Alkali–silica reaction

Characteristic crack pattern associated with the alkali–silica reaction affecting a concrete step barrier on an US motorway (photograph courtesy of the Federal Highway Administration, US Department of Transportation).^[1][2]

For less common types of alkali-driven concrete degradation see alkali-aggregate reaction (disambiguation page).

The alkali–silica reaction (ASR) is a reaction which occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture.

This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water, and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure.

ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.^[3]

Chemistry

ASR is the same as the pozzolanic reaction, which is a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)₂), and silicic acid (H₄SiO₄, or Si(OH)₄). For the sake of simplicity, this reaction can be schematically represented as following:

Ca(OH)₂ + H₄SiO₄ → Ca²⁺ + H₂SiO₄²⁻ + 2 H₂O → CaH₂SiO₄ · 2 H₂O

Mechanism of concrete deterioration

The mechanism of ASR causing the deterioration of concrete can be described in four steps as follows:

1. The alkaline solution attacks the siliceous aggregate, converting it to viscous alkali silicate gel.
2. Consumption of alkali by the reaction induces the dissolution of Ca²⁺ ions into the cement pore water. Calcium ions then react with the gel to convert it to hard C-S-H.
3. The penetrated alkaline solution converts the remaining siliceous minerals into bulky alkali silicate gel. The resultant expansive pressure is stored in the aggregate.
4. The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.^[4]

Mitigation

ASR can be mitigated in new concrete by several complementary approaches:

1. Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na₂O" content of cement.
2. Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass (obsidian) and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one. In principle, limestone (CaCO₃) is not expected to contain a high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones (a.o., Kieselkalk found in Switzerland)^[5] may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction, as observed with some Tournaisian siliceous limestones exploited in quarries in the area of Tournai in Belgium.^[6] So, the use of limestone as aggregate is not a guarantee against ASR in itself.
3. Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by deliberately provoking a controlled pozzolanic reaction at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.g., pozzolan, silica fume, fly ash, or metakaolin.^[7] These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.
4. Another method to reduce the ASR is to limit the external alkalis that come in contact with the system.

In other words, as it is sometimes possible to fight fire with fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with large siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.

As part of a study conducted by the Federal Highway Administration, a variety of methods have been applied to field structures suffering from ASR-affected expansion and cracking. Some methods, such as the application of silanes, have shown significant promise, especially when applied to elements such as small columns and highway barriers, whereas other methods, such as the topical application of lithium compounds, have shown little or no promise in reducing ASR-induced expansion and cracking.^[8]

ASR test

Some ASTM Tests that screen aggregate for the potential of ASR include:

• ASTM C227: “Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method)”
• ASTM C289: "Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method)"
• ASTM C295: “Guide for Petrographic Examination of Aggregate for Concrete”
• ASTM C1260: “Test Method for Potential Reactivity of Aggregates (Mortar-Bar-Test)”
• ASTM C1293: “Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction”
• ASTM C1567: "Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)"
• The concrete microbar test was proposed by Grattan-Bellew et al. (2003) as a universal accelerated test for alkali-aggregate reaction.

^[9]

See also

• Alkali-carbonate reaction
• Carbonatation
• Energetically modified cement (EMC)

External links

• Understanding cement website treatise on ASR
• PCA treatise on ASR
• Concrete Construction Net treatise of ASR
• US Federal Highway Administration treatise on the use of lithium to prevent or mitigate ASR
• Association of German Cement Works – Alkali-silica reaction - overview

References

1. FHWA (2010-06-22). "Alkali-Silica Reactivity (ASR) – Concrete – Pavements – FHWA". Alkali-Silica Reactivity (ASR) Development and Deployment Program. Archived from the original on 8 August 2010. Retrieved 2010-07-28.
2. Faridazar, Fred (2009-02-10). "TECHBRIEF: Selecting candidate structures for lithium treatment: What to provide the petrographer along with concrete specimens, FHWA-HRT-06-069 – Pavements – FHWA". FHWA-HRT-06-069. Retrieved 2010-07-28.
3. "Alkali–silica reaction in concrete". Understanding Cement. Archived from the original on 10 August 2007. Retrieved 2007-08-11.
4. Ichikawa T. and Miura M. (2007) Modified model of alkali-silica reaction. Cement and Concrete Research, 37, 1291–1297
5. Funk, Hanspeter (1975). "The origin of authigenic quartz in the Helvetic Siliceous Limestone (Helvetischer Kieselkalk), Switzerland". Sedimentology 22 (2): 299–306. Bibcode:1975Sedim..22..299F. doi:10.1111/j.1365-3091.1975.tb00296.x.
6. Monnin, Y.; Dégrugilliers P.; Bulteel D.; Garcia-Diaz E. (2006). "Petrography study of two siliceous limestones submitted to alkali-silica reaction". Cement and Concrete Research 36 (8): 1460–1466. doi:10.1016/j.cemconres.2006.03.025. ISSN 0008-8846. Retrieved 2009-03-17.
7. Ramlochan, Terrence; Michael Thomas; Karen A. Gruber (2000). "The effect of metakaolin on alkali-silica reaction in concrete". Cement and Concrete Research 30 (3): 339–344. doi:10.1016/S0008-8846(99)00261-6. ISSN 0008-8846. Retrieved 2009-03-18.
8. "Publication Details for Alkali-Aggregate Reactivity (AAR) Facts Book - Pavements - FHWA". dot.gov.
9. Grattan-Bellew, P.E.; G. Cybanski; B. Fournier; L. Mitchell (2003). "Proposed universal accelerated test for alkali-aggregate reaction: the concrete microbar test". Cement Concrete and Aggregates 25 (2): 29–34.