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


However, for aggregates CA-1 and CA-4, in which expansion was slightly above 0.2% and no alternate sources of coarse aggregates were available locally for construction of dams, the use of supplementary cementitious materials along with OPC was investigated to mitigate the phenomenon of potential alkali-aggregate reactivity. For coarse aggregate CA-1, the accelerated mortar bar test as per ASTM C-1567 on coarse aggregate sample CA-1 was carried out with cementitious combination of 80% OPC + 20% flyash + 8% silica fume. The expansion at 14 days after casting was found to be 0.02%, which is less than the prescribed limit of 0.1% (maximum), and is likely to produce acceptable expansion when tested in concrete (i.e. test method ASTM C-1293) and have a low risk of deleterious expansion when used in concrete under field conditions. Hence, it was suggested that coarse aggregate sample CA-1 should be used with the aforementioned cementitious combination for preparation of concrete in the field. For coarse aggregate CA-4, alkali-aggregate reactivity in combination with cementitious materials (OPC + flyash) and aggregate was determined as per ASTM C-1567 (Accelerated Mortar Bar Method). The flyash was used as 20% and 25% by weight of cementitious materials in the experiment, and expansion after 14 days was found to be less than 0.1% when replacement level was kept at 20%, which is termed as safe against alkali-silica reaction. Expansion was further reduced with the replacement level of 25% flyash. Hence, it was suggested that coarse aggregate sample CA-4 should be used with the aforementioned cementitious combination for preparation of concrete in the field.


5. Conclusions Based on the aforementioned studies the following conclusions were drawn:


i) Before using aggregates in mass concrete structures such as dams, in addition to their physical and mechanical properties, other performance characteristics such as their expansion potential due to alkali-aggregate reaction, which is directly related to the mineral composition, texture and petrogenesis of the rock particles that make up aggregates, should be evaluated.


ii) The aggregates which were found potentially reactive and deleterious from the accelerated mortar bar test either contained high amounts of amorphous silica in the form of quartz (CA-1 and CA-4), or high amounts of carbonates (CA-2) as confirmed from the petrographic studies.


iii) The mitigation of alkali-aggregate reaction in the case of doubtful and potentially less reactive aggregates to be used in dam structures can be achieved using supplementary cementitious materials such as flyash, silica fume, etc. in different proportions, as seen from the study. It is recommended that individual combinations of aggregate be evaluated with the proposed mitigative materials before acceptance of the aggregate for use in dams. These mitigative materials have generally been found to be effective in making many potentially reactive aggregates safe for use. However, dam authorities should carefully evaluate reactive aggregates before their approval for use.


Vol XXXI Issue 3 DAM ENGINEERING 181


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