Testing ASR in concrete before and after construction

written by Nirmal Raj Joshi | September 9, 2025

Alkali-silica reaction (ASR) is a deleterious chemical reaction that occurs between the alkaline pore solution in concrete and reactive silica present in certain aggregates. This reaction forms an expansive gel that can induce cracking, reduce durability, and compromise structural integrity (Thomas, 2011). Given its long-term implications, ASR is a critical concern in concrete design and post-construction assessment. To mitigate ASR, various standardized tests are employed during the concrete mix design phase. These tests assess aggregate reactivity and the effectiveness of preventive measures such as supplementary cementitious materials (SCMs) or lithium-based inhibitors.

Accelerated Mortar Bar Test (ASTM C1260 / C1567)

The Accelerated Mortar Bar Test (AMBT), as standardized in ASTM C1260 (2020), is a widely adopted method for assessing the alkali-silica reactivity (ASR) of aggregates. In this procedure, mortar bars incorporating the aggregate under investigation are immersed in a 1N sodium hydroxide (NaOH) solution at a controlled temperature of 80°C for a duration of 14 days. The degree of expansion is then measured to quantify reactivity. According to established criteria, an expansion equal to or exceeding 0.10% at 14 days is indicative of potentially deleterious aggregate reactivity (Lindgård et al., 2012).

ASTM C1567 (2020) provides a modification of the AMBT protocol to evaluate the effectiveness of supplementary cementitious materials (SCMs), such as fly ash or ground granulated blast furnace slag, in mitigating ASR. In this version of the test, the mortar mix includes SCMs alongside the reactive aggregate. A reduction in expansion to below 0.10% at 14 days is generally interpreted as evidence of the SCM’s efficacy in suppressing ASR-related expansion.

Testing before construction

Concrete Prism Test (ASTM C1293 / RILEM AAR-3)

The Concrete Prism Test (CPT), as specified in ASTM C1293 (2020), is a long-term evaluation method designed to assess the alkali-silica reactivity (ASR) of aggregates under conditions that closely simulate field exposure. In this procedure, concrete prisms are stored at 38°C and 100% relative humidity for a period of up to two years. An expansion equal to or exceeding 0.04% at one year is generally considered indicative of deleterious reactivity (Fournier & Bérubé, 2000). Due to its extended duration and use of concrete rather than mortar, the CPT is regarded as a more reliable indicator of field performance when compared to accelerated tests such as ASTM C1260.

The RILEM AAR-3 (2016) procedure adopts a similar methodological framework but employs elevated storage temperatures (typically 60°C) to accelerate the reaction kinetics. Despite the increased temperature, the test maintains a strong correlation with field performance, offering a balance between accelerated assessment and predictive reliability.

Petrographic Examination (ASTM C295 / C1778)

Petrographic analysis, as outlined in ASTM C295 (2019), entails the microscopic examination of aggregate particles to identify potentially reactive forms of silica, such as chert, opal, and strained quartz. This method provides critical insight into the mineralogical composition, texture, and fabric of aggregates, thereby aiding in the prediction of their susceptibility to alkali-silica reaction (ASR).

Complementing this, ASTM C1778 (2020) offers a comprehensive guideline for the petrographic assessment of hardened concrete affected by ASR. It facilitates the differentiation of ASR-induced distress from other concrete deterioration mechanisms by examining features such as gel presence, crack patterns, and reaction rims. Together, these standards support both the preventive evaluation of aggregates and the diagnostic investigation of ASR damage in existing structures.

Chemical Methods (ASTM C289)

ASTM C289 (2007) outlines a chemical method for evaluating the potential alkali-silica reactivity (ASR) of aggregates by measuring the amount of silica dissolved and the corresponding reduction in alkalinity when the aggregate is immersed in a sodium hydroxide (NaOH) solution. The underlying premise is that reactive aggregates will release significant amounts of silica into the solution while simultaneously consuming alkalis, indicating a potential for deleterious reaction in concrete.

However, the reliability of this method has been questioned due to its susceptibility to false positives. Certain non-reactive aggregates may exhibit high levels of silica dissolution or alkali reduction without causing ASR in service conditions. As a result, the test is considered less definitive than other physical or petrographic methods and is primarily used as a preliminary screening tool (Nixon & Sims, 2016).

Performance-Based Approaches (RILEM AAR-4, AAR-5)

RILEM AAR-4 (2016) assesses alkali release from aggregates, while AAR-5 evaluates the effectiveness of preventive measures. These methods complement traditional tests by considering material interactions in real-world conditions.

Detecting ASR in Existing Structures

Once concrete is in service, ASR detection requires a combination of visual inspection, mechanical testing, and advanced analytical techniques.

Visual and Macroscopic Indicators

Several visual and macroscopic features are commonly associated with alkali-silica reaction (ASR) damage in concrete structures, providing preliminary evidence prior to detailed laboratory analysis.

