Performance of Nevada’s aggregates in alkali-aggregate reactivity of Portland cement concrete

摘要

Alkali-aggregate reaction (AAR) is a form of distress that occurs in concrete and results in serviceability problems, cracks, spalling, and other deterioration mechanisms. There are two categories of AAR, namely, alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). Alkali-silica reaction is one of the most recognized deleterious phenomena in concrete, and has been a major concern since its discovery in the 1940s. The reaction which occurs between reactive silica or silicates present in some aggregates and alkalis of Portland cement produces an alkali-silica gel that expands in the presence of moisture resulting in concrete cracks. ACR is also a chemical reaction between reactive carbonate rocks and the alkalis present within the cement paste. Alkali-carbonate reaction is not as well-spread as alkali-silica reaction.The aim of the research study was to (1) examine the extent of the reactivity (expansion) of the aggregates quarried from fourteen different sources in the Southern and Northern Nevada, (2) investigate the ASR-induced losses in the unconfined ultimate compressive strength and stiffness of the concrete cylinders made with the trial aggregates, and (3) apply various mitigation methodologies to suppress the excessive expansion of the reactive aggregates.The reactive aggregates were identified by conducting different accelerated laboratory testings; namely, ASTM C 1260, modified C 1293, and ASTM C 1105. Afterward, a number of mitigation techniques were employed to control the adverse effect of alkali-aggregate reactivity. The selected mitigation methodologies included the use of the (i) Class F fly ash as a partial replacement of Portland cement, (ii) lithium nitrate in the mixing water, and (iii) combined Class F fly ash and lithium nitrate. Four different dosages of Class F fly ash; namely, 15, 2 0, 25 and 30% by weight of cement replacement were considered. Up to six dosages of lithium nitrate resulting in the lithium-to-alkali molar ratio (Li/Na+K) of 0.59, 0.74, 0.89, 1.04, 1.18 and 1.33 were used to suppress the excessive expansion of the reactive aggregates. For the third mitigation technique, a constant amount of lithium nitrate with a Li/Na+K molar ratio of 0.74 was combined with two dosages of Class F fly ash (15% and 20% by weight of cement replacement) to alleviate the excess expansion of the reactive aggregates that could not be controlled by the utilization of lithium nitrate with a Li/(Na+K) molar ratio of higher than 0.74.The laboratory test results reveal that the ASR-induced expansions depend on the aggregate mineralogy, soak solution type and concentration, cement alkalis, and immersion age. The ASR classification of the trial aggregates based on the late-age failure criteria of the test specimens produces more reliable and consisted results when compared to those obtained from the early-age failure criteria. The study also shows that the loss in stiffness due to ASR is more severe than the loss in compressive strength at both early and late-immersion ages. The optimum dosages of Class F fly ash or lithium nitrate in suppressing ASR-induced excess expansions of the reactive aggregates at different immersion ages varies depending on the aggregate mineralogy, ASR-induced expansion of untreated mortar bars, and the physio-chemical compositions of pozzolan or admixture used. The combined use of standard lithium nitrate dose (lithium-to-alkali molar ratio of 0.74) and a moderate amount (15%) of Class F fly ash (with a CaO content of 7.4% or less) as a partial replacement of Portland cement by weight is sufficiently effective in arresting the excess ASR-induced expansions of the investigated reactive aggregates at all selected immersion ages.