AAR - a Q&A explainer

Alkali-Aggregate Reaction (AAR) and Structural Integrity

Alkali-Aggregate Reaction (AAR), notably Alkali-Silica Reaction (ASR), represents a serious risk to the durability and structural safety of concrete infrastructures. Gruner provides expert analysis, targeted interventions, and preventive strategies to safeguard the long-term performance and safety of dams, hydraulic structures, and other concrete infrastructures. Our integrated approach ensures proactive mitigation, protecting assets and extending their service life through detailed evaluation and specialized engineering techniques.

What is Alkali–Aggregate Reaction (AAR)?

Alkali-Aggregate Reaction (AAR) is a chemical reaction within concrete involving alkalis, primarily from cement, and certain reactive minerals in aggregates. The reaction produces an expansive gel, causing internal pressures, swelling, and cracking within concrete structures.

What are the main types of AAR?

The two main types are:

• Alkali-Silica Reaction (ASR): Caused by the reaction between alkalis and reactive forms of silica in aggregates, forming an expansive gel that leads to cracking.
• Alkali-Silicate Reaction: Less common, it involves specific silicate minerals such as phyllosilicates, but also generates expansive gel and structural distress.

 What structures are commonly affected by AAR?

AAR typically affects large, moisture-exposed structures such as dams, bridges, pavements, nuclear power plants, and marine structures due to their continuous exposure to moisture and varying environmental conditions.

Can you provide an example of a dam affected by AAR?

Yes, the Salanfe Dam (Switzerland) provides a well-documented example. Constructed in 1952, the Salanfe Dam, a gravity dam, began showing signs of concrete expansion and deformation by the early 1970s due to AAR.

• Symptoms observed included upstream deformation, significant cracking, and loss of structural integrity.
• Diagnostic methods involved detailed visual inspections, long-term deformation monitoring, and petrographic analysis confirming AAR gel presence.
• Mitigation measures included strategic "slot-cutting" (vertical concrete cuts) to relieve stress and accommodate swelling, effectively managing the structural stress and extending the dam's service life.

Slot cutting works on Salanfe dam

 How can engineers diagnose AAR effectively?

ASR diagnosis typically involves:

• Visual inspections: Identifying characteristic map-pattern cracking.
• Petrographic examination: Laboratory analysis of concrete cores confirming ASR gel and aggregate damage.
• Expansion monitoring: Installation of instrumentation (extensometers, crack gauges) to monitor ongoing deformation and expansion.

 What factors contribute to the occurrence of ASR?

Critical factors include:

• Reactive aggregate composition: Certain silica-rich aggregates, such as quartzite or chert.
• High alkali content in cement: Alkali-rich cement promotes the chemical reaction.
• Moisture availability: Continuous moisture exposure accelerates gel formation and expansion.

 What short-term management strategies are effective for existing AAR-affected structures?

Short-term strategies include:

• Reducing moisture ingress: Sealing cracks, applying waterproofing membranes, and improving drainage systems.
• Structural confinement: Reinforcing affected structures externally to limit expansive damage.

 Are there long-term solutions for AAR-damaged structures?

When AAR is advanced, structural rehabilitation or even reconstruction becomes necessary. Long-term strategies include:

• Selective replacement of severely affected concrete.
• Confinement or strengthening of critical areas.
• Slot-cutting: Strategic cutting of vertical slots (as in the Salanfe Dam) to relieve internal stresses caused by concrete expansion.

What preventive measures can engineers adopt to avoid AAR?

Prevention is critical for new structures and includes:

• Rigorous aggregate testing: Confirming aggregate reactivity through ASTM-standard tests (e.g., ASTM C1260, ASTM C1293).
• Material selection: Using low-alkali cement or supplementary cementitious materials (fly ash, silica fume, slag).
• Chemical admixtures: Lithium-based admixtures can  mitigate ASR potential.

 What lessons have been learned from real-world case studies?

Case studies, such as the Salanfe Dam, Alto Ceira Dam (Portugal), and Seabrook Nuclear Power Plant (USA), emphasize several critical lessons:

• Proactive monitoring is essential to detect and manage ASR early.
• Prevention through design (selecting non-reactive materials and low-alkali cements) is far more effective and economical than reactive interventions.
• Innovative repair techniques (like slot-cutting) can extend the life of structures even after significant deterioration.

Alkali aggregate reaction in the concrete around the penstocks at Inga II

Why is understanding AAR important for dam engineering?

For dam engineers, AAR represents a critical durability and safety issue. Understanding AAR enables proactive design decisions, informed structural maintenance, and timely interventions, safeguarding the structural integrity and operational reliability of dam infrastructure.

The case studies, such as Salanfe Dam, demonstrate the significance of addressing AAR proactively and comprehensively, ensuring long-term structural safety and performance.

Gruner's Expertise on Alkali Aggregate Reaction : Recommended Reading

Gruner has a long-standing commitment to advancing knowledge and best practices in the assessment and management of Alkali-Aggregate Reaction (AAR) in concrete structures, particularly in dams and hydraulic infrastructure. Our engineering experts have contributed extensively to industry knowledge through technical papers, case studies, and research publications.

These contributions focus on understanding the progression of AAR, effective diagnostic techniques, and long-term mitigation and rehabilitation strategies. From in-depth analyses of stress-relief methods, such as slot-cutting in large concrete structures, to predictive modeling using advanced chemo-mechanical simulations, Gruner’s technical insights are designed to support both industry professionals and infrastructure owners in safeguarding the longevity of critical assets.

If you are interested in exploring our research, we invite you to review the conference and technical papers authored by Gruner’s specialists. These publications provide detailed case studies, practical methodologies, and lessons learned from projects involving some of the most complex AAR challenges.