The presence of secondary silica minerals had been known to be responsible for the reactivity of common aggregates for 40 years or more, and the first comprehensive assessment of certain Indian aggregates was made from this chemical point of view19. Accordingly, the following common rock types were identified as potentially reactive on the basis of their composition as well as results of the rapid chemical test (ASTM C289) and mortar bar expansion test (ASTM C227): 1
T RAP ROCK
Figure 11.13 Types of natural aggregates commonly used in India (schematic map-not to
(2) Sandstones containing secondary silica minerals such as chalcedony, crypto — to microcrystalline quartz, opal and quartzites having a reactive binding matrix and occurring in Madhya Pradesh, West Bengal, Bihar and Delhi.
(3) Granites and pigmetites containing opal, rhyolites and glasses and occurring in south India, notably Tamil Nadu and Karnataka.
(4) Trap aggregate containing reactive constituents occurring in Jammu and Kashmir and Deccan Plateau.
Since reactivity of aggregates due to the presence of secondary silica minerals was known at the time some of the major concrete dams were built in India in the early 1950s, such aggregates were used with due caution. Bhakhra Dam in Punjab, which was the highest concrete gravity dam in the world when completed, used river gravels composed predominantly of quartzites,
metasandstones, greywackes and sandstones, which contained cherts, chalcedonic sandstones and glassy andesites—2.3% of the total mass on an average-as well as limestone and dolomites. Similarly, the natural sand contained less than 2% cherts. These aggregates were proven to be innocuous after exhaustive laboratory evaluation and were used without any detrimental effect till now20. In the case of Hirakud Dam spillway, the quartzite river shingles containing cherts and chalcedony, which proved to be potentially reactive in laboratory evaluation, were used inadvertently.
Gogte in 1973 postulated that the reactivity of common aggregates could also be due to mineralogical and textural features of the crystal rocks, in which the commonly accepted susceptible forms of silica, such as opal and chalcedony, were absent2. He ascribed the reactivity of such aggregates to the presence of strained quartz. As a modification of the then existing test procedures of the Indian Standards (IS) and ASTM, Gogte recommended that mortar bar tests be carried out at a temperature of 50°C and suggested a criterion of mortar bar expansion above 0.05% in 6 months as indicative of reactivity. Accordingly, Gogte identified a number of granites, charnockites and quartzites and schistose rocks, mostly from Andhra Pradesh, Karnataka and Tamil Nadu in south India, as well as basalts from Maharashtra and Gujarat in western India, as being potentially reactive because of the presence of strained quartz. In most of these rocks, samples showed strongly undulatory, fractured and granulated quartz, nearly 35-40% of quartz grains showing a UE angle between 18 and 20°. On the other hand, rocks which contained less than 20% strained quartz, or in which most of the quartz showed uniform or faint undulatory extinction, were considered ‘innocuous’. Sandstones from Andhra Pradesh, Rajasthan, Himachal Pradesh and Madhya Pradesh owed their reactivity to presence of cherts as a detrital constituent and sometimes as binding matrix. Sandstones devoid of cherts but containing a few grains of strongly undulatory quartz showed expansion in mortar bar tests within tolerable limits2.