Moisture-heat Coupling Effect And Damage Analysis Of Isothermal Drying Process Of Concrete

The pore water and migration of concrete have an important influence on the durability of concrete.Because concrete is a heterogeneous material,the properties of aggregate and mortar are different.During the drying process,the different thermodynamic properties of each component affect the overall mechanical properties of concrete.The evaporation and overflow of water will inevitably lead to damage or even cracking of the internal structure of the concrete.In order to study the law of temperature and humidity transmission and internal damage evolution of the isothermal drying process of concrete,a two-dimensional three-phase mesoscopic model of concrete was established,taking into account the thermal physical parameters and moisture content of the aggregate,mortar,and interface layer,and the parameters of wet physical properties and temperature.Correlation,push the heat-moisture coupling control equation,establish the PDE mathematical module to solve,and study the effect of concrete microstructure on the heat-moisture coupling result.On this basis,first calculate the stress distribution caused by the mismatch between the thermal expansion coefficient and the wet expansion coefficient of each component,Then define the damage variables,select the damage evolution equation,establish the isothermal drying heat-wet-force-damage coupling model of concrete by analyzing the relationship between the thermal parameters,humidity diffusion parameters,mechanical parameters and damage variables,and obtain the stress field and damage under the damp-heat coupling Field distribution.Then iterate the damage variables into the model to study the evolution process of each parameter under drying conditions,and provide theoretical support for the determination of the optimal drying process of concrete.The main work and conclusions of the paper are:(1)The free-form curve and surface deformation technology based on the scaling factor establishes a two-dimensional random parameterized aggregate model to generate mesosco-pic geometry of three-phase pebble concrete with different aggregate content,different aggregate gradation,different maximum aggregate particle size,and different interface layer thickness model.By considering the mesoscopic heat transfer numerical simulation of the temperature dependence of the thermal parameters of various phase materials,the macroscopic thermal parameters of concrete are obtained and compared with the experimental and theoretical results.On the basis of verifying the effectiveness of the numerical method,the mesostructure of the concrete is studied on the temperature field,and the influence law of macroscopic thermal parameters.The results show that the thermal conductivity and density of concrete decrease with the increase of temperature,the specific heat capacity of concrete increases with the increase of temperature,and the growth rate gradually slows down;the mesoscopic structural parameters have a great influence on the thermal conductivity and density of concrete,and the contrast heat capacity has little effect.(2)Using temperature and relative humidity as driving potentials,a transient model of heat and moisture coupling transfer in the isothermal drying process of concrete was established according to the laws of conservation of mass and energy,the coupling relationship between mesophase thermal and moisture parameters was analyzed,and the boundary conditions were determined.Based on the multi-physics coupling simulation software COMSOL custom partial differential equation module(PDE)to solve the variable coefficient partial differential equations.The accuracy of the model and the solution method is demonstrated by comparing with the HAMSAD verification example.Based on this,the influence law of concrete microstructure on the coupling result of moisture and heat is discussed.The results of the study show that as the drying time becomes longer,the drying rate slows down.The more aggregate content,the shorter the drying time;the greater the thickness of the interface layer,the shorter the drying time;the larger the particle size of the aggregate,the longer the drying time.(3)The coupling relationship among thermal parameters,humidity diffusion parameters,and mechanical parameters is combed to calculate the stress distribution due to heat and moisture transfer during the isothermal drying of concrete.Then,based on the elastic damage theory,the damage variables are defined,the coupling relationship between the thermal conductivity,humidity diffusion coefficient,elastic modulus and damage variables is established,and the damage field is solved.Finally,the evolution of the main parameters of each field with drying is studied.The results show that: during the drying process of concrete,the maximum tensile stress is distributed diagonally at the four vertices of the concrete specimen;due to the heterogeneity of the concrete,the first principal stress fluctuates greatly at the aggregate.The damage caused by drying occurs at the four vertices of the concrete,which is a diagonal evolution distribution;due to the large elastic modulus of the aggregate,the damage first develops in the mortar and finally extends to the aggregate;the distribution of damage and the first A main stress distribution trend remains consistent.As the damage accumulates,the elastic modulus and thermal conductivity also begin to decrease from the four vertices of the concrete and gradually decrease along the diagonal,which is consistent with the damage distribution trend.The evolution of the humidity diffusion coefficient has nothing to do with the mesostructure of the concrete.From the four boundaries,the uniformity increases.