Cracking in shotcrete swimming pools can stem from multiple sources, including defects in planning, execution, construction, or the materials used. A critical issue to consider when examining cracks in shotcrete swimming pools is the Alkali-Silica Reaction (ASR).
What is Alkali-Silica Reaction (ASR)?
ASR is a chemical reaction in concrete between the alkaline cement paste and reactive silica found within certain aggregates. This reaction can cause the concrete to expand, crack, and ultimately lead to structural failure. For swimming pools, these failures can be severe, necessitating extensive repairs or even complete reconstruction. Understanding the roles and responsibilities of each party involved in the construction process is essential to mitigate the risks associated with ASR in concrete pools.
Alkali-Silica Reaction is a significant concern for the durability and longevity of concrete structures, including swimming pools constructed with shotcrete. It is my expert opinion that preventing ASR-related failures in swimming pool construction is the responsibility not of the pool contractors but of the ready-mix concrete suppliers and their aggregate suppliers.
Role of Pool Contractors in Managing ASR Risks
Pool contractors are responsible for the design and construction of swimming pools, relying on third-party materials to meet project specifications. Their standard of care involves ensuring the correct application and installation of materials, adherence to design specifications, and compliance with building codes and standards. However, pool contractors generally lack the specialized expertise required to analyze the chemical properties of aggregates used in concrete for ASR potential. This type of analysis falls outside the scope of their professional duties.
The Critical Role of Ready-Mix Concrete and Aggregate Suppliers
Ready-mix concrete suppliers and aggregate suppliers play a pivotal role in controlling the quality and composition of the concrete mix, including the prevention of ASR. They have direct access to and control over the selection of aggregates, the mix design, and the addition of admixtures that can mitigate ASR. These suppliers are expected to conduct or commission tests on aggregates to assess their reactivity before use in concrete mixtures, guided by standards such as ASTM C1260 and ASTM C1567.
During the Covid-19 pandemic, the supply of Fly Ash — commonly used as an admixture in concrete to combat ASR — was critically low. Consequently, many ready-mix companies dispatched shotcrete and concrete mixes without the usual Fly Ash content, making the shotcrete more susceptible to ASR.
Cracks Tell the Story: ASR in Action
The pattern and prevalence of cracks can indicate stress points, each telling its own story. With extensive experience in analyzing cracks resulting from various causes such as shrinkage, poor excavation compaction, expansive soils, and inadequate structural reinforcement, I can often rule these out when ASR is evident. ASR-related cracks typically appear in a crazing pattern, are numerous, and sometimes even circular, distinctly different from structural cracks alone.
In cases where a swimming pool and spa require complete removal due to ASR-related shotcrete failure, the investigation focuses on the source of the reactive aggregates. Given that the pool contractor relies on the ready-mix supplier for shotcrete material, the liability for ensuring the non-reactivity of aggregates falls squarely on the supplier. A failure to identify and mitigate the presence of reactive aggregates represents a lapse in the supplier’s duty to provide a product fit for its intended use.
It is my opinion that the pool in the photo below is suffering from ASR.