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Alkali contribution by aggregates to long-term alkali-silica reaction development in concrete dams

pubblicazioni - Memoria

Alkali contribution by aggregates to long-term alkali-silica reaction development in concrete dams

Per meglio capire lo sviluppo a lungo termine della reazione alcali-silice nelle dighe in calcestruzzo è stata condotta un’apposita ricerca sperimentale, su tre differenti aggregati contenenti alcali, con l’obiettivo di valutare la cinetica del rilascio degli alcali in diverse condizioni di prova di lisciviazione (tipo di agente lisciviante e temperatura) e di prevedere il contributo di alcali che questi aggregati possono apportare al contenuto totale degli alcali del calcestruzzo, durante la vita in servizio della diga.

Deleterious expansion associated to Alkali-Silica Reaction (ASR) has been widely reported in the last decades as the main cause of deterioration for several concrete dams, in some cases even bringing their structural integrity into question. The time of the first expansion appearance is, on the average, about 20 years after construction and there is an increasing number of dams in which the expansions are continuing after 40 or more years. These expansions tend to increase the more the aggregate is alkalireactive and the higher is the concrete alkali content. There is some evidence that certain types of aggregates with alkali-bearing minerals (e.g. feldspars) may enhance the ASR in field concrete as a result of long-term alkali release from such aggregates, although the rate of alkali release and the relative mechanism are still debatable. In order to better understand the long-term ASR development in concrete dams, an experimental research has been carried out on three different alkalibearing aggregates, whose results are reported in this paper. The main objectives were to evaluate the rate of alkali release under different leaching test conditions (type of leaching medium and temperature) and to predict the alkali contribution by such aggregates to the total alkali content of concrete (LacT) during the dam service lifetime. The results of leaching tests in saturated calcium hydroxide solution and in deionised water did not show appreciable alkali release at the three temperatures tested (20°, 38°, and 80°C). On the contrary, an appreciable alkali release was obtained using the leaching tests in 0.7M NaOH solution (for the K release) and in 0.7M KOH solution (for the Na release), especially at 80°C. The experimental data from NaOH and KOH solution tests well fitted the van’t Hoff-Arrhenius equation relating the effect of temperature to the rate of alkali release. Assuming these alkaline solutions as sufficiently reliable simulations of the concrete pore solution and taking into account the specific test conditions used in this study (e.g. higher liquid-to-solid ratio (L/S=1 dm3/kg dry solid) compared to field conditions) it was likely to think that the leaching tests in NaOH and KOH solutions at 80°C over 90 days may be appropriate for predicting the alkali release from aggregates in concrete over a very long period of time (beyond 60 years for certain aggregates). Even if the amount of alkali released from the tested aggregates was a minor part (0.2-3.4 % Na2Oeq) of their total alkali content, for a typical dam concrete composition (2100 kg/m3 of fine and coarse aggregates and 200 kg/m3 of Portland cement), such a release corresponded to an alkali contribution of 0.41-0.55 kg Na2Oeq/m3 to the total alkali content, LacT, of concrete. This contribution can not be neglected in computing the value of LacT for a dam concrete, as it can represent more than 25% of the alkali contributed by a Portland cement with an alkali content of 1.0% Na2Oeq. Therefore, due to the slow alkali release from the aggregates, the alkali content in a concrete dam may progressively increase with time and cross the Threshold Alkali Level (TAL) of the aggregate used in the construction. As a result, the alkali-silica reaction is triggered and deleterious expansion starts to develop, in spite of the low concrete alkali content at the stage of dam construction.

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