ALKALI-AGGREGATE REACTIVITY EVALUATION OF AGGREGATES FOR APPLICATION IN INDIAN CONCRETE DAMS


Muscovite grains are also uniformly distributed in the sample. Subhedral orthoclase grains with corroded margins are partially shattered. Subhedral to anhedral iron oxide grains with corroded margins are randomly distributed in the sample. Iron oxide grains are brittle and fragile in nature.


(a)


(b) Figure 6. Micrographs showing distribution of mineral grains in CA-6 sample


4.3 Evaluation of potential alkali reactivity of aggregate using accelerated mortar bar test: as per ASTM-C 1260


Alkali-silica reaction and alkali-carbonate reaction (together known as alkali-aggregate reaction) are basically chemical reactions between certain reactive minerals in aggregate and alkalis in the pore solution of concrete. This reaction can result in expansion and cracking of the concrete, leading to a shortened service life. To determine whether an aggregate is potentially reactive, ASTM C1260 Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method) is often used as a quick screening test. Accelerated mortar bar test, as per ASTM C-1260, was carried out for all the coarse aggregate samples. The accelerated mortar bar test consists of preparing mortar bar 25mm × 25mm × 285mm (having gauge length of 250mm) in the same way as for conventional tests, as per IS: 2386 (Part VII)[19], i.e. by proportioning one part of Ordinary Portland Cement (OPC) to 2.25 parts of graded aggregates by mass and a fixed water to cement ratio, i.e. 0.47. After 24 hours the samples were de-moulded and cured in hot water at 80ºC for 24 hours. Finally, the specimen was stored in 1N NaOH solution at 80ºC for 14 days. The length change observations were taken under hot conditions, i.e. within 15+5 seconds after removal from the solution. The samples were stored in plastic containers; the use of glass or metal container for this purpose is not recommended as they will be corroded by the NaOH solution. As per ASTM criteria, the aggregate showing expansions less than 0.10% at 16 days after casting are classified as innocuous, whereas the aggregates showing more than 0.20% expansion are


Vol XXXI Issue 3 DAM ENGINEERING 179


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