Mass change, petrography and damage ratings of bars at the completion of the concrete microbar test

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ConferenceProceedings of the 12th International Conference on Alkali-Aggregate Reaction in Concrete: 15 October 2004, Beijing, China
Pages257265; # of pages: 9
Subjectalkali-aggregate reactivity, test methods, mass change, petrography; Concrete
AbstractDuring the course of the concrete microbar test the mass changes of the bars were measured at the same time as the lengths. The mass changes of the bars are characteristic of the petrography of the aggregates. Microbars made with ?inert? alumina grinding balls showed continuous mass gain for 50 days, but virtually no expansion. In contrast, microbars containing 50% opal and a non-reactive dolostone exhibited mass loss for about 10 days that was proportional to expansion. Thereafter, the mass loss leveled off. The gel formed by the reaction migrated into the NaOH solution where it precipitated. The greywackes and sandstones, in which microcrystalline quartz is the reactive component, exhibited a mass gain of about 0.4% in the first 8 days followed my mass loss. Only one unaltered volcanic rock, an andesite, was evaluated. It showed mass gain for 15 days but thereafter showed no significant change. Cristobalite is the main reactive component in the andesite. The mass change graphs of the gravels reflected the composition of the major component of the gravel. The damage rating indices (DRI's) were determined at the end of the tests. All the microbars showed extensive fine cracking, without gel, in the cement paste. This pattern of cracking is quite different from that observed in concrete prisms or in cores from field concretes. No correlation was found between the microcrystalline quartz content of many aggregates and expansion in the microbar test. The lack of correlation between expansion in the concrete prism and concrete microbar tests raises serious concerns about the advisability of using the results of the microbar test to evaluate the potential expansivity of aggregates for use in concrete without first confirming the results in a concrete prism test.
Publication date
AffiliationNRC Institute for Research in Construction; National Research Council Canada
Peer reviewedYes
NRC number47358
NPARC number20377796
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Record identifierb631d6dc-c93c-4a02-ac54-6eb5156b8f7a
Record created2012-07-24
Record modified2016-05-09
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