Macro-mesoscopic Numerical Simulation Of Temperature Crack In Mass Concrete Based On The Heat-fliud Coupling Method

In recent years, along with a large number of high concrete dam project been built or implemented, the controlling of the temperature-induced crack in mass concrete is one of the main concerns in the design and construction of concrete dams. There are few mature theoretical results on the concrete cracking mechanism and dynamic expansion simulation method of temperature cracks so far. In this research, the heat-fluid coupling method, combined with the layer-merged algorithms, is introduced to perform the thermal analysis of a pipe cooling system in mass concrete structures, so as to precisely simulate the distribution of temperature. In the study of concrete crack problem, the heterogeneous mesoscopic mechanical modelbased on the random aggregate-interface-mortar three-phase concrete is established to actually simulate the whole crack propagation process, including microcracks development, visible cracks formation, and finally the eventual failure of concrete. And findings in this research could be of great significance for the development of cracking mechanism research of mass concrete and safety evaluation standardfor crack prevention.Heat-fluid coupling method can take into account the water flow characteristics well and simplify the pretreatment by pipes modeling. In this paper, the FE program based on heat-fluid coupling method to simulate the water cooling process in mass concrete is compiled. To verify the rationality and applicability of the heat-fluid coupling method, a comparison with several widely used methods is presented, by means of a2-D FE numerical example and a3-D FE numerical example. The simulation results indicate that the proposed method can accurately simulate the cooling pipe state, the temperature rise along the water flow, and the effect of directional changes of flow in temperature distributions. Thus, the prediction of the thermal fields associated with cooling pipes can be successfully used in practice.As for the heat-fluid coupling method, its pre-processing is very complex and calculation is too large. Therefore, this paper explores rapid ways to get a global-local consistency model of mass concrete, and compiles an automatic program which could mesh pipes and generate heat flow pipe elements quickly. Considering the calculation efficiency, herein takes layer-merged algorithms to carry out multi-scale simulation and analysis for the temperature stress field based on heat flow pipe elements. The results show that, on the premise of ensuring correct calculation results, reasonably using layer-merged analysis could greatly reduce grid scale and control mesh quantity while calculating, which can effectively reduce the time and improve efficiency. So it's possible to carry out large fine calculations.Concrete material, as is a kind of extremely heterogeneous and discontinuous composite material, is difficult to be studied from the macroscopic scale, which neglects concrete's complicate inner meso-structure and is hard to reveal the physical mechanism of deformation and damage of material. To research the cracking process and mechanism of mass concrete under the effect of temperature like pipe cooling, numerical experiments of concrete water process are taken with a three-phase heterogeneous model. Through analyzing numerical simulation results, crack propagation law is obtained. Through systematically analyzing several groups of mesoscopic numerical simulation results under different conditions, the generating mechanism of temperature crack is explored. In the same initial conditions and boundary constraint conditions, concrete temperature cracks are mainly affected by heterogeneity of material, non-uniformity of temperature, and overall range of temperature dropping. To be more specific, the heterogeneity of concrete causes the existence of internal weak parts of material; The overall range of temperature dropping in concrete is a direct reason of producing tensile stress by shrinkage; The non-uniformity of temperature generates the heterogeneous distribution of temperature stress in time and space, which causes stress concentration. The combination of above three effect factors is the most unfavorable to produce crack in the concrete interior. According to the characteristics of cracks formation, the key points of mass concrete temperature control and crack prevention are summarized below:The pouring quality in concrete construction should be critically controlled; To ensure the safety against concrete cracking, it is effective to strictly control the range of temperature dropping on the whole and control the temperature gradient locally to guarantee a uniform distribution of temperature.On the basis of the mesoscopic numerical simulation of concrete, a research idea about anti-crack safety factor is proposed. On account of traditional anti-crack safety factor, the temperature non-uniform coefficient, material non-uniformity coefficient and safety coefficient are adopted, which can consider the influence of the temperature non-uniform, material non-uniform and the real structural safety, separately. In addition, the critical cooling rate and the critical anti-crack safety factor are derived for the28d and90d aged concrete under water cooling effect. The critical cooling rate shows a decreasing trend with the increase of the pipe temperature difference. Overall, the new safety factor, to some extent, reflects the influence of water temperature and concrete non-uniformity property, and also is a supplement for the traditional anti-crack safety factor. A macro-mesoscopic model is introduced to perform the temperature crack simulation in mass concrete structures in complicated conditions, to further discuss the suitable pipe temperature difference in final stage cooling. Macro-mesoscopic method is proposed for the simulation of temperature crack under complicated boundary conditions in mass concrete, so as to be used as the basis for further study of anti-crack safety standards.