The alkali–silica reaction (ASR), more commonly known as "concrete cancer", is a swelling reaction that 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 soluble and viscous gel of sodium silicate (Na2SiO3 • nH2O, also noted Na2H2SiO4 • nH2O, or N-S-H (sodium silicate hydrate), depending the adopted convention). This hygroscopic gel swells and increases in volume when absorbing water: it exerts an expansive pressure inside the siliceous aggregate, causing spalling and loss of strength of the concrete, finally leading to its failure.
ASR can lead to serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure. The expansion of concrete through reaction between cement and aggregates was first studied by Thomas E. Stanton in California during the 1930s with his founding publication in 1940.
The reaction can be compared to the pozzolanic reaction which would be catalysed by the undesirable presence of too high concentrations of alkali hydroxides (NaOH and KOH) in the concrete. It is a mineral acid-base reaction between NaOH or KOH, calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4). When complete and to simplify, this reaction can be schematically represented as following:
- Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42− + 2 H2O → CaH2SiO4 • 2H2O
Catalysis of ASR by NaOH or KOH
However, the ASR reaction significantly differs from the pozzolanic reaction by the fact that it is catalysed by soluble alkali hydroxides (NaOH / KOH) at very high pH. It can be represented as follows using the classical geochemical notation for fully hydrated dissolved silica (Si(OH)4 or silicic acid: H4SiO4), but an older industrial notation also exists (H2SiO3, hemihydrated silica (does not exist), by analogy with carbonic acid):
- 2 Na(OH) + H4SiO4 → Na2H2SiO4 • 2H2O
The sum, or the combination, of the two above mentioned reactions gives a general reaction resembling the pozzolanic reaction, but it is important to keep in mind that this reaction is catalysed by the undesirable presence in cement, or other concrete components, of soluble alkaline hydroxydes (NaOH / KOH) responsible for the dissolution of the silicic acid at high pH:
- Ca(OH)2 + H4SiO4 → CaH2SiO4 • 2H2O
Without the presence of NaOH or KOH responsible for a high pH (~13.5), the amorphous silica would not be dissolved and the reaction would not evolve. Moreover, the soluble sodium or potassium silicate is very hygroscopic and swells when it absorbs water. When the sodium silicate gel forms and swells inside a porous siliceous aggregate, it first expands and occupies the free porosity. When this latter is completely filled, if the soluble but very viscous gel cannot be easily expelled from the silica network, the hydraulic pressure raises inside the attacked aggregate and leads to its fracture. It is the hydro-mechanical expansion of the damaged siliceous aggregate surrounded by calcium-rich hardened cement paste which is responsible for the development of a network of cracks in concrete. When the sodium silicate expelled from the aggregate encounters grains of portlandite present in the hardened cement paste, an exchange between sodium and calcium cations occurs and hydrated calcium silicate (C-S-H) precipitates with a concomitant release of NaOH. In its turn, the regenerated NaOH can react with the amorphous silica aggregate leading to an increased production of soluble sodium silicate. When a continuous rim of C-S-H completely envelops the external surface of the attacked siliceous aggregate, it behaves as a semi-permeable barrier and hinders the expulsion of the viscous sodium silicate while allowing the NaOH / KOH to diffuse from the hardened cement paste inside the aggregate. This selective barrier of C-S-H contributes to increase the hydraulic pressure inside the aggregate and aggravates the cracking process. It is the expansion of the aggregates which damages concrete in the alkali-silica reaction.
Portlandite (Ca(OH)2) represents the reserve of OH– anions in the solid phase. As long as portlandite, or the siliceous aggregates, has not become completely exhausted, the ASR reaction will continue. The alkali hydroxides are continuously regenerated by the reaction of the sodium silicate with portlandite and thus represent the transmission belt of the ASR reaction driving it to completeness. It is thus impossible to interrupt the ASR reaction. The only way to avoid ASR in the presence of siliceous aggregates and water is to maintain the concentration of soluble alkali (NaOH and KOH) at the lowest possible level in concrete, so that the catalysis mechanism becomes negligible.
