ALKALI-AGGREGATE REACTIVITY EVALUATION OF AGGREGATES FOR APPLICATION IN INDIAN CONCRETE DAMS
(ii) Petrographic examination (ASTM C 295)[11]
Mineral composition is a good indicator of potential aggregate reactivity. The Guide for Petrographic Examination of Aggregates for Concrete, ASTM C 295, is used to determine mineral composition and form. Petrographic examination is basically a qualitative analysis, and should be used as a preliminary examination for aggregates. A petrographic evaluation of an aggregate sample by an experienced petrographer will identify potentially reactive minerals in an aggregate sample. It gives quick results to help predict possible aggregate reactivity, but is unable to provide quantitative information about the aggregate’s actual behaviour in concrete. Correlating the results of a petrographic analysis with the service record of the aggregate in concrete is very useful. The results of a petrographic analysis will not reveal whether an aggregate will cause deleterious expansion in concrete. This needs to be evaluated using other test methods.
(iii) Accelerated Mortar-Bar Test (ASTM C 1260)[12]
Assessing aggregate reactivity by accelerated methods is becoming increasingly common. The shortcomings of ASTM C 227 and C 289 prompted development of a rapid test method for assessing the potential reactivity of aggregate. ASTM C 1260, a test method for the potential alkali reactivity of aggregates, offers a relatively rapid test to supplement lengthier test methods. It involves measuring the length change of mortar bars made with sample aggregate stored in a strongly alkaline solution at an elevated temperature. This test measures potential aggregate reactivity, not reactivity of specific cement-aggregate combinations. ASTM C 1260 classifies an expansion of 0.10% or less, at 14 days, as innocuous behaviour. Expansions of more than 0.20% are indicative of potentially deleterious expansion. Between 0.10% and 0.20%, aggregates may exhibit either innocuous or deleterious behaviour in field performance. ASTM C 1260 (AASHTO T 303) should be used as a screening test in conjunction with other tests to determine the potential reactivity of an aggregate. ASTM C 1260’s severe test conditions make it useful for identifying slow reacting aggregates, which may not be identified by ASTM C 289.
(iv) Concrete Prism Test (ASTM C 1293)[13]
The Standard Test Method for concrete aggregates by determination of length change of concrete due to alkali-silica reaction, ASTM C 1293, measures the expansion of concrete prisms made
with the fine or coarse aggregate in question, and a Type I high-alkali (0.90% ± 0.10% Na2O equivalent) cement plus added NaOH to simulate a 1.25% Na2O equivalent cement. Concrete proportions are strictly defined, including a water-cement ratio of 0.42 to 0.45. Measurements
of concrete prisms are taken at different specified intervals for 12 months. This standard offers the following interpretation of results: potentially deleterious behaviour is indicated if the one-year expansion is greater than or equal to 0.04%. ASTM C 1293 is currently considered the most representative test method when compared to structures in the field. C 1293 should be used to supplement results obtained from C 289, C 295, C 1260 (AASHTO T 303), or
Vol XXXI Issue 3 DAM ENGINEERING 169
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