Study On Thermodynamic Evolution Of Concrete With Chemo-thermo-mechanical Couplings And Temperature Control And Cracking Prevention Method

Concrete has been widely used in civil engineering because of its unique advantages. However, the crack is often accompanied by the whole life of concrete structure. The prevention of crack in concrete has always been the focus of attention in engineering and academia. Recently, with the construction of a large number of high concrete dams and nuclear power projects, there is still no uniform understanding and mature research results about the complex aging effect on thermodynamic evolution of concrete, the precise simulation of temperature field of mass concrete with water pipe cooling, the formation and evolution mechanism of temperature crack and the simulation method of crack propagation process. The temperature control and crack prevention of concrete are still the key technical problems of concrete need to solve. In this paper, the numerical method is mainly used to study problems about the chemo-thermo-mechanical coupling effect on thermodynamic evolutions during the hydration process in concrete, the causes and influencing factors of early age concrete cracking, the precise simulation of temperature field, the formation mechanism of cracking induced by water pipe cooling and the crack evolution pattern of surface temperature crack in concrete.First of all, based on the basic characteristic that the free water is gradually transformed into the bound water during the hydration reaction in concrete, the three control equations of chemical, thermal and mechanical properties are analyzed according to the basic laws of thermodynamics. Hence, the complex relationships among the aging effect on the evolution of thermodynamic of concrete and the temperature variation process, the hydration degree and the external environmental factors are revealed, which also provide the basic data and for subsequent elastoplastic damage and fracture analysis of concrete.According to the thermodynamic evolution of early age concrete based on the chemo-thermo-mechanical couplings, the early age elastoplastic damage model in which the composite yield criterions used is developed to study the mechanism and causes of early age cracking of concrete. In the calculation, the Kelvin chain creep model is adopted to describe the influence of concrete creep deformation, which do not need to storage the stress-strain history. The age effect of mechanical properties caused by the creep deformation, temperature effect, self-desiccation shrinkage are also taken into consideration. The calculation results show that the elastic and plastic damage model based on composite yield criterion can predict change process of concrete stress more accurately; both creep deformation and creep caused by temperature change have obvious stress relaxation effect. However, the stress relaxation effect have different action mechanism at different age. The precise prediction and control of early age concrete creep deformation have significant influence on concrete crack prevention.Then, according to requirement and developing of the intelligent temperature control technology in actual engineering, an improved particle swarm algorithm for parameter identification of the Dittus-Boelter equation is proposed to improve the heat-fluid coupling method in the precise simulation of temperature field of concrete with water pipe cooling to solve engineering problems in this paper, which reveals the mechanism of heat exchange between cooling water and concrete from the physical perspective and reflects temperature gradient distribution. Based on the results of the heat-fluid coupling analysis, the optimization layout of cooling water pipe with double circulation is put forward in this paper to effectively improve the distribution of internal temperature and obviously decrease the internal temperature gradient and the maximum temperature tensile stress in mass concrete. Besides, the air cooling effect of concrete is also analyze by the proposed heat-fluid coupling method. The cooling effect calculated by the heat-fluid coupling method is almost the same as the equivalent method which taken the air cooling as the third boundary condition. The same cooling effect can be obtained both by water cooling and air cooling, as long as the appropriate air cooling parameters used. But compared with the water pipe cooling, the effect of air cooling will be weakened with the increase of the pipe length and it is not suitable to bury too long cooling pipelines. Moreover, air cooling can easily cause a large temperature gradient along the direction of cooling pipe, which is harmful to temperature control and crack prevention of concrete.In the simulation of cracking, it is difficult to reveal the physical mechanism of deformation and damage of material from the macroscopic scale because concrete is a kind of extremely heterogeneous and discontinuous composite material. In this paper, the mesomechanical method with the equivalent probability model is introduced to study temperature cracking of concrete. The parameters of the mesomechanical method and the calculation reliability are verified through uniaxial tension and three point bending beam numerical test. Compared with the conventional macroscopic method, the mesomechanical method can more accurately describe the internal micro crack and damage, which are the initial cause of macro cracking, as well as the crack's initiation and propagation and the whole developing process. In the same initial conditions and boundary constraint conditions, the temperature cracking induced by water cooling pipes mainly affected by heterogeneity of material, temperature gradient and overall drop range of temperature. To be more specific, the heterogeneity of concrete causes the existence of internal weak parts of material. The overall rage of temperature dropping in concrete in a direct reason of producing tensile stress by shrinkage. The temperature gradient generates the heterogeneous distribution of temperature stress in time and space, which causes stress concentration in concrete. The combination of the above three effect factors is the most dangerous situation to produce temperature cracking in concrete. Furthermore, the mesomechanical method can predict the appearance of damage area and time in advance, which has important engineering application values.At last, the combined finite-discrete coupling simulation method is proposed and the corresponding program is developed to study the surface crack propagation process of concrete based on the advantages of mesomechanical method in the simulation of temperature cracking problems. By inserting interface elements in the finite elements, in which the contact relationship is solved by the discrete element method, the temperature crack propagation process can be modeled by removing the complete damaged cohesive interface elements. Hence, the simulation results can intuitively reflect the surface temperature crack initiation and propagation process. Different temperature dropping processes of the external temperate outside the concrete surface have different influencing mechanisms of temperature cracking. Using the heat preservation material can effectively reduce the possibility of cracking of concrete and prevent the expansion of temperature cracking during a cold wave.