Study On The Engineering Mechanical Properties Of Hydraulic Concrete By Mesoscopic Analysis Method

Hydraulic concrete plays a significant role in modern hydraulic engineering practice,and a throughout knowledge about its mechanical behaviors and engineering performances is of great importance for realistic service status prediction and safety assessment for high concrete dams.It has been well known that the nonlinear mechanics behaviors(e.g.,deformation,damage and cracking)of concrete may be traced back to its strong heterogeneity at the mesoscale level,i.e.,multi-phase composite consisting of coarse aggregates,mortar,interfaces and defects.Comparing with the traditional experimental means,the mesoscopic analysis method has shown particular advantages in predicting the engineering mechanical properties of concrete material,revealing the strength difference between fully-graded concrete with the two-grade wet-screened specimen,and analyzing the initiation,propagation and junction of microcracks under thermal actions or mechanical loads.Development of the mesoscopic analysis method to investigate the complex mechanical properties and the damage and fracture mechanisms of concrete,has become a fronted issue in the fields of engineering,materials and solid mechanics.This dissertation is intended to develop and apply the mesoscopic analysis method to study the mechanical behaviors(including deformation,creep,strength,damage and cracking)of hydraulic concrete.With mesoscale models generated for resembling the real concrete,numerical simulations based on the mechanics theory can be undertaken to relate the macro properties of concrete material with its mesostructure.The main achievements of this dissertation are summarized as follows.(1)A state-of-the-art overview of the mesoscopic analysis method for concrete with respect to mesostructure generation,experimental investigation on mesoscale components,effective material properties prediction and numerical methods for modeling the damage and fracture of concrete,is given.Some key technical problems not well solved in literatures have been summarized,and the main contents of this dissertation are presented.(2)A software package for modeling the 3D mesotructure of concrete is developed,which incorporates the generation and packing of random aggregate particles with high volume fraction,multi-gradation and various shapes,the simulation of micro pores and micro cracks,and the visualization of the mesoscale model,as well as the direct mesh discretization of the mesostructure.The multi-phase components(e.g.,aggregates,mortar,ITZ and defects)can be carefully modeled in realistic size.The proposed mesoscale model shows good performance and powerful capability which can meet the basic need of 3D mesoscopic numerical analysis,and it is the essential foundation for the following numerical investigations.(3)Based on the 3D mesoscale model,a numerical test system for predicting the effective properties of fully-graded concrete is established.The prediction results of the effective modulus,effective heat conductivity and effective thermal expansion coefficient of concrete show reasonable accordance with several experimental or analytical results,which have verified the validity and reliability of the numerical test.Further numerical results show that the representative volume element(i.e.,RVE size)for effective properties of concrete is about 3.5-4 times the maximum size of aggregate.(4)A comparative study between laboratory test and mesoscopic numerical simulation is performed,focusing on the effect of aggregate characteristics on concrete strength and the size effect.Based on the numerical results,the strength converting coefficient between fully-graded concrete and two-grade wet-screened concrete is estimated,which can provide a preliminary assessment of the strength of fully-graded concrete.(5)A mesoscopic thermo-mechanical model for concrete sample based on the theories of unsteady heat transfer and creep thermal stress analysis is proposed.With this model,creep of concrete samples with different aggregate volume fractions is predicted.The development of early-age self-restraint thermal stress in three-grade concrete under standard curing condition is simulated,and a parametric study is carried out.The production mechanism of self-restraint thermal stress and its main influential factors are revealed.It is shown that the presence of this kind of stress in early-age concrete may cause micro cracking that jeopardize its load-bearing capacity and durability.(6)The application of the phase field fracture model and its staggered finite element algorithm to trace the crack initiation and propagation in the mesostructure of concrete are developed.The simulated results show that the failure of concrete sample under uniaxial compression occurs in a mixed mode of tensile and shear cracking,while only tensile cracking takes place in concrete sample under uniaxial tension.It is also shown that the mesostructural characteristics(i.e.,aggregates,ITZ and initial defects)have a substantial effect on the cracking behavior of concrete:aggregates,ITZ,initial pores and micro cracks tend to cause crack initiation in concrete easily,and their content and distribution are the predominant factors that affect the location,path and geometry of cracks.Finally,general conclusions of this dissertation are drawn and several issues concerning the mesoscopic analysis method are discussed to attract more research interest in the future.