Research On Size Effect Of Concrete And RC Columns

The size effect of concrete material and RC components has become a frontier issue as the dimensions of engineering structures getting larger and larger.The size effect is understood as any dependence of nominal strength or brittleness on the structural size.The research on size effect of concrete structures has fallen into two major directions,1)material size effect,and 2)component size effect.Studies on material size effect mainly focused on the influences of aggregate size,aggregate distribution,concrete strength grade and initial defects on the global mechanical properties,which is a complex materials science problem.For component size effect,it is a structural mechanics problem,which should be caused and affected by 1)the inherent heterogeneous and nonlinear features of concrete,and 2)the complex mutual effect between concrete and steel rebar.Based on theoretical analysis,the achievements of experimental research on the size effect of concrete structures have been summarized,and a series of numerical tests on the size effect of RC members based on finite element analysis and numerical simulation method have been conducted.The main efforts are summarized as follows.Part ?: Theoretical analysis on the size effect of concrete materialThe size effect behavior of concrete structures would be affected by several factors,such as the randomness of material strength,the fracture failure pattern of concrete,the fractal characteristics of crack propagation and the boundary effect,etc.It has to be noted that,different factors have different impacts on the size effect behavior of concrete structures at different circumstances.One objective of this thesis is that,establish a global size effect law which is able to describe the entire range of component size,based on the framework of fracture mechanics size effect theory in conjunction with the randomness of material strength and the fractal characteristics of cracks.First,the basic theories of concrete mechanics,including the fracture mechanics and the damage mechanics,are introduced and the basic concept of mesoscopic analysis of concrete damage-fracture mechanics is proposed.Second,the fracture failure process of concrete,including uniaxial and biaxial tensile/compressive properties,is simulated at meso-scale.A macro-knowledge of the size effect of concrete has been established from failure modes,crack propagation behavior and other perspectives.Third,a general expression of the length of the crack propagation path and a theoretical solution for the uniaxial tensile strength for different graded concretes are established based on the recognition of failure patterns and influential factors of concrete material.A fictitious stress model is set up at meso-scale for the analysis of stress at crack tip,which is suitable for the description of aggregate cracking behavior at high strain rate.Finally,based on the framework of concrete damage-fracture mechanics and the distribution characteristics of the initial defects in concrete material,several global size effect laws which are able to describe the entire range of component size are established.Part ?: Experimental and numerical studies on the size effect of RC membersThe other objective of this thesis is that,lay the foundation for the size effect theory of RC members,based on the deep analysis of experimental and numerical achievements of the size effect behavior of larger-sized RC members.A series of systematically designed size effect tests of large-scale RC members are carried out,as for the current situation of size effect tests of RC members mostly located into the small-scale models.Take RC columns as examples,the influential factors have been analyzed thoroughly within a broader size range.First,the influential factors of normal-strength RC columns,including the slenderness ratio,the longitudinal reinforcement ratio,the transversal reinforcement ratio,the grade of concrete strength and the load eccentricity,are discussed.Second,the size effect behavior of the highstrength RC columns,mainly focused on the transversal reinforcement effect and the loading pattern,are analyzed.Besides,in order to understand other factors which also have impact on the size effect of RC members,the numerical tests are adopted.First,a 2D meso-scale mechanical analysis model for RC columns is set up,and the size effect behavior of the RC columns under axial centric and axial eccentric compression is investigated.Second,a 3D meso-scale mechanical analysis model for RC columns is set up,and the influence of slenderness effect on the mechanical properties of RC columns is discussed.