The Study On Thermal Stress And Temperature Cracks Control Of Roller Compacted Concrete Gravity DAM In Severe Cold Area

Roller Compacted Concrete dam is a new type dam developed in recent 20 years. Cracks occur on many built dams, especially on RCC dams in harsh climate region. Cracks decrease the integrity, anti-seepage, durability and safety of concrete dams. Research indicates that thermal stress is the chief load leading to cracks in RCC dam, and stresses caused by other loads are smaller. Therefore, research on thermal stress during construction period and temperature control is very important to prevent cracks and guarantee the safety. Dam materials and structure type also play an important role in causing cracks.In harsh climate region, the air temperature changes greatly between winter and summer. RCC dam are usually placed without bulk and longitudinal joints, and left free in winter. Thus, it's unique thermal stress time-space distribution rule increases the difficulty of temperature control and crack control of RCC dam. About thermal stress and temperature control of RCC dam in harsh climate region, this paper emphasizes mainly on the following aspects:1. Research on space-time distribution rules of the temperature and thermal creep stress of RCC gravity dam in harsh climate region; analyzing the reason of formation and influencing factors of the horizontal cracks at the dam upstream and downstream face. This paper establishes a model simulating dam actual placing process and considering the non-continuity of initial temperature on the boundary of new and old concrete, daily air temperature change, concrete-placing temperature, hydration heat, heat insulation on boundary, water-storing process, placing intermission and curing with water. The calculation of stress field considers the concrete weight, static water pressure, thermal stress, different autogenous volume change between RCC and normal concrete, change of elastic modulus of concrete with age and creep action.2. Based on artificial neural network, thermal parameter feedback analysis model of RCC dam during construction period has been established. According to the observation data of RCC dam temperature, the thermal parameters were back calculated, and the results could meet the requirements of the project. Adaptive learning BP algorithm and Weight Decay Method are applied to training network. This makes the complication of network and the complication of actual system inosculated well and avoids over-learning. It is feasible that neural network is applied to the feedback analysis of RCC thermal parameters during construction period.3. Research on the crack control measures of RCC dam in the view of structure. In harsh climate region, mere exists obvious local stress concentration near the upstream and downstream of the exposed-in-winter horizontal surface of the RCC dam, and the surface of spillway bucket Though heat insulation is applied on the surfaces, tensile stress still exceeds the allowable one of concrete. This paper puts forward to adopt preset crack to avoid occurring more cracks. The special crack model is applied to simulate the dam preset crack, and the Joint Model of Fracture Mechanics is induced into strain-soften model. The research results werecompared to and verified with practical observation data, and it proved to be reasonable. The measure gives a new way to treat the cracks on the surface of exposed-in-winter placing layer of RCC dam in harsh climate region. 4. Research on the crack control measures in the view of material. In the dam foundation restraint area, MgO can be mixed into concrete to prevent dam longitudinal crack. Research on autogenous volume change of RCC mixed with MgO, and research on thermal stress compensation of darn foundation prove that RCC mixed with MgO plays a role in dam crack control.