it creates internal pressure within the rigid concrete.
Eventually, that pressure can become strong enough to crack the concrete from within.
Why Pools Are Especially Vulnerable
ASR requires three key ingredients:
High internal moisture exposure
Aggregates containing reactive forms of silica
A high-alkali concrete environment
Unlike other structures with less moisture exposure, pool shells remain in a near-constant state of saturation. This sustained moisture allows the reaction to continue over time.
Additional factors, such as water chemistry, maintenance practices, and temperature, can influence how quickly the damage develops.
Because ASR develops gradually, damage may not become visible until years after construction.
In many cases, the pool may appear to perform normally for years before signs of distress emerge.
What Does ASR Damage Look Like?
One of the challenges with ASR identification is that concrete cracking in pool shells can have other possible causes.
Common visual indicators of late-stage ASR include:
- Fine, map-like or “spiderweb” cracking patterns
- Cracks that intersect in irregular, angular shapes
- Surface discoloration or deposits in some cases
- Progressive widening of cracks over time
In water-retaining structures like pools, these cracks can eventually lead to leakage, even if they begin as purely cosmetic concerns.
Cracking alone is not enough to diagnose ASR.
Many other factors, such as reinforcement issues, poor placement practices, curing conditions, or soil movement, can also cause pool shells to crack. Even though ASR exhibits unique distress signatures, this overlap is part of what makes ASR difficult to identify through visual observations alone.
How Is ASR Confirmed?
A visual examination can hint at whether ASR is likely, but petrographic examination is needed to confirm ASR damage.
This process involves extracting core samples from the pool shell and subjecting them to microscopic analyses. Petrography can evaluate the internal composition of the concrete, including siliceous aggregates, the presence of ASR gel, and the characteristic microcracking associated with the reaction.
By examining polished sections and other prepared specimens, the analyses can reveal whether the damage is driven by an internal chemical reaction or another mechanism entirely, which cannot be seen with the naked eye.
This level of evaluation helps move from observation to confirmation, especially when multiple causes of cracking may be possible.
Can ASR Be Avoided or Repaired?
Preventing ASR begins long before a pool is ever filled with water. The risk is largely influenced by decisions made during material selection for the concrete.
Prevention focuses on one or a combination of the following:
- Limiting the alkali content of the concrete
- Using low-alkali cement
- Incorporating supplementary cementitious materials, such as fly ash
- Reducing cement content
- Selecting non-reactive aggregates
- Using lithium-based admixtures
Once a pool is constructed, however, the options for repair are limited to trying to minimize moisture exposure to the concrete, which is generally not feasible or cost effective for most pools.
For ASR, prevention during material selection is far more effective than trying to repair the issue after construction.
