Alkali-Aggregate Reaction

Alkali-Aggregate Reaction

Definition

• Alkali-Aggregate Reaction (AAR): A chemical reaction in concrete between alkali hydroxides in the cement and reactive aggregates, often resulting in expansion and cracking of the concrete.

Types of AAR

1. Alkali-Silica Reaction (ASR):

   • Involves silica in aggregates reacting with alkali hydroxides.
   • Produces a gel that can absorb water and expand, leading to cracking.

2. Alkali-Carbonate Reaction (ACR):

   • Less common, involves carbonate aggregates.
   • Causes deleterious expansion but with different mechanisms from ASR.

Key Factors Influencing AAR

• Reactive Aggregates: Specific types of aggregates, especially those containing amorphous silica, can react with alkalis.
• Alkali Content: Higher alkali content in the cement increases the risk of AAR.
• Moisture Presence: Necessary for the reaction and subsequent expansion.
• Temperature: Higher temperatures can accelerate AAR.

Symptoms of AAR in Concrete

• Microcracking.
• Map cracking on surfaces.
• Misalignment of structural elements.
• White gel exudations on concrete surfaces (specific to ASR).

Preventive Measures

• Use of Non-Reactive Aggregates: Selecting aggregates that do not react with alkalis.
• Low-Alkali Cement: Using cement with low alkali content.
• Supplementary Cementitious Materials (SCMs): Incorporating materials like fly ash, slag, or silica fume can mitigate AAR.
• Chemical Admixtures: Lithium-based admixtures can be effective in reducing the risk of ASR.
• Proper Concrete Mix Design: Ensuring optimal ratios and ingredients to control alkali levels and moisture.

Remedial Measures

• Surface Treatments: Applying coatings or sealants to reduce moisture ingress.
• Structural Rehabilitation: Repairing or replacing affected concrete elements.
• Internal Humidity Control: Reducing internal humidity to slow down the reaction.

Significance

Understanding and mitigating AAR is crucial for:

• Durability: Extending the lifespan of concrete structures.
• Safety: Maintaining structural integrity.
• Cost-Effectiveness: Reducing repair and maintenance costs.