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Background

Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition.

Although first recognised in the 1940s in the USA, it wasn’t until the 1970s that ASR really became the focus of attention of concrete science worldwide. It also came to the attention of the media, who promptly dubbed it “concrete cancer”.

Extensive research was undertaken, which later led to better practise in the production of concrete to minimise the risk of ASR in the future. Of course, it wasn’t purely scientific interest that had led to research funding suddenly being available; rebuilding major structures damaged by ASR was becoming expensive.

The reaction

ASR is caused by a reaction between the hydroxyl ions in the alkaline cement

reactive forms of silica are typically chert, quartzite, opal and strained quartz crystals. Other materials, such as glass can also produce ASR. Pyrex glass and calcined flint have been used as test reactive aggregates in experimental

concrete mixes. Ordinary glass (as in household jars and bottles) is also reactive.

Undispersed agglomerations of microsilica can also cause ASR.

Silicate anions are detached from the reactive aggregate by hydroxyl ions in the cement pore fluid; sodium and potassium ions are the ions most readily-available to balance the silicate anions and an alkali-silicate gel is formed. This can take up (“imbibe”) water and is mobile. The gel is unstable in pore fluid containing

dissolved calcium, and calcium silicate hydrate (C-S-H) is produced. This releases sodium and potassium from the gel into the pore fluid, increasing the pore fluid hydroxyl ion concentration, with the potential for continued reaction.

Figure 9.1 Alkali-silica reaction in multi-storey car park, UK. (Photo courtesy The Concrete Society).

Figure 9.2 Alkali-silica reaction in highways structure (Photo courtesy The Concrete Society).

The alkali-silicate gel increases in volume by taking up water, and so exerts an expansive pressure. If the reactive particle is impermeable to the pore fluid, gel forms at the surface. Some types of silica (eg: chert and opal) are permeable to alkali in pore fluid and gel can form inside the aggregate, often near the centre of the particle. Where gel has formed inside an aggregate particle, the aggregate particle usually cracks, with the crack extending into the surrounding concrete (Figures 9.3-9.5).

Figure 9.3 ASR in concrete viewed in thin-section;

alkali-silica gel (arrowed) almost fills the crack extending from the chert particle at the right beyond the left of the image. Yellow material is resin used in specimen preparation.

Figure 9.4 SEM image of ASR in chert particle (centre); internal microcracking is widespread and some cracks extend into the surrounding concrete.

Figure 9.5 Detail of Figure 9.4 showing gel extruded from chert particle into a crack in the concrete. Ettringite is also present (lower right) in a gap between the cement paste and a sand particle.

In unrestrained concrete (that is, without any reinforcement), ASR typically causes the characteristic 'map cracking' associated with ASR, with a repeating pattern of three radiating cracks 120 degrees apart.

Usually, the best method to confirm that ASR has occurred is to examine the concrete in either thin section using a petrographic microscope, or in polished section using a scanning electron microscope (SEM). Gel may be seen in cracks and in aggregate particles.

The process of alkali-silica reaction is believed to be broadly similar to the pozzolanic reaction, as occurs normally in concrete containing fly ash, for example. However, there are important differences.

replacement, the particles are small. As there is much calcium available in young concrete, the alkali-silicate gel forms in a thin layer around the pozzolanic

particle and quickly converts to C-S-H and no expansion results.

In the case of alkali-silica reaction, the reaction usually occurs much later, possibly years after the concrete was placed. Large aggregate particles (large, that is, compared with cement-sized pozzolan) generate a significant volume of gel, which then takes up water and expands within the hardened, mature, concrete.

Because the concrete is mature, calcium availability is limited, as most of the calcium is bound up in stable solid phases. The rate of supply of calcium is therefore insufficient to convert the gel quickly to C-S-H, especially if the gel has formed within an aggregate particle such as opal or chert, where calcium is scarce.

