Petrographical investigation on alkali-reactive aggregates in Japan.

As the first step of investigation to avoid alkali-silica reaction, a study was carried out on the characteristics of reactive rocks in Japan, especially andesites of the Tertiary and Quaternary Periods.The test methods used were as follows.(1) Observation under a microscope(2) X-ray diffraction analysis(3) ASTM C 289 Standard Test Method for Potential Reactivity of Aggregates (Chemical Method)It was recognized that the rocks containing a large quantity of reactive silica (such as cristobalite, tridymite, and amorphous silica) were susceptible to alkali-silica reaction. Most of the andesites of the Quaternary were classified as ‘deleterious’. Many cracks and cavities, which occurred during the cooling of the magma, were observed at the inner part of andesite aggregates in deteriorated concrete structures.