Analogy with the soda lime carbonation
The alkali-silica reaction mechanism catalysed by a soluble strong base as NaOH or KOH in the presence of Ca(OH)2 (alkalinity buffer present in the solid phase) can be compared with the carbonation process of soda lime. The silicic acid (H2SiO3 or SiO2) is simply replaced in the reaction by the carbonic acid (H2CO3 or CO2).
(1) CO2 + 2 NaOH → Na2CO3 + H2O (CO2 trapping by soluble NaOH) (2) Na2CO3 + Ca(OH)2 → CaCO3 + 2 NaOH (regeneration of NaOH) sum (1+2) CO2 + Ca(OH)2 → CaCO3 + H2O (global reaction)
In the presence of water or simply ambient moisture, the strong bases, NaOH or KOH, readily dissolve in their hydration water (hygroscopic substances, deliquescence phenomenon) and this greatly facilitates the catalysis process because the reaction in aqueous solution occurs much faster than in the dry solid phase. The moist NaOH impregnates the surface and the porosity of calcium hydroxide grains with a high specific surface area. Soda lime is commonly used in closed-circuit diving rebreathers and in anesthesia systems.
Mechanism of concrete deterioration
The mechanism of ASR causing the deterioration of concrete can thus be described in four steps as follows:
- The very basic solution (NaOH / KOH) attacks the siliceous aggregates (silicic acid dissolution at high pH), converting the poorly crystallised or amorphous silica to a soluble but very viscous alkali silicate gel (N-S-H, K-S-H).
- The consumption of NaOH / KOH by the dissolution reaction of amorphous silica decreases the pH of the pore water of the hardened cement paste. This allows the dissolution of Ca(OH)2 (portandite) and increases the concentration of Ca2+ ions into the cement pore water. Calcium ions then react with the soluble sodium silicate gel to convert it into solid calcium silicate hydrates (C-S-H). The C-S-H forms a continuous poorly permeable coating at the external surface of the aggregate.
- The penetrated alkaline solution (NaOH / KOH) converts the remaining siliceous minerals into bulky soluble alkali silicate gel. The resulting expansive pressure increases in the core of the aggregate.
- The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.
Structural effects of ASR
- Expansion: The swelling nature of ASR gel increases the chance of expansion in concrete elements.
- Compressive Strength: The effect of ASR on compressive strength can be minor for low expansion levels, to relatively higher degrees at larger expansion. (Swamy R.N 1986) points out that the compressive strength is not very accurate parameter to study the severity of ASR; however, the test is done because of its simplicity.
- Tensile Strength / Flexural Capacity: Researches show that ASR cracking can significantly reduce the tensile strength of concrete; therefore reducing the flexural capacity of beams. Some research on bridge structures indicate about 85% loss of capacity as a result of ASR.
- Modulus of Elasticity/UPV: The effect of ASR on elastic properties of concrete and ultrasound pulse velocity (UPV) is very similar to tensile capacity. The modulus of elasticity is shown to be more sensitive to ASR than pulse velocity.
- Fatigue: ASR reduces the load bearing capacity and the fatigue life of concrete (Ahmed T 2000).
- Shear: ASR enhances the shear capacity of reinforced concrete with and without shear reinforcement (Ahmed T 2000).
ASR can be mitigated in new concrete by several complementary approaches:
- Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na2O" content of cement.
- 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) may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction, as also observed with some Tournaisian siliceous limestones exploited in quarries in the area of Tournai in Belgium. In Canada, the Spratt siliceous limestone is also particularly well known in studies dealing with ASR and is commonly used as the Canadian ASR reference aggregate. So, the use of limestone as aggregate is not a guarantee against ASR in itself.
- 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. 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.
- 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 larger 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. Beside initially lowering the pH value of the concrete pore water, the main working mechanism of silica fume addition is to consume portlandite (the reservoir of hydroxyde (OH–) in the solid phase) and to decrease the porosity of the hardened cement paste by the formation of calcium silicate hydrates (C-S-H). However, silica fume has to be very finely dispersed in the concrete mix, because agglomerated flakes of compacted silica fume can themselves also induce ASR if the dispersion process is insufficient. This can be the case in laboratory studies made on cement pastes alone in the absence of aggregates. However, most often, in large concrete batches, silica fume is sufficiently dispersed during mixing operations of fresh concrete by the presence of coarse and fine aggregates.
