P N OJHA, SURESH KUMAR, ABHISHEK SINGH, BRIJESH SINGH & B N MOHAPATRA


and certain clay minerals have also been reported to be alkali reactive. Depending on which minerals are involved, alkali-aggregate reaction is sub-divided into alkali-silica reaction and alkali-carbonate reaction. However, alkali-silica reaction is the more common form of alkali- aggregate reaction. Forms of silica that have disordered crystalline structure such as opal, cristobalite, moganite, chalcedony, flint, chert, tridymite, strained quartz, fragmented/ sheared/shredded quartz, are most susceptible to alkali-silica reaction[1,2]. The varying aggregate properties, particularly those related to the rate of the alkali reaction, make it necessary to apply standard testing methods and assessment criteria for aggregate reactivity[3]. Despite the slow course of reaction in the case of aggregates containing the aforementioned types of reactive minerals, the reaction leads to delayed concrete expansion and destruction[4]. In recent years, many researchers have noted an increase in new types of rocks which are alkali-reactive in concrete. Dolar-Mantuani[5] collected information and presented a list of potentially reactive rocks. In the case of potentially reactive aggregates, regarding the rock type as the only criterion is inadequate, particularly for assessing poly- crystalline, slow-reactive rocks. French[3] also suggests that a complete description of potentially reactive rocks is required. In a like manner, Jensen[6] claims that alkali reactivity of slow- expansive rocks may be estimated through microstructure analysis and well-known field observations, rather than based on a list of rocks derived from traditional classification. AAR test data[7,8,9] generally indicates that many aggregates found in the Pacific Northwest have a moderate to high AAR potential.


Various standard test methods are available to evaluate the alkali-aggregate reactivity of an aggregate. A few of the most popular methods are listed here:


(i) Chemical Method (ASTM C 289)[10]


This is a test method for evaluating the potential reactivity of aggregates (Chemical Method), ASTM C 289. Commonly called the quick chemical test, it estimates the potential reactivity of siliceous aggregate. In this method, aggregate is crushed and sieved to yield three samples of 25 grams each. This material is then reacted with an alkaline solution (1 N sodium hydroxide) at 80ºC (176ºF). After 24 hours the amount of dissolved silica from the aggregate, and the reduction in alkalinity of the solution, are measured. By plotting this data against a provided curve, it is possible to estimate reaction potential. The aggregate falls into one of three ranges: innocuous, deleterious or potentially deleterious. ASTM C 289 identifies highly reactive aggregates fairly reliably; however, it fails to identify slowly reactive aggregates. Also, certain aggregates have high amounts of soluble silica present, but produce only small expansions in service. Thus, the test does not always give reliable results (Stark 1993 and 1994). This method is also not usually applicable for testing carbonate aggregates, but is a helpful research tool which may be useful for the initial screening of aggregate. However, other tests should be relied upon to better define which aggregates are potentially reactive.


168 DAM ENGINEERING


Vol XXXI Issue 3


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95