R.p Pathak.et al. Int. Journal of Engineering Research and Applications www.ijera.com ISSN: 2248-9622, Vol. 6, Issue 4, (Part - 1) April 2016, pp.46-52 www.ijera.com 46|Page Effect of Grain Size and Reaction Time in Characterisation of Aggregates for Alkali Silica Reaction Using Chemical Method R.P. Pathak 1 , Pankaj Sharma 2 , Kachhal Prabhaakar 3 , Sameer Vyas 4 , N.V. Mahure 5 , N Sivakumar 6 1-6 Central Soil and Materials Research Station, Hauz Khas, New Delhi-110016 ABSTRACT Concrete can deteriorate as a result of alkali aggregate reaction, an interaction between alkalis present in alkaline pore solution originating from the Portland cement and reactive minerals in certain types of aggregates. Potential reactivity of aggregates with regard to alkalis present in concrete mix can be determined by Mortar Bar method, Chemical Method and Petrographic analysis. Of these the chemical method though is quick and does not require a large quantity of material for testing yet have its own inherent limitations. It does not ensure completion of reaction as the observations are limited to 24hour only and also does not assess the effect of varying the combination of coarse and fine aggregates. A study on chemical method by allowing the reaction for a prolonged time up to 96 hours and also on different grain size ranged matrix was carried at Central Soil and Materials Research Station, New Delhi. Simultaneously the test results of the modified method are compared to the existing Mortar Bar method, Chemical Method and Petrographic analysis The outcome of the studies clearly reflects that the grain size play an important role in the reaction, the reaction time has a demarked impact on reactivity, in the cases having a high value of silica release the choice of reduction in alkalinity as an indicator of degree of reaction is not reliable, instead measuring remaining Na 2 O concentration in Sodium hydroxide solution after the reaction seems to be much more meaningful in justifying the silica release. Keywords: Aggregate, Petrographic, Mortar Bar Expansion, Alkali, Silica, I. INTRODUCTION Alkali aggregate reaction (AAR) occurs when there is interaction between the alkalis present in the pore solution and silicious minerals in some aggregates forming a calcium alkali silicate gel (1) . Potential reactivity of aggregates with regard to alkalis present in concrete mix can be determined by chemical method (CM) developed by Milenz and co-workers of US Bureau of Reclamation (USBR) in USA (2,3,4) . For determining potential reactivity of aggregates CM - ASTM C 289 -81 and IS 2386 (Pt VII) (5, 6) are being followed. These methods cover the chemical determination of the potential reactivity of an aggregate with alkalis in a Portland cement concrete as indicated by the amount of reaction during 24 hours at 80 o C between 1N NaOH solution and aggregate that has been crushed and sieved to pass a 300μm sieve and retained on a 150μm sieve. This chemical method has had a word wide success, because it is quick, easy to do and does not require a large quantity of material. However, it suffers from several defects like Non Representative Grain Size of Sample (150μm to 300μm) The material passing the 150μm sieve is discarded. The desired fraction is washed. The reactive silica present in amorphous state in the aggregates passes with the fraction smaller than 150μm. Washing of the desired fraction removes any traces of amorphous silica. Abnormality in Measurement of Reduction in Alkalinity (Rc) The Reduction in Alkalinity is the measurement of remaining OH - ions concentration after the reaction. The choice of measuring OH ion concentration is not correct as it is involved in two reactions. Apart from OH - ion the HCl titration using phenolphthalein indicator, is also influenced by some other anions such as carbonates, silicates, chlorides and sulphates. Inadequate Reaction Time Strained quartz is a slow reacting material, thus the reaction time of 24 hours is not adequate to ensure release of total silica content. Anomaly in Estimation of Nature of Aggregate in Siliceous Limestone Samples Siliceous limestone does not show deleterious nature because of low silica content compared to the very high CaCO 3 level and a very high Rc because of carbonates. RESEARCH ARTICLE OPEN ACCESS