Development of Finite Element Analysis for Coupled Thermal and Stress Analysis for Conventional Gravity Concrete Dams

Abstract

During the process of setting-out and hardening in concrete, the temperature profile shows a gradual nonlinear distribution due to the development of heat of hydration of cement. At early ages of concrete structures, this non-linear distribution can have a large influence on crack evolution, especially in mass concrete such as that used in concrete gravity dams. It is thus important to study the factors affecting the amount of heat generated in hydration heat process trying to minimize it as much as possible in order to prevent the generation of undesired cracks through the dam’s body. A finite element analysis has been performed on a full scale concrete gravity dam to determine the impact of changing the time intervals in concrete placing schedule on the thermal/stress response of the dam. The significance of time, material and environmental factors has been scrutinized in this numerical investigation. These investigated parameters are the construction schedule of casting concrete, the cement content and the ambient temperature. Coupled finite element thermal-stress analysis has been adopted in the present study using Midas Gen Software. Mass gradient analysis using ACI 207-5R charts and equations gives an adequate estimation of using construction joints, through dam length. The methodology depends on minimizing the crack width especially at the restraint boundary near rock foundation. CHAPTER 6: Conclusions 146 Verification for hydration analysis has been conducted by simulating a mass concrete lift. By comparing thermal and induced stress of an early age concrete monolith as a part of experimental and numerical research program conducted by Ayotte at al.(1998). The finite element modeling technique gave accepted results, which reflected the real behavior of heat hydration and its mechanical consequences such as tensile stresses induced that was also compared to the experimental program conducted by Ayotte et al. (1998). Parametric study was performed to monitor the most significant factor that could decrease the maximum temperature induced inside dam body. Construction scheduling, environmental and material factors were studied; construction scheduling was performed with different time intervals. Intervals ranging between from 4 days to 16 days have been observed reaching to a conclusion that the maximum temperature of dam core decreased to 14% only by increasing the time interval between each cast to 4 times. Also, changing the construction sequence by dividing one cast to two adjacent casts to be casted simultaneously with time interval ranging from 2 days to 8 days did not have a significant difference than casting at once, as the rate of dissipated heat depends on the volume to surface ratio. The environmental factor presented in ambient temperature showed that a decrease in temperature of 37.7% has been noticed. Regarding material property presented in cement content; maximum reduction could be achieved is 17% when cement content decreased 40%.