Study On The Thermal Cracking Mechanism And Simulation Analysis Method Of Concrete Dams

In recent years, high concrete dam construction entered a new stage of development in China, a group of high concrete dams around300m height have been built. There are many difficult technique problems in the high dam construction. The thermal cracking analysis of dam concrete is one of the key technical problems and difficulties. Because there is no mature design method, technique, specification, experience for reference, to ensure the safety of the project, the research of this problem should be studied and the thermal cracking mechanism of high concrete dam should be explored, which has important scientific value and economic and social benefits.In this dissertation, the thermal cracking mechanism and simulation analysis method of concrete dams are carried on the deep discussion. The research on thermodynamics and meso-mechanics, fracture mechanics analysis of structure, spatial and temporal simulation of concrete dams, and the stability analysis of dam body cracks have been achieved. The dissertation mainly contains following contributions:(1) The cracking analysis methods of concrete dams, macro-mechanical constitutive and the meso-mechanical damage fracture of concrete material, the significance of spatial and temporal simulation of concrete dams and the research achievements of domestic and oversea are reviewed in detail. Some critical scientific and technical issues to be explored in the research are put forward, and the dissertation research contents are introduced.(2) Considering the temperature rise of the cooling water in the pipe and the dynamic simulation of pouring block of concrete dams, the composite element method (CEM) for simulation and feedback analysis of concrete structure containing cooling pipes is proposed. The CEM allows for the cooling pipes embedded in composite elements for discrete simulation. The method does not need to refine or change the analysis mesh, and the pre-processing of the CEM is quite simple. Then, the CEM is used to simulate the cooling effect of a concrete cylinder and a multi-lifts concrete model. Finally, the method is applied in the simulation and feedback analysis of the temperature field of the Xiaowan22#dam monolith during the construction period. The research shows that the method proposed in this dissertation is suitable for the simulation and feedback analysis of the temperature field.(3) The meso-mechanics research of concrete material is carried out, the implementation of building, packing and validity checking of spherical aggregates numerical simulation model has been achieved. The element cutting method is proposed in this dissertation for the mesh generation of the three dimensional numerical specimen of concrete. Then, the generation of random convex polyhedral aggregate model is realized and the equivalent elastic modulus of three dimensional numerical specimen of concrete is calculated. The viscoelastic damage theory is used as the criterion for three dimensional mesoscopical damage of a concrete specimen under uniaxial compression condition. Finally, the whole stress-strain of the mechanical test of concrete standard cube specimens is carried out, and the damage process of concrete specimens is revealed.(4) The composite element method is used to conduct the static crack analysis. The element containing cracks is defined as a composite element, which is divided into sub-elements by the crack plane. The displacement of the crack composite element is interpolated by the multiple nodal displacement. The remarkable advantage of the composite element method is that the crack can be simulated explicitly while the difficulty of mesh generation containing cracks can be avoided, which provides significant convenience for pre-processing of the crack analysis. The stress intensity factors (SIFs) at the crack tip are calculated, at the same time. The calculation precision of the composite element method is verified to the finite element method and the analytical solution by numerical examples, which shows the proposed method is suitable for the static crack analysis.(5) The dynamic fracture analysis of structure based on the composite element method is presented. To implement the dynamic propagation of cracks, the maximum circumferential stress criterion is used as the propagation criterion, and the cracking direction at the crack tip is calculated. The difficulties in pre-processing are reduced by eliminating mesh refinement or mesh modification in the dynamic propagation simulation. Numerical examples show the superiority of the composite element method for crack dynamic propagation analysis.(6) Spatial and temporal simulation method of concrete dams is studied. Firstly, the simulation method of the temperature field and the stress-strain field of dam body, excavation-induced relaxation field of rock foundation are introduced. Then, key simulation technologies of high concrete dams are introduced also. Finally, the Xiaowan project is taken as an example, the large scale three-dimensional finite element model of Xiaowan project is established and the simulation analysis of the whole process of Xiaowan concrete dam during construction and operation period is carried out. The research results show that the temperature, deformation and stress are close to the monitoring data, and reveal the spatial and temporal evolution of the temperature field and the stress-strain field of the arch dam. The whole working state of the arch dam and foundation is evaluated base on the above research.(7) The implicit crack element method is used to simulate the effects of cracking process. Firstly, the causes of the dam body cracks are analyzed based on the analysis of the actual anti-cracking performance of concrete material of the dam body, the simulation of the temperature field and the stress-strain field during the construction period. The main reason of the crack initiation and propagation in the Xiaowan arch dam has been confirmed to be the high tensile stress caused by the temperature drop during the second stage post cooling process before joint grouting. Then, the working state of the existing cracks at the high water level is analyzed. Finally, combined with fine synchronous simulation method, two typical dam sections, the21#and25#dam monolith, are selected to carry out the sub-model analysis. Analysis results show that the crack is under pressure during the water storage process, and there is a decrease of the stress intensity factors at the crack tip, and the temperature cracks in the Xiaowan arch dam are proved to be compressed, closed and stable.Finally, some researches to be explored in future are put forward after summarizing research achievements of the dissertation.