Aggregates alkali silica reaction

Potential volume change of cement—aggregate combinations Accelerated detection of potentially deleterious expansion of mortar bars due to alkali-silica reaction... 

The different types of admixtures, known to reduce alkali-aggregate reactions, can be divided into two groups those that are effective in reducing the expansion due to the alkali-silica reaction, and those that lower expansions resulting from the alkali-carbonate reaction. For the alkali-silica reaction, reductions in the expansion of mortar specimens have been obtained with soluble salts of lithium, barium and sodium, proteinaceous air-entraining agents, aluminum powder, CUSO4, sodium silicofluoride, alkyl alkoxy silane,... 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

Alkali-silica reaction is an expansive reaction in concrete that can occur when a solution of sodium or potassium hydroxide reacts with a siliceous aggregate to form a gel of hydrated alkali silicate. 

It has been shown earlier that cathodic protection creates hydroxyl ions and will also attract positive ions such as sodium and potassium to the steel. This will increase the alkalinity around the reinforcing bar. In principle this could cause ASR or accelerate ASR in susceptible mixes. This has been demonstrated in the laboratory. However, there are no recorded cases of ASR being caused or accelerated by CP in field structures. In a review of field structures by SHRP, a structure with ASR showed no acceleration in the ASR in areas where CP was applied. The issue is also discussed in Mietz (1998). In the European standard on cathodic protection BSEN 12696(2000) Appendix A.5 states that cathodic protection applied in accordance with this standard has been demonstrated to have no influence on alkali silica reaction/alkali aggregate reaction (ASR/AAR). ... 

The deterioration of concrete can be the consequence of the presence of some aggregates components which, for example, as iron sulphide, decompose to give iron(lll) hydroxide and sulphuric acid [71]. This phenomenon will be presented later. Let us discuss now the studies of concrete deterioration mechanism caused by alkali silica reaction, the most important in practice. The two types of reactions can be distinguish ... 

Linking of the preventing action of mineral additions, with the sodium and potassium concentration lowering in the liquid phase, is obvious. The progress of alkali silica reaction is strongly dependent on this concentration, as well as on the related pH of this solution. Low alkali content is a warrant of the lack of concrete expansion with reactive aggregate. 

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. 

The undulatory extinction of strained quartz has become important in engineering geology as it serves as an indicator for aggregates to undergo Alkali-Silica Reaction (ASR). The amount of strained quartz found in the rock samples varied between 18 and 50% and their respective undulatory extinction angle varied from 16° to 36° Table 3. 

Alkali-silica reaction. The second destructive problem is attributed to a reaction of the silica-rich aggregates (e.g., cherts, obsidian, opal, quartzite), and alkalis present in the cement paste. In theory, any aggregate containing silica has the potential to participate in the alkali-silica reaction. 

Beside the alkali-silica reaction described above, similar phenomena may occur in the case of reactive dolomites and limestones. The so-called alkali-carbonate reaction (ACR) is less frequent and not completely understood. When the effects of ACR are observed, two similar remedies are also necessary either to keep the content of alkalis in concrete as low as possible, or to decrease the percentage of deleterious aggregate in the concrete mix. 

The characterisation of damage, e.g. in aggregates due to alkali-silica reaction (ASR) (this is discussed later)... 

Limestones generally do not contain sufficient reactive silica or silicates to cause expansion, and damaging alkali-carbonate reaction has rarely been reported. The reactions involving carbonate rocks can be either expansive or non-expansive and are more likely to occur when the limestone contains appreciable quantities of dolomite and clay minerals [8.1]. ASTM C586 [8.8] gives a test method for determining the potential alkali reactivity of carbonate rock aggregates. 

Side effects. During chloride extraction, hydroxyl ions are formed around the reinforcing steel, locally increasing the pH and sodium and potassium ions are enriched around the steel. These changes might stimulate aUcah-silica reaction (ASR, Section 3.4). In the framework of COST 521, the possibility of ASR was checked as a side-effect of chloride extraction [28,36,80,81]. The aggregates studied were reactive and the alkali content of the cement was just below the critical values. The results obtained with non-carbonated concrete showed that, under the worst conditions, chloride extraction induced concrete expansion, but no cracking was observed. 

A.B. Poole, Alkali silica reactivity mechanisms of gel formation and expansion, in 9th International Conference on Alkali-Aggregate Reaction in Concrete 2, 1992, pp. 782-789. 

The physico-mechanical properties of low-alkali Portland cements do not differ significantly from corresponding cements with normal alkali contents. Their marketing and use make sense only in regions with deposits of rocks that may be susceptible to alkali-silica or dedolomitization reactions, if employed as concrete aggregates. 

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