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- Malm, Richard, et al.
(författare)
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Cracking in the concrete foundation for hydropower generators : Analyses of non-linear drying diffusion, thermal effects and mechanical loads
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An extensive program for improvement of the hydropower plants in Sweden is currently on-going. The aims are to secure future production and to maintain and further develop an already high dam safety.During inspection, cracks were discovered in the concrete foundation, near the stator and rotor spider supports, at some hydropower stations in Sweden. The cracks were believed to be related to the function of the stator supports and to new patterns of generator operation. In earlier times, the generators ran continuously, while nowadays there are many stops and starts, sometimes even several times during one day. The objective of this study is to understand the complex interaction between the power generating system (stator, rotor, turbine, etc.) and the supporting concrete structure. It is important from a dam safety perspective to determine the causes of the structural cracks that have been found in-situ.A three dimensional non-linear finite element model has been developed in order to analyse formation and propagation of the cracks. Several different load effects have been studied in this project in addition to the mechanical loads during operation. The new pattern of generator operation with several starts and stops lead for instance to variations in temperature which have been studied. Besides this, the uneven drying shrinkage of concrete has also been studied in this project. Thereby, the structural behaviour of a concrete foundation for the power generating system has been analysed taking into account the transient thermal and moisture gradients in combination with dead loads and some of the operational loads imposed to the foundation.The analyses shows that reinforced concrete structure that constitute a support to the generator is subjected to cracking due to the loads considered in this study, where the cracks near the supports are caused by a combination of mechanical loads, long-term drying shrinkage and temperature variations. The analyses showed that even after 20 years, the moisture content in the centre of the thicker part in the concrete foundation still had a high relative humidity. At the same time the concrete close to the free surfaces and the slender parts of the concrete foundation had reached the same relative humidity as the environment. Thereby, a large difference in drying shrinkage is obtained between different parts of the concrete foundation and thereby large forces due to restrain. The analyses showed that the drying shrinkage induced cracking inside the concrete foundation and especially close to the supports of the stator and the rotor spider which coincides with location of the cracks found in-situ.The results show that the cracks found in-situ can be simulated and explained with advanced numerical methods. The results also indicate that the dynamic effect from the loads caused by the power generating system have to be studied further, since a reduced structural stiffness due to cracking may result in larger loads imposed on the structure from the magnetic eccentricity and turbine imperfections or alternatively lead to a fatigue failure of for instance the reinforcement.
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