Mesoscopic Numerical Model Of Thermo-hygro Cracking Process In Concrete

The influences of temperature and humidity on the mechanical behavior of concrete structure exist throughout its period of service.On the one hand,the change of temperature results the structure not only subjected to external loads,but also bearing the effects of thermal stress;on the other hand,as the environmental humidity is lower than that of concrete, the concrete would loss its water and then have a drying effect.The deformations of concrete caused by change in temperature and humidity significantly affect its stress state,and often leading to cracks which would influence the integrity and stability of structures.Therefore, investigations on the initiation and propagation processes of thermo-hygro type of cracks are helpful to understand the failure mechanism of structure,as well as providing the necessary guidance for engineering.Up to now,the classic macroscopic heat and mass transfer models, which characterize the concrete as a continuum material at macroscopic level,use the tested data at laboratory scale as input parameters to analyze the thermo,hygro and mechanical response of structures with greater size.This kind of homogenization is very important to study the stability of concrete structures.However,it is difficult to use these models to study the complete initiation,propagation and coalescence of fracture processes in concrete structures as they subjected to external loading,temperature and humidity variation.At present,the international investigations on thermo-mechanical behavior of concrete structure are prefer to set up the numerical model on the bases of mesoscopic level,which are able to reflect the variation of internal states of material with the change of environment.However, there are a few numerical studies on heat and mass transfer characteristics of concrete at mesoscopic level,especially lack of investigations on the initiation,propagation and coalescence processes of thermo-hygro type of cracks.Accordingly,in this paper,a numerical model is proposed to investigate the thermo-mechannical-damage coupling processes during the initiation,propagation and coalescenece processes of thermo-hygro type of cracks.The work in the present paper can be summarized as follows:(1) In order to reflect the heterogeneity feature of concrete at mesoscopic level,the concrete is firstly assumed to be a three-phase composite composed of matrix,coarse aggregates and bonding interface between them,and additionally the thermo,hygro and mechanical parameters of each phase are assumed to be conform to the Weibull distribution law.And then the mathematical model of thermo,hygro,mechanical interaction in concrete is established on the basis of heat and mass transfer theory.Additionaly,the parameters among these equations are discussed and calibrated for the following studies.(2) The damage mechanics is used to establish the relationship between damage processes and thermo-hygro diffusion of meso-elements.And then the numerical model,which is able to reflect the heterogeneity feature and failure processes of material,is proposed on the bases of finite element method at spatial scale and finite difference method at time scale.In order to speed up the computing efficiency,the numerical model use the paralle computing method on multi-core PC for two-dimensional model,while the three-dimensional numerical model use the LAN- and MPI-based parallel computing technology.(3) The numerical model is used to analysis the feature of thermal transfer in concrete at the mesoscopic level.The discussion of different thermo-mechanical performances between macro and mesoscopic level of concrete indicate that the mesoscopic-based numerical model is more efficient than that of macroscopic-based model to analsy the thermal behavior of heterogeneous materials.The crack growth processes of sample with multi-phases but different in thermal expansion coefficient indicate that heterogeneity of material significantly influence the behavior of concrete.In addition,the different crack growth processes casued by thermal stress and other external loads,as well as the temperature indication method in seepage monitoring of hydraulic structures engineering are also discussed.(4) The numerical mode is used to analysis the influences of humidity,temperature and coarse aggregate on the humidity diffusion in concrete;Studies about the influence of heterogeneity degree of material on its effective diffusion coefficient is well to establish the relationship of mass diffusion between macro- and mesoscopic level;The effects of existing crack on mass diffusion has also been carried out,and then the analysis of crack depth,as well as cracks caused by change of humidity effect on humidity diffusion process are disscussed; Finally,the cracks initiation,propagation and coalsescence process in mortar overlays caused by hygro-stresses are numerically simulated to reveal the failure mechanism of concrete structure as it subjected to changes in humidity.(5) The initiation,propagation and coalescenes processes of surface cracks at three-dimention caused by variation of temperature are numerically simulated,which is helpful to understand the formation of honeycomb cracks of solids.The effect of restriction condition and heterogeneity feature of material on crack growth mechanism is also studied by the three-dimensional numerical model.