Effect of the casting defects on the ageing of the natural draught cooling towers

摘要

Natural draught cooling towers often develop cracks as a consequence of the ageing of concrete. In situ observation indicates that the degradation rate depends on the quality of the tower construction. The towers presenting segregation or zones with increased porosity are damaging faster. Aware of this phenomenon, EDF is involved in several research projects, in order to assess the damage processes and to propose new concepts for the extension of the service life. This contribution studies the effect induced by a casting defect on the long time behaviour of the structure. This defect is represented by a locally reduced value of the Young modulus and by a greater permeability of the concrete. The mechanical behaviour of a cooling tower was studied under the service load induced by temperature and moisture cycles. The concrete is represented as a non-saturated porous media according to thermo-hydro-mechanical theory of Coussy. Moisture migration is described by non-linear diffusion equations, and liquid and gaseous transport mechanisms are taken into account. The mechanical state of the concrete is evaluated in the frame of the poroelasticity theory. The modelling is achieved using Code_Aster~®, which is a free FE code developed by EDF (www.code-aster.org). The results consist of in time evolutions of moisture gradient, strain and stress across the tower section. The simulation confirms that cracking occurs in the thicker parts of the tower as observed on real structures. The presence of a casting defect leads to increase deformation and damage in the surrounding zones, as a result of the reduced rigidity. The increase of the permeability of the defect seems to have no effect with respect to cracking but hydraulic equilibrium is reached faster. Further research is needed on this subject in order to improve the modelling of the concrete behaviour under cyclic loading (wind loading, moisture variations), the effective damage of concrete induced by transport phenomena and to take into account the real initial state of the building.