Study On Meso-scale Tensile Mechanical Properties Of Concrete Based On Cohesive Zone Model

Concrete is a kind of complex heterogeneous material composed of cement mortar,aggregates and various defects,in which each component material has a marked effect on the mechanical behavior of concrete material.The internal structure of concrete directly affects the cracking of concrete material.For convenience,concrete is usually assumed to be a homogeneous material at macro level in traditional researches.The macroscopic model of concrete is very efficient for engineering problems,but can not reveal the failure mechanism of concrete in mesoscopic theory.In this paper,the non-uniformity of concrete material from the mesoscopic level is considered,and regardsed as a three-phase composite composed of cement mortar,aggregates and the interfacial transition zones.The main research works and conclusions of this study are as follows:(1)In this paper,the research status of concrete micro-mechanics is introduced.Twodimensional random aggregate packing programs of circular,elliptical and polygon are compiled based on aggregate gradation theory,Monte Carlo method and related aggregate packing algorithm.The program controls aggregate packing through " area share".(2)In this paper,the cohesive zone model and the embedding method of zero-thickness cohesive element are elaborated in detail.The cohesive element is embedded in the random aggregate model by the program.The embedding area of cohesion element is the potential area of crack,and the cohesion element is endowed with the traction-separation constitutive relationship to establish the cohesive zone model of concrete.(3)The failure modes of concrete without pore under uniaxial tensile is simulated.The effects of aggregate content,aggregate shape and interface mechanical parameters(interfacial strength,interface fracture energy)on the mechanical properties of concrete are studied.The results show that,with increasing of aggregate content,the elastic modulus of concrete increases linearly and the tensile strength decreases exponentially approximatively.With the same aggregate content,the tensile strength of concrete is affected by the aggregate shape.The change of interfacial strength will affect the position of crack initiation and crack morphology of concrete.With increasing of interfacial strength,the tensile strength of specimen increases linearly.The change of interfacial fracture energy will not affect the position of crack initiation,main crack shape and tensile strength of concrete specimen.With increasing of interfacial fracture energy factor,the softening resistance of concrete is stronger.(4)The failure mode of concrete with pore defects under uniaxial tensile was simulated by introducing pore defects into the mesoscopic model.The effects of porosity,pore size and pore shape on the mechanical properties of concrete were studied in this paper.The results show that,with increasing of porosity,the elastic modulus of the specimen decreases linearly,and the tensile strength decreases exponentially.In the case of a certain porosity,with the growth of the pore size of the concrete,the tensile strength decreases exponentially,the elastic modulus does not be changed with the changing of the pore size.When the ratio of short axis and long axis of elliptical pores become larger,the elastic modulus and tensile strength of specimen with circular pore are close those of that with elliptical pore.