Alkali-Aggregate Reactivity Evaluation of Aggregates for Application in Indian Concrete Dams


P N Ojha(1), Suresh Kumar(2), Abhishek Singh(3), Brijesh Singh(4) & B N Mohapatra(5)


Abstract: The availability of aggregates suitable for use in construction and maintenance of dams is a continuing concern to the engineering and resources-management community. In addition to the physical and mechanical properties of an aggregate, its other performance characteristics specifically depend on its mineral composition, texture and petrogenesis of the parent rock from which the aggregate originated. These properties can result in deleterious chemical reactions such as alkali-aggregate reaction when aggregate is used in concrete mixes having cements with high alkali content, which can pose a major problem for concrete structures, reducing their service life and requiring expensive repair or even replacement of the concrete. In the present study six different types of coarse aggregates, which were proposed to be used in different concrete dams, were collected from different sources across India. These coarse aggregate samples were subjected to petrographic and mineralogical examinations for evaluation of their modal compositions. The potential of alkali-aggregate reactivity (expansion potential) of all six coarse aggregate samples was evaluated using ASTM standards. The study covers the mitigation measures adopted for coarse aggregate samples which were found to be potentially reactive, as per ASTM standards, and no alternate sources of coarse aggregates were available locally for construction of the particular dam in which the aggregate was proposed to be used.


Keywords: Alkali-aggregate reaction, mineralogy, petrography, modal composition. 1. Introduction


Alkali-aggregate reaction is a chemical reaction which occurs in Portland cement concrete, involving alkalis which are present in the pore solution of concrete and certain types of mineral phases present in the aggregates used to prepare the concrete. The typical pH of pore solution in concrete (except cover region) is about 12 to 13. Several minerals are easily soluble at a pH greater than 9 to 10 and thus are highly susceptible to attack by alkalis. Many natural aggregates contain some quantity of amorphous silica, which react with the alkalis present in the concrete mix. Apart from various forms of amorphous silica, some carbonate species, zeolites


Author Details: (1)Joint Director & Head, (2)Manager, (3)Project Engineer, (4)Group Manager, (5)Director General, National Council for Cement & Building Materials, Ballabgarh, Haryana, India.


Vol XXXI Issue 3 DAM ENGINEERING 167


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