Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition. This is a short introduction to ASR - for more information, see the Understanding Cement e-book.
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'.
Gel may be present in cracks and within aggregate particles. The best technique for the identification of ASR is the examination of concrete in thin section, using a petrographic microscope. Alternatively, polished sections of concrete can be examined by scanning electron microscopy (SEM); this has the advantage that the gel can be analysed using X-ray microanalysis in order to confirm the identification beyond any doubt.
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 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.
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 (see
Figure 4). By then, though, the damage to the concrete may have already
The "Understanding Cement" book or e-book contains even more information!
Compared with this web site, the book has, for example, about two-and-a-half times as much on ASR, one-and-a-half times as much on sulfate attack and nearly three times as much on carbonation. It has sections on alkali-carbonate reaction, frost (freeze-thaw) damage, steel corrosion, leaching and efflorescence on masonry. It also has about four-and-a-half times as much on cement hydration (comparisons based on word count).
NEW! Now available as a printed book
See also the blog page on asr and glass: Alkali-silica reaction ( asr ) and glass