Figure 9.6 This is the same image as in Figure 9.5, with X-ray spectra superimposed showing how alkali-silica gel composition changes with time to become more like that of the surrounding calcium silicate hydrates. At A the gel spectrum shows large peaks due to silicon and potassium and only a very weak peak due to calcium. At B the calcium peak has become much stronger and the potassium peak much weaker. At C the potassium peak has disappeared entirely and the gel has approximately the same composition as the normal calcium silicate hydrate comprising the bulk of the cement paste. Clearly, the gel is older with increasing distance from the aggregate particle in which it originated - the 'oldest' gel has had more time in which to take up calcium from the surrounding paste, and has now become calcium silicate hydrate.

Expansion of the gel as water is taken up, is likely to result in damage to the surrounding concrete. Over time, the gel will slowly take up calcium and

eventually the composition of the gel may become similar to that of the calcium silicate hydrate in the cement paste (see Figure 9.6). By then, though, the concrete may already be severely damaged.

Conditions necessary for ASR and how to limit expansion Three conditions are necessary for ASR to occur in concrete:

 A sufficiently high alkali content of the cement pore fluid.

 A reactive aggregate, such as chert.

 Water - needed for the gel to expand; expansion due to ASR will not occur if there is no available water in the concrete.

Given these three conditions, ways in which we can prevent expansion in concrete due to ASR may appear obvious; we avoid using reactive aggregates and we limit the available alkalis. However, restrictions that are too severe will in themselves cause other problems.

Restricting the alkali in the cement

The cement is usually the main source of alkali in the pore fluid in concrete, derived from alkali sulfate and from alkalis in the main clinker minerals, mainly in belite and aluminate. So if we restrict the alkali content of the cement, won’t that limit the alkali in the pore fluid? Yes, it will, but this does not allow for different cement contents in concrete, so simply imposing a limit on cement alkalis is not a complete answer to the problem. Also, we need to remember that higher alkali cements tend to give better early concrete strengths; alite and belite hydrate faster as pore fluid alkalinity increases.

Imposing limits on cement alkali content that were “too low” (however defined) may therefore affect cement performance. It would also increase the cost of cement production, as bypass dust and precipitator dust high in alkalis could not be returned to the kiln and would have to go to landfill. Additionally, some raw materials high in alkali could not be used and would have to be replaced by possibly more expensive alternatives. So, although restricting the cement alkali content makes an important contribution to controlling ASR in concrete where potentially reactive aggregate is to be used, it is not necessarily a complete answer to the problem.

Alkali limits are usually expressed as “sodium equivalent”. This conveniently combines the potassium and sodium oxides, and is useful for other purposes as

Upper limits for sodium oxide equivalent on low alkali cement are typically of the order of 0.60%-0.75%; check the standards or codes of practice applicable to your location.

Restricting the alkali in the concrete

An alternative approach is to limit the alkali content of the concrete. This would then take into account that different concretes have different cement contents.

Limiting the alkali content can be achieved by using lower alkali Portland cement, by using a lower cement content in the concrete or by partially replacing the Portland cement with slag, fly ash or microsilica.

The use of slag or a pozzolan in the concrete mix as a partial cement

replacement can reduce the likelihood of ASR occurring as these reduce the alkalinity of the pore fluid. Slag, fly ash, microsilica and metakaolin have all been found in various studies to reduce or prevent expansion due to ASR. However, this is a complex area because some of these cement replacement materials also contribute alkali when they react.

For example, using some typical Figures from Taylor, fly ash typically contains 1.5% Na2O and 4.2% K2O (1) and slag contains approximately 0.4% Na2O and 0.7% K2O (2). Compared with the alkali levels in Portland cement, slag is therefore broadly similar and fly ash is considerably higher. However, both are less reactive than Portland cement and even in mature concrete some slag or fly ash will remain unreacted.

Only the reacted fraction of the slag or fly ash will have released alkali into the pore fluid. As this proportion is not known precisely, and anyway will vary between different concrete mixes, it is difficult to calculate just how much alkali from the slag or fly ash has become available for reaction. National Rules will determine what substitution levels are effective and what allowance to make for slag or PFA alkalis.

