Study On Meso-scopic Model And Analysis Method Of Concrete

Concrete structure is one of the engineering structures and it has been utilizedwidely in civil engineering constructions. The security and performance of theconcrete structure should be closely associated with its important part, i.e. theconcrete material. Compared with those of steel reinforcement material, the physicaland mechanical properties of concrete are much more complicated, and there are stilllots of efforts needed to be solved. Consequently, they have being the very importantproblems in the studies of concrete structures.Concrete is a typical composite material, whose physical and mechanicalproperties are determined by the ingradients, proporttions, mechanical properties andthe bonding effects of the two major components invloing coarse aggregates andhardened cement mortar. Accordingly, to describe the physical and mechanicalproperties of concrete, the micro-/meso-scale analysis should be employed. Theobjective of the present work is to solve the two concerns, i.e. the static and dynamicmechanical properties of concrete and, the physical behavior of stress corrosion ofconcrete induced by chloride penetration, based on meso-scale mechanical methods.The main efforts are summarized as follows.Part I: Meso Element Equivalent Method (MEEM) and its applicationsIn reality, the material nonlinearity and the size effect of concrete essentiallyoriginate from the heterogeneity of meso constituents. In light of this viewpoint, todescribe the intrinsic characteristic—heterogeneity of concrete, the present studytakes the dispersion coefficient of elastic modulus of meso elements as the index ofheterogeneous character of concrete, and develops a meso-mechanical numericalmethod with a high computational efficiency, i.e the so called Meso ElementEquivalent Method (MEEM).There are two key issues emphasized in the present MEEM, they are theequivalence of the mechanical behaviors of the aggregated concrete material and thedetermination of the mesh-element size, respectively. The two important issues arestudied systematically, and the failure process and macro-mechanical properties ofconcrete are explored using the MEEM. The detailed efforts are given as follows:1. The characteristic element size is introduced and determined by means of astatistical analysis of the aggregate sizes. Based on the Weibull distribution, theheterogeneous statistical properties of elastic modulus of meso elements ofconcrete material are studied. Finally, the magnitudes of the characteristicelement sizes of concrete with different grades are determined. And they aretaken as the appropriate mesh sizes of concrete specimens.2. In the present study, the equivalent static constitutive relationships for four kindsof meso elements are evaluated and derived based on the homogenization theoryof composites. The four cases are illustrated respectively as follows: aggregate/mortar matrix, concrete matrix/initial defects, concrete matrix/pore-water, andaggregate/mortar matrix/interfacial transition zones. Considering the influenceof strain-rate effect of concrete meso components, the equivalent dynamicconstitutive relationships of concrete meso-elements are then developed. Thegood agreement between the obtained theoretical models and the relatedexperimental results indicates the effectiveness of the obtained equivalentconstitutive models. Moreover, the strength criteria of concrete meso constituentsused in the simulation of meso-failure of concrete subjected to complex loadingsare explored preliminarily.3. Based on the proposed meso element equivalent method, the static and dynamicfailure process and macro-mechanical properties of two-and three-dimensionalconcrete specimens subjected to uniaxial tension, uniaxial compression andflexural loadings are investigated by a number of numerical simulations. Thestatic and dynamic failure mechanisms are studied according to the simulationresults. Subsequently, the meso-scopic failure process and macro-mechanicalproperties (including the averaged stress-strain relationship) of a reinforcedconcrete column under uniaxial compression are simulated by using the MEEM,and the size effect in concrete members are discussed preliminarily. Therefore,the application of the developed MEEM in the failure behavior of RCcomponents is realized. In addition, combining the MEEM with the eXtendedFinite Element Method (X-FEM), the simulations of the static tensile fractureprocess of three-dimensional concrete specimens are conducted. All the abovesimulation results show that the present meso-mechanical method (i.e. theMEEM) can describe the damage process of concrete effectively. Part II: Meso-scale numerical method for the study on stress corrosion ofconcrete under chloride environment1. Considering the influence of meso-structural heterogeneities on the chloridediffusivity in concrete, a meso-scale numerical method is developed andemployed to simulate the chloride diffusivity behavior. The quantitativerelationship between the effective chloride diffusion coefficient of saturatedcement paste and initial porosity as well as external loadings (herein they aredescribed by volumetric strain) is derived, and the influence of external loadingson chloride diffusivity behavior in cement paste is explored. Based on thetheoretical approach, the chloride diffusivity in heterogeneous saturated concretesubjected to compressive loads is numerically modeled by using the randomaggregate model of concrete, and then the effect of compressive stress level onthe apparent diffusion coefficient of concrete is studied and discussed.2. The cracking behavior of concrete cover caused by the corrosion-expansions ofreinforcing bar is simulated and explored using a meso-scale mechanical model.Furthermore, the effects of uniform and nonuniform corrosion-expansions of steelreinforcement on the cracking pattern and the critical corrosion rate when thecover cracks are illustrated and discussed.