In this article, we discuss Alkali-Silica Reaction (ASR) in concrete, which is a chemical reaction between alkalis in Portland cement and reactive forms of silica-containing minerals in aggregates. ASR is a long-term deterioration process, often manifesting 20 to 50 years after construction, but sometimes as early as 2 years. Here is a summary of the article. You can also download the full article.
ALKALI-SILICA REACTION (ASR) HAS VARIOUS EFFECTS:
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Effects on Serviceability: ASR typically leads to concrete cracking due to gel formation and internal expansion when exposed to water. This expansion exceeds the concrete’s tensile capacity, resulting in unsightly and sometimes alarming cracks, as well as features like gel discharge and staining.
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Effects on Structural Integrity and Performance: Map cracking from ASR can significantly affect concrete’s tensile and compressive strength, leading to bending and deflection issues. It can also create pathways for other deterioration mechanisms like corrosion, further compromising structural integrity.
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Cost of Infrastructure Management: ASR-induced damage can prevent structures from reaching their full design life, incurring significant costs for rehabilitation and replacement. Precautionary measures for preventing ASR in new structures also add to expenses. Additionally, ASR damage often co-occurs with other damage mechanisms, like corrosion, complicating maintenance and repair costs.
ASR requires specific conditions to occur, including a sufficient alkali content in the concrete pore solution, aggregates with reactive minerals, and the right environmental conditions. The reaction between alkalis and reactive silica forms produces an alkali-silica gel that swells and exerts tensile stress, leading to concrete cracking when it surpasses the concrete’s tensile strength.
CONDITIONS REQUIRED FOR ASR:
Adequate Alkali Content in Concrete Pore Solution: Alkalis in cement are measured in terms of the sodium oxide equivalent (%Na2O-eq), often expressed as a percentage by mass of cement or in kilograms per cubic meter of concrete (Na2O-eq/m3). Alkalis primarily come from cement, existing as neutral sulfates such as Na2SO4, K2SO4, or mixed salts (Na,K)2KO4, making the concrete pore solution highly alkaline with a pH of 13-14. Alkalis can also come from external sources like mixing water, sea water, or alkali-containing aggregates. High-alkali cement, with Na2O-eq greater than 0.6%, may not be suitable when used with reactive aggregates.
Reactive Aggregates: ASR occurs when aggregates contain sufficient alkali-reactive constituents and are dense. Porous aggregates can accommodate the gel formed during the ASR reaction. The expansiveness depends on the content of reactive constituents and aggregate reactivity. Various minerals and rocks have been identified as potentially reactive with alkalis, and caution is necessary when using them.
Environmental Conditions: ASR requires specific environmental conditions, primarily temperature and moisture. ASR expands more rapidly at higher temperatures, and the expansion rate doubles for every 10°C increase in mean annual ambient temperature. Moisture is essential for ASR, and it typically occurs when the internal relative humidity is above 85%.
ASR IN NAMIBIA:
The article highlights cases of ASR in Namibia, where granite and gneiss aggregates have been associated with ASR-related damage to structures, particularly in coastal areas. ASR damage has led to structural deterioration and concerns as many structures in Namibia approach or exceed 20 years of service life.
ASR AVOIDANCE MEASURES:
To prevent ASR, one or more of the necessary conditions (reactive silica in aggregates, sufficient alkali content, and adequate moisture) should be eliminated. Avoidance measures include using non-reactive aggregates, limiting alkali content in concrete, using lithium and sodium compounds, fine lightweight aggregates, supplementary cementitious materials, steel fibers, and managing environmental exposure.
Using Non-reactive Aggregates: Employing non-reactive aggregates is a straightforward method to prevent ASR, but it depends on the availability of such aggregates in the construction area.
Limiting Alkali Content: To control ASR, the alkali content in concrete should be limited. The alkali content can come from various sources, including cement, aggregates, deicing salts, and chemical admixtures.
Using Supplementary Cementitious Materials (SCMs): Supplementary cementitious materials like Fly Ash, Ground Granulated Blastfurnace Slag (GGBS), and Condensed Silica Fume (CSF) can be used to reduce the risk of ASR in concrete. Blending SCMs with high-alkali cement dilutes the active alkali content in the concrete, effectively preventing deleterious expansion. Recommended SCM content depends on the type of SCM and the alkali content of the cement.
Managing Environmental Exposure: Controlling environmental conditions is an effective ASR avoidance method. Moisture is a crucial factor, and keeping concrete dry can mitigate ASR expansion. Concrete elements with reactive aggregates and high-alkali cement can be protected from moisture through improved drainage, cladding, or hydrophobic materials. Exposure to sea spray can increase expansion, so considering its effects on the sodium content is essential.
Using Lithium and Sodium Compounds: The addition of lithium and sodium compounds to concrete can reduce ASR expansion. Research has shown that ASR gel with sodium compounds is less expansive, and lithium compounds, like lithium nitrate and lithium hydroxide monohydrate, can decrease the total ASR gel produced. The effectiveness of lithium compounds depends on the reactive aggregate type and the molar ratio of [Li]/[NA+K]. These compounds can be added through chemical admixtures during concrete mixing.
The Use of Fine Lightweight Aggregates (FLWAs) and Steel Fibers: These methods have shown promise in preventing ASR expansion, but practical applications are still under evaluation, and further research is needed to determine their viability in construction.
ASR Testing Methods: Various testing methods are employed to assess ASR susceptibility in aggregates and concrete mixes. Integrated schemes according to RILEM and North American guidelines involve petrographic examination, accelerated mortar-bar tests, and concrete prism tests to identify ASR potential in aggregates. Accelerated tests are used to detect susceptible aggregates, allowing the implementation of ASR avoidance measures.
Petrographic Examination: This method involves examining the siliceous phases in specific aggregates. It requires prior knowledge of the types of reactive silica in the region. However, its precision can be low, especially when assessors are not familiar with certain aggregates. Different standards bodies have produced guidelines and methodologies for petrographic examination.
Screening With Accelerated Mortar Bar Test (AMBT): The AMBT is a widely used screening method to evaluate the potential for ASR reactivity in aggregates. Results can be obtained within a few weeks, making it a preferred choice. However, very high acceleration levels can lead to unreliable results.
Concrete Prism Test (CPT): These longer-term laboratory tests take about one year for accurate results but can be accelerated to around 15 weeks. They provide a more accurate reflection of behavior in real concrete structures.
Concrete Performance Test with Specific Concrete Mix: These tests assess the actual concrete mix used in construction. While reliable, they face challenges in achieving the right balance between acceleration and reliability.
In conclusion, ASR is a widespread issue causing damage to concrete structures globally. Prevention methods include using non-reactive aggregates, limiting alkali content, using lithium and sodium compounds, fine lightweight aggregates, supplementary cementitious materials, steel fibers, and managing environmental exposure. The use of supplementary cementitious materials and lithium compounds has proven effective in preventing ASR when applied correctly. The article also emphasizes the importance of understanding the local geology and employing the right testing methods to assess and address ASR risks in construction projects. The accelerated mortar bar test is widely used to assess aggregate reactivity and determine whether ASR avoidance measures are necessary.
