Alkali-aggregate expansion

Although all aggregates can be considered reactive, only those that actually cause damage to concrete are cause for concern. Experience has shown that the presence of an excessive amount of alkalis enhances the attack on concrete by an expansion reaction. Use of marginal quality aggregate and the production of high strength concrete may also produce this effect. 

The alkali-aggregate reaction in concrete may manifest itself as map cracking on the exposed surface, although other reactions may also produce such failures. The alkali-aggregate reaction, known as alkali-silica type, may promote exudation of a water gel, which dries to a white deposit. These effects may appear after only a few months or even years. 

Three t) es of alkali-aggregate reactions are mentioned in the literature  

The preventive methods to counteract alkali-aggregate expansion include replacement of cement with pozzolans orblast-ftimace slag and use of low alkali cement. 

Frost action is defined as the freezing and thawing of the moisture in materials and the resultant effects on these materials. Essentially, three kinds of defects are recognized  

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Methods of preventing alkali-aggregate expansion reactions in concrete consist of avoidance of reactive aggregates, use of cement with alkalis less than 0.6% Na20 equivalent, the use of pozzolanic materials, sealing of... 

McCoy, W.J. and Caldwell, A.G. (1951). New approach to inhibiting alkali aggregate expansion. Journal of the American Concrete Institution, 47, 693-706. 

The work carried out to date indicates that not all lithium salts are effective in diminishing the alkali-aggregate expansion reaction [10]. Optimum dosage requirements and long-term effects have to be evaluated. Before the relative effects of various lithium compounds such as hydroxide, carbonate, nitrate, fluoride, perchlorate, chloride, etc., can be substantiated much more work employing reliable predictive tests has to be carried out. 

Information from previous work suggests that air-entrainment offers a measure of protection against alkali-aggregate expansion. An air-entrainment of 3.6% can cause a 60% reduction in expansion [10]. In Fig. 6.2 the... 

Alkyl alkoxy silanes have been found to be very effective in reducing alkali-aggregate expansion [11] (Fig. 6.4). Of the silanes used in the study, hexyl trimethyl siloxane and decyl trimethoxyl silane were found to be more effective in decreasing the expansion than the others. In the same study, Ohama et al. [11] investigated the effect of sodium silicofluoride, alkyl alkoxy silane, lithium carbonate, lithium fluoride, styrene-butadiene rubber latex and lithium hydroxide on compressive strength and the expansion of mortar containing cement with 2% equivalent Na20. The reduction of the level of expansion shown in Fig. 6.4 with the siloxanes was attributed to... 

Fig. 6.4 The effect of silanes on alkali-aggregate expansion (Ohama [45]).

Fig. 6.3 Influence of dual admixtures on alkali-aggregate expansion (Ramachandran).

Reduced concrete deterioration due to alkali-silica reaction in mixes in which Portland cement has been partially replaced by fly ash has been widely reported (Hobbs, 1986, 1989 Meland, 1986 Shayan et ai, 1996). Fly ash seems to act mainly as an alkali diluter, lowering the amount of available alkalis in the system. The capability to reduce the alkali-aggregate expansion may vary in different ashes, and depends on their own alkali content and fineness. 

Concrete may deteriorate if adequate precautions are not exercised to protect it from adverse effects that could result from exposure to natural or artificial conditions. Several physical, chemical, and electrochemical processes are known to induce cracking of concrete. Concrete can have durability problems as a consequence of its exposure to seawater, sulfates, chlorides, freeze-thaw action, carbon dioxide, etc., or when it is attacked by artificially induced processes such as exposure to acids and salts in chemical plants or to fire. In recent years, a new type of durability problem was encountered that involved use of steam cured concrete products. The distress was caused by the formation of delayed ettringite. If the raw materials in concrete are not carefully controlled, there may be an eventual failure of concrete elements, eg., the presence of excess alkali in concrete that promotes alkali-aggregate expansion reaction, harmful impurities in the aggregates, or the presence of excess amounts of dead-burnt MgO. Thermal techniques in combination with others have been employed with success to examine the raw materials as well as the failed concrete. The knowledge gained from such work has been applied to produce more durable concrete. 

Miscellaneous admixtures Those other than what have already been described and are known to impart many benefits to concrete. Examples are latexes, corrosion inhibiting admixtures, alkali-aggregate expansion reducing admixtures, pigments, pumping aids, shotcreting admixtures, etc. 

Kawamura, M., Takemoto, K., and Hasaba, S., Effectiveness of Various Silica Fumes in Preventing Alkali-Aggregate Expansion, Concr. Durability, (J. M. Scanlon, ed.), Vol. 2, SP-100 1809-1819 (1987)... 

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