ALKALI SILICA REACTIONS IN CONCRETE STRUCTURES OF WATER RESOURCES SECTOR

It is a well known fact that alkali aggregate reactions are one of the predominant causes of concrete deterioration. Swelling phenomenon in the form of expansion of the cement matrix by formation of ettringite due to sulfate-rich water and some external factors such as cycles of freezing and thawing or the attack on concrete by water in some forms such as extremely pure water, acidic water play a deleterious role for concrete durability. In alkali silica reaction, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment. These gels can swell and induce enough expansive pressure to damage concrete. Such damage has been reported in many countries, particularly in those with hot-wet climates. Quality of construction materials especially coarse and fine aggregates contribute towards durable structures. Several methods have been formulated to characterize the aggregates viz. mortar bar method, chemical method, petrographic examinations and accelerated mortar prism test methods. The present paper discusses the characterization of aggregate samples from various projects across the country as per chemical method, methods to prevent ASR in the hydraulic structures and some of the case histories from India and abroad where the alkali silica reaction was reported also have been mentioned.