| Abstract | The Concrete Microbar Test is modified after a Chinese test, for alkali-carbonate reactive aggregates, described in 2000. The protocol for the test is essentially the same as for ASTM C 1260, the accelerated mortar bar method except for the size of the bars, the grading of the aggregate, the water to cement ratio and the length of the test. The concrete microbars are 40 by 40 by 160 mm. The aggregate is graded to pass a 12.5 mm sieve and be retained on a 4.75 mm sieve. The water to cement ratio is 0.33. The length of the test is 30 days in 1 M NaOH at 80°C. The test results show that the method is applicable to both alkali-carbonate and alkali-silica reactive aggregates. Moderate correlation was found between the expansions measured in this test at 30 days, and in the concrete prism test (CSA A23.2-14A) at 1 year. When expansions in the concrete prism test and in the concrete microbar test are plotted on a graph the siliceous limestones fall on a separate line from all the other aggregates tested so far including the alkali-carbonate reactive aggregate from Kingston. Some tests were also carried out with some bars measuring 40 by 40 by 300 mm. The longer bars gave, on average, 22% more expansion. Alkali-carbonate reactive aggregates may be distinguished from alkali-silica reactive aggregates in this test by replacing a portion of the portland cement by a supplementary cementing material. The expansions of alkali-silica reactive aggregates are significantly reduced by the presence of the supplementary cementing material but expansion of alkali-carbonate reactive aggregates is largely unaffected. Opal does not appear to exhibit the pessimum effect in this test. It would be premature to establish expansion limits based on the limited number of aggregates tested to date. However, it is tentatively suggested that the expansion limit for siliceous limestones is ~0.13% at 30 days; the limit for all other aggregates is ~0.06%. |
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