Effect of heat generation from cement hydration on mass concrete placement

Construction of a concrete dam requires large volume of concrete. Due to the small surface area-to-volume ratio, concrete dams are often subjected to high potential of thermal cracking, caused by the heat generation from cement hydration. To reduce the thermal cracking and ensure dam structure safety, a concrete dam is often constructed with low heat-generating cement and separated blocks having relative thin lift thickness. In Korea, low heat Type IV has been mainly used to reduce the temperature rise. Although benefits of using supplementary cementitious materials (SCMs) in concrete have been recognized, the practice of SCMs in dam concrete is not very common due to a concern for safety. On the other hand, increasing lift thickness of concrete blocks to accelerate dam construction has always been a demand. The purpose of this research is to explore the potential use of fly ash as Type IV cement replacement in Korea concrete dams and to determine the proper lift thickness of concrete blocks in dam construction. In the present study, the chemical compositions and fineness of cement and fly ash are characterized. The heat hydration of fly ash replacement for Type I cement and Type IV cement are studied and compared. The temperature distributions and thermal cracks of a concrete block having four different lift thicknesses (1.5m, 2m, 2.5m, 3m) are analyzed using FEM commercial software ANSYS. The results indicate that 40% fly ash replacement for Type I cement shows a similar heat generation and compressive strength at 28 days with that of Type IV cement and also has cost savings of 25%. A lift thickness equal or less than 1.5m showed little potential for thermal cracking. Construction placement with a lift thickness greater than 2m had high probability of thermal cracking