Alkari-silica reactions have a harmful effect on some andesites. In the present paper, the alkali -silica reactions in andesitic rocks were examined by means of optical and X-ray diffraction analyses and the standard test with mortar bars (ASTM C227). Furthermore, by the microscopic examination of the dyed thin section made from a mortar bar, various types of reaction products including cracks and spherules were identified and distinguished. The contents of alkari-reactive minerals such as cristobalite, tridymite, and glass, which are present in andesites, are irregular and change very much with the location. The results of the test on mortar bars had no direct correlation with any kind of alkali -reactive minerals . It is difficult to determine the role of these alkali-reactive minerals in the expansive alkali-silica reaction, because there are many factors in the reaction.

The mortar-bar expansion test (ASTM C227), chemical test (ASTM C289) and measurement of specific gravity were performed on the aggregates of which the mineral characteristics were discussed in the previous report. The cement used for the mortar bar test was an ordinary portland cement with an alkali content of 1.6% (equivalent Na2O) adjusted by NaCl reagent. The results of the tests on 27 mortar bars showed that the rate of expansion was in the range of 0.02-0.65% in 6 months. Twenty -one aggregates were classified as being reactive according to the ASTM criteria . In the chemical test, the determined concentration of silica in solution ranged from 190mmol/l to 670mmol/l. The reduction in alkalinity of the NaOH solution ranged from 65mmol/l to 290mmol/l. All of the 36 aggregates belonged to the reactive group. The specific gravity of the aggregates was in the range of 2.19 -2 .68. Although no relationship was observed between mortar bar expansion and the result of the chemical test, a negative correlation was found between mortar bar expansion and specific gravity. To prevent excessive expansion, the isopleth map of the rate of expansion was drawn based on the test results on a number of mortar bars to cover variables in alkali content of the cements and in mixing ratio of the reactive aggregate of andesite and the nonreactive aggregate of greywacke.