The Pozzolanic reaction ratio of fly ash in concrete specimen taken from the 34 years old dam is determined by several methods. Experiments on laboratory prepared concrete and paste experiments have been carried out for 3 years as reference data for the Dam specimen. The reaction rates of fly ash in the paste are determined with elapsed time. Ca/Si and Ca/Al moler ratios of hydration products of pozzolanic reaction in the paste and the concrete are determined by EPMA (area analysis). The change of these moler ratios with time are correlated with the pozzolanic reaction rate of fly ash. It is determined that 50% of the fly ash content in the Dam concrete has undergone pozzolanic reaction in 34 years. Furthermore it is found that the evaluation of reaction rate of fly ash in old concrete structures using EPMA is convenient and practical.