Restricting the water

Can we restrict water availability? In most cases, not really, unless the concrete is entirely under cover and will never get wet. While expansion due to ASR is unlikely in concrete that remains dry inside buildings, even building interiors can become saturated in flood conditions. In any case, most concrete is used in foundations, roads, bridges or other open structures where it will be exposed to water.

1The constant 0.658 is derived from the ratios of atomic weights of sodium and potassium oxides: Na 22.99; K 39.10; O 15.99; therefore Na2O (22.99 x 2)+15.99=61.97; KO (39.10 x 2)+15.99=94.19; 61.97/94.19 = 0.658.

Tests for aggregate reactivity

If concrete does not contain reactive aggregate, it follows that there will be no expansion due to ASR. So, can we prevent expansion by not using reactive aggregate in concrete? Yes, we can, but we need to decide what we mean by

“reactive” and we also need some method by which we can determine whether or not a particular aggregate is reactive.

With some aggregates, expansion due to ASR increases broadly in proportion with the amount of reactive aggregate in the concrete. The majority of

aggregates (eg: chert) show what is called a “pessimum” effect; if the proportion of reactive aggregate in test mixes is varied while other factors are kept

constant, maximum concrete expansion occurs at a particular aggregate content.

Higher or lower proportions of reactive aggregate will give a lower expansion.

In some areas, chert represents the bulk of aggregate used in concrete, with no resulting expansion due to ASR because the proportion of chert in the concrete is well above the pessimum. If we excluded all potentially reactive aggregate, we would not be able to use chert, but such a restriction is unnecessary since it can clearly be used safely. We just need to avoid aggregate that contains chert near the pessimum proportion.

There are broadly three types of test for alkali reactivity of aggregate:

 Examination in thin-section using optical microscopy (ie: petrography)

 Chemical tests (eg: immersion in an alkaline solution such as sodium hydroxide)

 Expansion tests using mortar bars or concrete prisms

Variations on all three of these approaches have been described in different standards.

Standard microscopy procedures include: ASTM C295-08 Standard Guide for Petrographic Examination of Aggregates for Concrete and British Standard BS 812: Part 104: 1994 Testing aggregates - method for qualitative and quantitative petrographic examination of aggregates.

Chemical tests include ASTM C289 - 07 Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method).

Expansion tests include ASTM C227-03 Standard Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method) and British Standard BS 812: Part 123: 1999 Testing aggregates - method for determination of alkali-silica reactivity – concrete prism method”.

diagnostic test, assuming of course that the microscopist is able to recognise potentially reactive forms of silica and also that the sample examined is representative of the aggregate as a whole.

Chemical tests can also be effective in identifying potentially reactive aggregate but may not be a good predictor of expansion where the aggregate shows a pessimum effect.

Conventional expansion tests with mortar bars or concrete prisms are the closest simulation to ‘real concrete’ but may require two years or more to show results;

this may not always be convenient. Accelerated expansion tests can shorten this timescale but do not simulate real conditions as closely.

In summary, all these tests for aggregate reactivity can be very useful – indeed essential - but all have potential problems and the perfect test for ASR

susceptibility has yet to be developed.

So, is it practicable to limit expansion due to ASR?

Yes, definitely. Despite the limitations we’ve looked at above, the incidence of expansion due to ASR has declined markedly over the last 10-20 years where these different approaches to limiting expansion have been applied. For example, in the UK, there have been no reported cases since the current rules were

introduced in the 1980s. Generally, codes of practice and national standards define how these approaches are used in practice in different parts of the world.

Usually, several strategies to limit expansion are applied at the same time.

For example, aggregate producers will routinely have their aggregate examined petrographically to determine the rock types and their potential reactivity. These aggregates then go into concrete that contains cement that has limits on alkali content specifically to minimise the risk of ASR; the concrete may well also contain fly ash or slag or other mineral addition to limit the available alkali.

Of course, these relatively recent ways of dealing with the problem only control the problem in concrete produced using these methods. Instances of ASR still occur in some older concrete because there were fewer controls over what was used to produce it. Even where good controls are apparently in place, there is always the potential for things to go badly wrong.