Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition.
Cause of alkali-silica reaction
ASR is the most common form of alkali-aggregate reaction (AAR) in concrete; the other, much less common, form is alkali-carbonate reaction (ACR). ASR and ACR are therefore both subsets of AAR.
ASR is caused by a reaction between the hydroxyl ions in the alkaline cement pore solution in the concrete and reactive forms of silica in the aggregate (eg: chert, quartzite, opal, strained quartz crystals).
A gel is produced, which increases in volume by taking up water and so exerts an expansive pressure, resulting in failure of the concrete. In unrestrained concrete (that is, without any reinforcement), ASR causes characteristic ‘map cracking’ or ‘Isle of Man cracking’.
The use of pozzolans in the concrete mix as a partial cement replacement can reduce the likelihood of ASR occurring as they reduce the alkalinity of the pore fluid.
With some aggregates, expansion due to ASR increases in proportion with the amount of reactive aggregate in the concrete. Other aggregates 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.
The process of ASR is believed to be in many respects similar to the pozzolanic reaction, such as occurs normally in concrete containing Fly Ash, for example. However, there is an important difference. In the pozzolanic reaction small pozzolanic particles are reacting in a Ca-rich environment, while ASR occurs in mature concrete and involves larger particles of aggregate.
The pozzolanic reaction mechanism is believed to be a process in which silicate anions are detached from the reactive aggregate by hydroxyl ions in the 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 alkali-silicate gel is unstable in the presence of calcium, and calcium silicate hydrate (C-S-H) is formed.
In the pozzolanic reaction where a pozzolan is used as a partial cement 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. No expansion results.
In the case of ASR, 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. Expansion of the gel as water is taken up, may result in damage to the surrounding concrete. Over time, the gel slowly does take up calcium; eventually the composition of the alkali-silica gel may become very similar to that of the calcium silicate hydrate in the cement paste. By then, though, the damage to the concrete may have already been done.
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