Numerical Simulation On Temperature And Thermal Stress Of MgO Concrete Arch Dam

The concrete arch dam is a super-static structure subject to multi-directional constraints.The temperature will greatly affect the stress of the dam body,excessive changes in different temperature usually produce temperature stress that exceeds the strength of the concrete,which in turn leads to cracks in the dam body.That is the reason why analyzing and con-trolling the temperature becomes the key to preventing cracks in the dam.Traditional-tem-perature control-measures such as lowering the storage temperature,reducing-the-adiabatic-temperature rise,cooling water pipe,and surface insulation can effectively control the max-imum temperature rise of the concrete dam,reduce the temperature gradient,and reduce the temperature stress,thereby preventing the arch dam cracking.However,these temperature control and crack prevention methods mentioned above usually have the disadvantages of poor economy and large construction interference.In recent years,the method of preventing cracks in dam body by adding Magnesium Oxide(MgO)to concrete has attracted more and more attention.This method makes full use of the delayed micro expansion characteristics of MgO to reduce or compensate the shrinkage deformation of concrete,reduce the tensile stress of concrete dam,and then effectively reduce the possibility of dam cracking.In this dissertation,the temperature control simulation analysis and cracking prevention research are carried-out on concrete arch dams mixed with MgO through the four following main-contents:Firstly,the dissertation introduced the technology of controlling thermal stress and preventing cracking in concrete mixed with MgO.Secondly,an autogenous-volume-defor-mation model of MgO-concrete was established.This model was based on the experimental study of deformation characteristics,and also took-into-account the effects of temperature,age,and MgO content.Then the heat exchange experiment of the concrete surface under different roughness-conditions was studied,and the heat transfer coefficient model of the concrete surface affected by various factors was derived and established.Next,the study established the simulation calculation model of arch dam temperature control based on the background of the project.The effects of MgO content and heat exchange coefficient on the temperature and stress field of the dam body were studied.Finally,the construction process of the concrete arch dam was optimized by comprehensively considering the MgO content,the construction interval and the initial concrete placement temperature.The following re-sults have been achieved:1)An experimental study on the deformation-characteristics of MgO concrete was carried out,and the mathematical model of autogenous volume deformation of MgO concrete is developed.The model considers the relationship between temperature,age,MgO content and other factors on the volumetric deformation of concrete.Comparison with experimental test data shows that the model can accurately predict the aututogenous volume deformation of concrete with different MgO content with temperature and age,which can provide better parameter model support for temperature control simulation analysis of the construction scheme optimization of the MgO concrete arch dam.2)A heat exchange experiment of concrete surface under different roughness conditions was conducted,and the heat exchange coefficient model of concrete surface influenced by many factors was deduced and developed.The model comprehensively considers the effects of concrete surface roughness,wind speed,air humidity and thermal conductivity on the surface heat exchange performance,which can better reflect the heat exchange of the con-crete arch dam surface under complex boundary conditions,provide support for more accu-rate simulation analysis of dam temperature field,and provide the possibility for better cal-culation of concrete temperature stress and control of temperature cracks.3)Taking the Linxihe project as the background,a three-dimensional finite element method numerical simulation model was established to consider the thermal stress compen-sation efficiency of MgO concrete by different contents.The model considers two important parameters that have been previously studied:the AVD and the heat exchange coefficient and combined with consideration of influences from other parameters such as the environ-mental,construction methods,and mechanical parameters of materials.Overcoming the weaknesses of available commercial software,this model can simulate the thermal stress behavior of the dam affected by the AVD of concrete mixed with MgO.4)An analysis of the characteristics of temperature control measures,the working mech-anism of MgO concrete,and the role of MgO was included in the simulation of temperature control by the FEM model.The calculation results show that MgO has a better effect on improving the stress of the dam body,the higher the amount of MgO,the better the thermal stress of the control dam.Specifically,under the amount of MgO used(3-5%),when adding5%MgO,concrete has the lowest tensile stress and satisfies the allowable tensile stress,proving that MgO concrete can compensate shrinkage and control thermal stress,and pro-vide an effective method of limiting concrete cracking.5)The temperature-and-thermal-stress_-distribution of the concrete dam and its devel-opment over time depend on not only\the mechanical parameters of concrete and surround-ing environmental factors,but also the construction plan such as the initial placement tem-perature and construction time for a concrete layer.These two values are determined by numerical simulation results.Calculation results show that different construction diagrams will give different values of maximum temperature and thermal stress,as well as the time and location of the appearance of these values are also different.It also shows the variation between temperature and stress fields for various construction schemes.The calculation re-sults have once again confirmed that under different material conditions and control methods,the temperature and thermal stress of the concrete dam will give different calculation results.6)Analysis of the multi-objective combination optimization problem was conducted to consider the construction measures optimization under the effect of MgO content,construc-tion interval,and initial concrete placement temperature as the dependent variable of the optimization problem.Furthermore,the temperature and tensile stress are considered as ob-jective functions that the dependent variables must satisfy.Calculation results promoted finding solutions of temperature control and cracking prevention of concrete dams,provided support for more accurate effective concrete dam construction measures.The optimization results showed that to achieve the overall best effect,the MgO content is mixed into the concrete of 5%content,the construction time for a concrete layer is not less than 8 days,meanwhile,the initial temperature of a concrete mixture should be lower than 11~oC.The research results promoted the search for solutions to control temperature and prevent crack-ing of concrete dams,and provided support for more accurate effective concrete dam con-struction measures.