Study On Failure Process And Mechanism Of Concrete With Mesostructure Based On Cohesive Zone Model

As a common construction material,concrete is generally classified as a multi-scale material that is mixed by granulars covering cement,mortar,and aggregates.These components crucially influence the macro physical and mechanical properties of concrete.Much research work has demonstrated that the meso-structure of concrete is essential to ensure the durability and strength of concrete.Therefore,it is seriously concerned how to design numerical concrete models that can describe the deformation performances of concrete from the mesoscopic point of view.In this thesis,a generic algorithm of batch cohesive element embedded into finite element mesh is presented firstly,which bases on the concept of "face identification number".This creates conditions for further research on the effect of meso-structure in concrete on its macroscopic behaviors by using numerical concrete methods combined with cohesive zone model(CZM).Then,the aggregate placement algorithm of constructing concrete mesoscopic models in 2D and 3D space is proposed,which is called as dot matrix method(DMM).With the actual aggregate information collected by laser scanning technology,real concrete numerical models can be succefully constructed by the present aggregate placement procedure,which are the basis of mesh generation for subsequently numerical simulation.Finally,the present numerical concrete model with CZM was used to investigate the sensitivities of mesh element size and computational parameters,the effect of spatial distribution of aggregates,the effect of aggregate volume content,and the pore structure on the physical and mechanical properties of concrete.The main conclusions are as follows:(1)The algorithm of batch embedding cohesive element based on the concept of "face identification numbers" is of universality,robustness,and high efficiency in applications of the numerical concrete model.(2)The basic idea of the present DMM is to transform overlap detection between polygons or polyhedra into checking the possibility of any intersection between the point sets within a trial placement aggregate and the already placed ones in mortar.The algorithm changes a complex three-dimensional problem into a one-dimensional one,which is easy to understand and implement.Through the arithmetic operation of integer point sets,the effciency of the underlying algorithm in DMM is higher.Combing with 3D laser scanning technology,the present DMM can create mesostructure concrete models conveniently and flexibly.(3)The crack initiation time,location,propagation path,and macroscopic failure pattern are all affected by mesh element sizes in the simulations.The time of crack initiation significantly delays if smaller element sizes are applied in the mesh.The fact is that crack initiation and propagation can only develop along the element boundary,which implies that there are fewer potential paths for crack propagation in the models with larger element sizes.In addition,different crack initiation positions lead to different crack propagation paths.With a relaatively coarse mesh,the prediction value of elastic modulus is relatively larger and the corresponding peak stress is smaller.(4)The stiffness K,damage initiation strength T,fracture energy G,and damage evolution rule of concrete play different roles in the crack initiation position,initiation time,and crack propagation path.Among these parameters,the entire process of crack initiation,propagation and penetration is little sensitivity to damage initial strength T.However,the crack initiation time,propagation rate and elastic modulus are highly sensitive to the stiffness K.As the stiffness K is larger,the crack initiation time early occurs.At the same time,the crack propagation rate becomes faster while the elastic modulus increases.Moreover,the crack initiation time and peak load are more sensitive to the fracture energy G.With the increase of fracture energy G,the crack initiation time delays and the peak load increases.If the exponential damage law is applied,the crack propagation rate is extremely fast and the stress-strain curve has a sudden drop.It shows that such damage law is suitable for describing the mechanical response of brittle materials.On the other hand,if the linear damage law is employed,the crack propagation rate is slow,which indicates that this law not suitable for characterizing the mechanical response of brittle materials.(5)In the case of the same volume fraction of aggregate,different aggregate distributions moderately influence the failure modes of concrete under uniaxial compression conditions.The relative positions of principal cracks have a little change and the extent of crack propagation and evolution is different.However,macroscopically,it shows an X-shaped failure mode.From the perspective of mechanical behavior,the stress-strain curves of the numerical models almost overlap before reaching the peak stress,and the changes begin after the peak stress.This change is mainly govened by the contact and collision between the discrete elements.In the case of different volume fractions of aggregates,the failure patterns of specimens are dominated by X-shaped modes either.From the perspective of mechanical properties,the increase of aggregate volume fraction can enhance the stiffness of the specimen,but it is not beneficial to the improvement of the toughness.Contrarily,it can increase the brittleness of specimen.Compared with the MCM elements,the mechanical properties of ACM elements have an important effect on the physical and mechanical properties of concrete,especially on the tangential parameters of ACM elements.(6)The uniaxial compression stress-strains of concrete are not sensitive to the change of pore shape.Instead,the effect of pore shape on the uniaxial compression failure of concrete can be better reflected in the macro failure pattern.Under the same porosity,both the macro failure pattern of the model and the stress-strain relationship curve are highly sensitive to the pore size.Under external loads,the path of crack propagation and evolution in the model with a larger pore size is relatively simple,and the crack develops rapidly from initiation to penetration.In the model with a smaller pore size,secondary cracks are more developed.Under the same volume fraction of aggregate,increasing the porosity can significantly reduce the elastic modulus and peak load of the model.Further,the pore compression deformation contributes to softening phenomenon before reaching the peak load.Moreover,the compressive bearing capacity of concrete is related not only to the volume fraction of aggregate,but also to the porosity.When the porosity is less than 5%,the uniaxial compression strength of the concrete decreases first and then increases as the aggregate volume fraction increases.When the aggregate volume fraction is 40%,the uniaxial compression strength is the lowest.When the porosity is greater than 5%,the uniaxial compression strength of concrete is almost always increasing as the volume fraction of aggregate increases.