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.
There are no treatments in general in affected structures. Repair in damaged sections is possible, but the reaction will continue. In some cases, drying of the structure followed by the installation of a watertight membrane can stop the evolution of the reaction.
Massive structures such as dams pose particular problems: they cannot be easily replaced, and the swelling can block spillway gates or turbine operations. Cutting slots across the structure can relieve some pressure, and help restore geometry and function.
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)”. It is a rapid test of aggregates: immersion of mortar bars in NaOH 1 M at 80 °C for 14 days used to quickly identify highly reactive aggregates or quasi non-reactive aggregates.
- ASTM C1293: “Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction”. It is a long-term confirmation test (1 or 2 years) at 38 °C in a water-saturated moist atmosphere (inside a thermostated oven) with concrete prisms containing the aggregates to be characterised mixed with a high-alkali cement specially selected to induce ASR. The concrete prisms are not directly immersed in an alkaline solution, but wrapped with moist tissues and tightly packed inside a water-tight plastic foils.
- ASTM C1567: "Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)"
- The Oberholster method on which the ASTM C1260 test is based.
- The Dungan method with superimposed additional thermal cycles.
- The concrete microbar test was proposed by Grattan-Bellew et al. (2003) as a universal accelerated test for alkali-aggregate reaction.
Known affected structures
- Adelaide Festival Centre car park, demolished in 2017
- Centennial Hall, Adelaide (1936-2007)
- Dee Why ocean pool, Dee Why, Australia.
- King St Bridge, demolished and replaced in 2011 (crossing the Patawalonga River, Glenelg North, South Australia).
- Manly Surf Pavilion, Manly, Australia (1939–1981).
- The MCG's old Southern Stand, demolished in 1990 and replaced with the Great Southern Stand which was completed in 1992
- Westpoint Blacktown car park
- Many bridges and civil engineering works of motorways because the improper use of highly reactive Tournaisian siliceous limestone during the years 1960 - 1970 when most of the motorways were constructed in Belgium. ASR damages started to be recognised only in the 1980s.
- Poorly conditioned radioactive waste from the Doel nuclear power plant: evaporator concentrates and spent ion-exchange resins (SIER) producing large quantities of sodium silicate gel.
- Many hydraulic dams are affected by ASR in Canada because of the wide use of reactive aggregates. Indeed, reactive frost-sensitive chert is very often found in glacio-fluvial environments from which gravels are commonly extracted in Canada. Another reason is also the presence of reactive silica in Paleozoic limestones like the Spratt siliceous limestone.
- Many bridges and civil engineering works of motorways.
- Building of the National Gallery of Canada at Ottawa.
- Former Térénez bridge in Brittany, built in 1951 and replaced in 2011.
- East German Deutsche Reichsbahn used numerous concrete ties in the 1970s to replace previous wooden ties. However, the gravel from the Baltic Sea caused ASR and the ties had to be replaced earlier than planned, lasting well into the 1990s.
- After reunification, many Autobahns in East Germany were refurbished with concrete that turned out to have been defective and affected by ASR, necessitating expensive replacement work.
- Keybridge House, South Lambeth Road, Vauxhall, London, England.
- Millennium Stadium North Stand (part of the old National Stadium), Cardiff, Wales.
- Merafield Bridge, A38, England. Demolished via implosion in 2016.
- Pebble Mill Studios, Birmingham. Demolished in 2005
- Royal Devon and Exeter Hospital, Wonford. Demolished and replaced in the mid-1990s.
- Steve Bull Stand, Molineux Stadium, Wolverhampton
- Soda lime: the mechanism of ASR catalysed by NaOH is analogous to the trapping mechanism of CO2 by Ca(OH)2 impregnated with NaOH
- 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
|Wikimedia Commons has media related to Alkali silica reactions.|
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