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

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.

Slot cutting an AAR-affected dam: case study of the Salanfe dam

Article

October 2013 - International Journal on Hydropower and Dams 20(5):102

Authors

Droz, Patrice & Menouillard, Thomas & Vallotton, Olivier & Leroy, Raphaël. (2013). Slot cutting an AAR-affected dam: case study of the Salanfe dam. International Journal on Hydropower and Dams. 20. 102.

Abstract

The Salanfe dam is a concrete gravity structure located in the Swiss Alps, close to Martigny. The crest length can be divided into four unaligned straight sections: a central part which is 260.65 m long; the right wing, composed of two sections of 72.5 m and 76 m respectively; and, the left wing measuring 189.5 m. The total length at the crest is 598.65 m and the maxi-mum height above the foundations is 52 m. The foun-dation of the dam is located on sound gneissic rocks presenting excellent geomechanical properties.

Monitoring of an AAR affected dam

Conference paper

June 2015 - Conference: International Commission on Large Dams  -  At: Stavanger  Volume: Q99-R33

Author

Droz, Patrice. (2015). 

Abstract

The Salanfe dam is a concrete gravity dam located in the Swiss Alps. The crest length can be divided into unaligned 4 straight sections: a central part which is 260.65 m long, the right wing composed of 2 sections of respectively 72.5 m and 76 m, and the left wing measuring 189.5 m. The total length at the crest is 598.65 m; the maximum height above the foundations is 52 m. The dam was built in 1952. Since early 1970's, monitoring of the behavior of the dam based on pendulums started to present an irremediable deformation upstream, in particular at the most pronounced kink between the left wing and the central part. After in-depth studies along the years, investigations proved that the dam was affected by alkali-aggregate reaction (AAR) inducing swelling of the concrete mass. In order to assure the structural safety of the dam in reducing the effects of AAR and to release the consecutive compressive stress state, slot cutting of the dam has been performed in 2012 and 2013. Additional monitoring instruments specifically dedicated to capture the behavior of the dam during and after slot cutting have been installed.

Rehabilitation of the monitoring system of Inga 1 and 2 dams

Article

April 2019 - International Journal on Hydropower and Dams 2019(2)

Authors

Droz, Patrice & Wohnlich, Alexandre

Abstract

The dams at the Inga 1 and 2 schemes in the Democratic Republic of Congo have been showing movement towards downstream since they were impounded decades ago. The authors describe the various theories about the potential causes of the problem, and a project financed by the World Bank and managed by SNEL (Société Nationale d’Electricité), the owner of the schemes, which identified the causes and improve future monitoring and maintenance. It was found that both dams had experienced alkali aggregate reaction, among other issues. The monitoring instruments at the dams, installed at the dams in the 1970s and 1980s, were found to be obsolete, and needed to be replaced, and the quality of surveillance of the structures has been improved.

Swelling Dams in Switzerland

Conference Paper

April 2019 - Conference: Dam Sweling Concrete 2017 - At: Chambéry - France

Authors

Amberg, F & Bremen, R  & Droz, P & Leroy, R & Maier, J & Otto, B. (2019). Swelling Dams in Switzerland.

Abstract

In 2013 the Swiss Committee on Dams created the Working Group “Alkali-Aggregate Reactions” with the aim to investigate the current situation on the Swiss dams in relation with this problem. One of the results provided by the Working Group is presented in this paper. It consist in the identification of concrete dams potentially concerned by expansive phenomena based on the analysis of their behaviour. In particular, the presence of a non-reversible drift, both in horizontal and in vertical directions, is identified and characterized. The members of the Working Group are listed as author of the present paper. The entire work is also exposed in a specific publication of the Swiss Committee of Dams.

For more information, please visit our website’s Combatting Alkali Aggregate Reaction Services page to find contact details of our specialists.