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Concrete Cracking Due to Alkali-Aggregate Reaction

The most common reaction in concrete having aggregate with deleterious chemical is reaction between siliceous minerals in the aggregate and alkaline hydroxides in pore water derived from the alkalis (Na2o and k2o) in cement. This reaction can be disruptive and manifest itself as cracking.

Size of cracks

The crack width can range from 0.1 mm to As much as 10 mm in extreme cases. The cracks are rarely more than 25 mm, or at most 50 mm, deep.


Pattern of surface cracking

The pattern of surface cracking induced by the alkali-silica reaction is irregular. Somewhat reminiscent of a huge spider’s web. However, the pattern is not necessarily distinguishable from that caused by sulfate attack or by freezing and thawing, or even by severe plastic shrinkage. Within the concrete, many of the cracks caused by the reaction can be seen to pass through individual aggregate particles but also through the surrounding hydrated cement paste.

Mechanism of cracking


Consequent of the reaction stated above is formation of alkali-silica gel. These gel takes its position in the planes of weakness or pores in the aggregate(where reactive silica is present) or on the surface of the aggregate particles.


In the later case, a characteristic altered surface zone is formed. This may destroy the bond between the aggregate and the surrounding hydrated cement paste.


Causes


Reactive aggregate with high alkali cement produce alkali-silica reaction and consequent cracking.


Source of Alkalis

The most common source of Alkalis in concrete is cement. Alkalis become concentrated in some locations, at the expense of others. Such concentration may be caused by moisture gradients or by alternating wetting and drying. The alkalis may also become concentrated by an electric current passed through the concrete which may occur when cathodic protection is used to prevent corrosion of embedded steel.


The additional sources of Alkalis in concrete include sodium chloride present in unwashed sand dredged from the sea or obtained from the desert. Other internal sources of alkalis are some admixtures, especially superplasticizers, or even the mix water. The alkalis from these sources, and also from fly ash and ground granulated blast furnace slag, should be included in the calculation of the amount of Alkalis present.

Location of Appearance


It is normally appeared in damp location.


Remedy


It has been found that expansion due to the Alkali-silica reaction can be reduced and eliminated by the addition to the mix of reactive silica in a finely powdered form. This finely divided siliceous material added to the coarse reactive particles already present would reduce expansion, although the reaction with the alkalis still takes place. These pozzolanic additions, such as crushed pyrex glass or fly ash, have indeed been found effective in reducing the penetration of the coarse aggregate particles. The fly ash should contain no more than 2 to 3 percent by mass of alkalis.

Pozzolanas in the mix are beneficial also because they reduce the permeability of concrete and therefore reduce the mobility of aggressive agents, both those present with in the concrete and those which may ingress. Furthermore, C-S-H formed by pozzolanic activity incorporates a certain amount of alkalis and thus lowers the value of PH. With the decrease in PH alkali-silica reaction rate also decreased.


Silica fume is particularly effective because the silica reacts preferentially with the alkali. Although the product of reaction is the same as that between the alkalis and the reactive silica in the aggregate, the reaction takes place at the very large surface of the fine particles of silica fume. In consequence, the reaction does not result in expansion.

Ground granulated blastfurnace slag is also effective in mitigating or preventing the deleterious effects of the alkali-silica reaction. It should be noted that the presence of ground granulated blastfurnace slag results in a reduced permeability of concrete.

Lithium salts may inhibit also expansion reactions.


Time of Arrival

These cracks normally appeared after 5 years of construction.

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