  • Map Cracking: One of the most characteristic indicators of ASR is the presence of a random, interconnected network of cracks on the concrete surface, often referred to as “map” or “craze” cracking. This pattern typically reflects internal expansion pressures resulting from ASR gel formation and subsequent moisture absorption (Figg, 1999).
  • Gel Exudation: ASR-affected concrete may exhibit white or dark-colored gel deposits at or near the surface, particularly around cracks or joints. These exudates are indicative of ASR gel migration and accumulation, a consequence of internal gel swelling and moisture interaction (Stark et al., 2002).
  • Structural Displacement: In advanced stages of ASR, expansion-induced stresses can result in noticeable structural distortions, such as the misalignment of joints, beams, or other load-bearing elements. Such displacements may compromise the serviceability and structural integrity of affected components (Fournier & Bérubé, 2000).

Petrographic Analysis (ASTM C856 / C1778)

Thin-section petrographic analysis, as prescribed in ASTM C856 (2018), is a powerful technique for identifying microstructural features associated with alkali-silica reaction (ASR) in hardened concrete. Through the preparation and microscopic examination of polished or thin sections, several diagnostic indicators of ASR can be observed:

  • Microcracks Containing ASR Gel: The presence of microcracks partially or fully filled with alkali-silica gel is a key indicator of ASR activity. These gel-filled cracks often radiate from reactive aggregate particles and may extend into the surrounding cement paste.
  • Reaction Rims: Reactive aggregate particles commonly exhibit distinctive reaction rims-concentric alteration zones-indicating the progressive dissolution of silica and interaction with alkaline pore solution. These rims serve as evidence of ongoing or past ASR processes.
  • Gel Deposits in Air Voids: ASR gel may also accumulate in nearby entrapped or entrained air voids, suggesting gel migration from reaction sites. The presence of gel within voids is often interpreted as a sign of significant internal expansion and transport phenomena (Thomas et al., 2008).

Expansion Monitoring

Long-term expansion monitoring is essential for evaluating the progression of alkali-silica reaction (ASR) in existing concrete structures. Mechanical and non-destructive techniques are commonly employed to quantify deformation and assess internal damage.

  • Mechanical Strain Measurement: Instruments such as Demec gauges and embedded strain sensors are routinely used to measure length changes in concrete elements over time. These tools provide precise, localized data on expansion behavior, facilitating the evaluation of ASR-induced deformation in field conditions (Nixon & Sims, 2016).
  • Ultrasonic Pulse Velocity (UPV): UPV is a non-destructive testing method that measures the velocity of ultrasonic waves transmitted through concrete. A reduction in wave velocity is generally associated with internal cracking and material degradation. While UPV is effective in detecting internal damage, it is not specific to ASR and may reflect other forms of deterioration such as freeze-thaw damage or corrosion-related cracking (ACI 228.2R, 2013).

Chemical and Spectroscopic Techniques

Advanced microanalytical methods play a critical role in confirming the presence and characterizing the composition of alkali-silica reaction (ASR) products at the microscale.

  • Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS): SEM-EDS enables high-resolution imaging combined with elemental analysis of ASR-affected regions. This technique is commonly employed to confirm the presence of ASR gel by identifying its characteristic elemental composition, typically rich in silicon, alkalis (sodium and potassium), and calcium (Lindgård et al., 2012). SEM-EDS also allows for the investigation of microcracks, reaction rims, and gel distribution within the cement matrix and aggregate-paste interface.
  • Raman Spectroscopy: Raman spectroscopy provides a non-destructive means of identifying ASR products based on their unique vibrational spectra. It is particularly useful for distinguishing amorphous ASR gels from crystalline phases within the concrete microstructure. Its ability to analyze uncoated and minimally prepared samples makes it a valuable tool for in situ characterization of reaction products (Leemann & Lura, 2013).

Core Expansion Testing (RILEM AAR-7)

RILEM AAR-7 (2016) outlines a methodology for evaluating the residual alkali-silica reactivity (ASR) of concrete through expansion testing of extracted core specimens. In this approach, cores obtained from in-service structures are exposed to accelerated environmental conditions-typically 38°C and high relative humidity-to promote potential expansion resulting from ongoing or latent ASR.

References

  • ACI 228.2R. (2013). Nondestructive Test Methods for Evaluation of Concrete in Structures.
  • ACI 364.1R. (2019). Guide for Evaluation of Concrete Structures Prior to Rehabilitation.
  • ASTM C1260. (2020). Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method).
  • ASTM C1293. (2020). Standard Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction.
  • Fournier, B., & Bérubé, M. A. (2000). Alkali-Aggregate Reactivity in Concrete: A Review. Cement and Concrete Research, 30(9), 1349-1359.
  • Leemann, A., & Lura, P. (2013). Raman Spectroscopy of ASR Products. Cement and Concrete Research, 47, 1-7.
  • Nixon, P. J., & Sims, I. (2016). RILEM Recommendations for the Prevention of Damage by Alkali-Aggregate Reactions. Materials and Structures, 49(10), 4093-4106.
  • Thomas, M. (2011). The Effect of Supplementary Cementing Materials on Alkali-Silica Reaction: A Review. Cement and Concrete Research, 41(12), 1224-1231.