Research And Application Of Concrete Coarse Aggregate Mesoscopic Model And Diffuse Meshing Method

At present,most large-volume concrete structures have cracks during their service life,and the complete theoretical system and mature technical methods have not been established.Therefore,exploring the cracking mechanism can provide safety guarantee for practical projects,and has great scientific research value and practical significance.Concrete is a highly heterogeneous and multiphase composite artificial material composed of coarse aggregates,cement mortar,and pores.Existing studies have revealed that the macro mechanical properties of concrete are inseparably related to their micro-structures,curing conditions,and the environment.With the rapid development of numerical analysis algorithms and computer technology,numerical analysis methods have gradually become an important means to understand and analyze the evolutionary relationship between meso-mechanical properties and macro-mechanical properties of concrete.For in-depth study the cracking mechanism,the dissertation focused on the generation algorithm of the concrete mesoscopic model with complex-shaped coarse aggregate and the matching meso-mechanical numerical analysis method,and applied the established method to analyze the self-restraint stress evaluation of concrete in early-age and the internal damage process under uniaxial loading.The main work and research results are summarized as follows:(1)Based on the implicit T-spline basic theory,a set of algorithms to develop meso-structure models of 3D concrete specimen with complex-shaped coarse aggregate was proposed systematically,and the corresponding computer program COCAG(Code Of Coarse Aggregate Generation)was complied on MATLAB.The threshold was used as the criterion for subdividing the T-mesh,which can better solve the contradiction between the number of control points,the constraint effect of the surface and the calculation efficiency,without increasing the control points,the open node vector was introduced to avoid the possible occurrence of spurious sheets,aiming at the uniqueness of the form of the implicit T-spline function,a method for determining the number and position of offset points which the value of the implicit function is not equal to zero was proposed,thus the reconstruction of complex-shaped surface based on discrete point sets was realized;a method for generating the surface point sets of coarse aggregate was proposed which a sinusoidal generatrix function was used to reflect the smoothness and size range of gravel aggregate and the number of cut planes was used to refect the damage degree of the crush aggregate.Combined the generation algorithm of complex-shaped coarse aggregate,the occupation and removal method with the layering disposition method,an improved method for packing coarse aggregates was proposed,which the meso-structural models for a given gradation and aggregate content was effectively established.(2)Based on elastic-viscoplastic potential theory and concrete temperature deformation,creep,autogenous volume deformation and damage model,the diffuse mesh method was developed which integrated the basic principles of the diffuse meshing method to the reconstruction algorithm of complex-shaped coarse aggregate,and the computer analysis program COCE3D-DM(Code Of Concrete Engineering-Diffuse Method)was complied.The sensitivity analysis with multi-scale grid to the calculation accuracy in the diffuse grid method was carried out,and the element size which is less than the third of the minimum diameter of aggregates was proposed for the stability of the results.This program can be applied to analyze the evolution of self-restraint stress in early-age and plastic damage in the process.It developed a novel method to analyze the meso-mechanical behavior of concrete.(3)The COCAG program and COCE3D-DM program were used to deeply analyze the self-restraint stress generation mechanism of concrete in early-age was in-depth discussed by using the COCAG program and COCE3D-DM program,and the effect of coarse aggregate shape and orientation on mechanical behavior in early-age was studied.Firstly,five equal volume aggregate models with different shape and orientation which is reconstructed by the COCAG program were generated.Next,cross-scale(macro-and meso-)numerical simulations were carried out.When performing single-factor analysis such as adiabatic temperature rise,creep strain,and autogenous volume deformation,the random coarse aggregate model was calculated by using the diffuse meshing method while the simplified concrete model was calculated by using the finite element method,further verifying the feasibility of the diffuse meshing method for thermo-mechanical mesoscopic numerical simulation.Meanwhile,the parameter equivalent method between the concrete mesoscopic model and the homogeneous concrete model was established.Then,numerical simulation was carried out under the combined effects of temperature,creep,and autogenous volume deformation.The results show that: the temperature is the main cause of self-restraint stress at the beginning of the hydration process,and the autogenous volume deformation plays a dominant role when the temperature gradually stabilizes.After curing,the residual principal tensile stress still exists in the cement mortar which cannot be ignore and the region of stress concentration will occur at the aggregate/ matrix “interface”.Finally,the effect of coarse aggregate shape and orientation on self-restraint stress was studied by comparing the numerical results of five models.The results show that the shape of the coarse aggregate has obvious influence on the global response and local behavior of concrete,especially during heating period;the orientation of the coarse aggregate has little influence on the global mechanical behavior,while influence the state of local spatial stress distribution through changing the distance or sparsity of coarse aggregate.(4)The macro-and micro-mechanical behavior of concrete specimen in simple uniaxial loading test was studied by a combination of macro-physical experiment and meso-numerical analysis(COCE3D-DM),and the process of deformation,damage evolution to failure of concrete were further recognized.Meanwhile,the influence of coarse aggregate shape,orientation and self-restraint stress on damage failure process were studied.Firstly,the MTS universal testing machine and non-contact deformation measurement technology were used to conduct uniaxial compression test for five test specimens.The evolution of the displacement field and strain field during the deformation process of the concrete specimens were analyzed.The macroscopic stress-strain curve under loading was consistent with the general law.Secondly,the COCE3D-DM program was used to conduct in-depth research on the damage evolution law of concrete under uniaxial loading tests.The mechanical law and damage evolution process of concrete under compression and tension were revealed,and the volume average method was used to predict the equivalent deformation modulus of concrete.The results of numerical experiment,physical experiment and literature research were generally consistent.For uniaxial compression tests,the effects of coarse aggregate shape and orientation on the equivalent deformation modulus,compressive strength,internal damage,and damage variables of concrete were analyzed.The results show that the coarse aggregate shape mainly affects the nonlinear stage and strain softening stage,compared with the spherical coarse aggregate model,the crushed coarse aggregate model has higher equivalent deformation modulus,bearing capacity and ductility than the spherical coarse aggregate model;the orientation of coarse aggregate has little influence on the macro strength and the equivalent deformation modulus,but has a significant effect on the initiation position and the propagation path of microcracks in concrete.Finally,the influence of the early age self-restraint stress field on the mechanical properties of concrete was studied.The results show that the self-restrained stress has very little effect on the elastic stage and can be ignored,but has obvious influence on the peak strength.Considering the self-restraint stress filed and strain softening stage,the compressive strength and tensile strength were reduced by 2.45% and 3.35%,respectively.In conclusion,the dissertation focuses on the numerical analysis method of concrete micromechanics,establishes a complex-model coarse aggregate concrete model algorithm,and combines the coarse aggregate algorithm to develop the diffuse meshing method,thus exploring an effective way to analyze the meso-mechanical problems of concrete.The COCAG and COCE3D-DM programs were used to conduct a meso-scale numerical analysis of the self-restraint stress in the early-age and damage process to further understand mechanical properties of concrete.