| Permeable concrete,also known as porous concrete,is a kind of porous lightweight concrete prepared with coarse aggregate,cement and water.Antifreeze permeable concrete is a new type of permeable concrete with good mechanical strength,water permeability and frost resistance obtained by adjusting mix ratio and adding fibers on the basis of ordinary permeable concrete.The complexity and disorder of the mesoscopic pore structure are the key factors that determine its mechanical properties,permeable performance and frost resistance.In this paper,a research method including physical experiment,theoretical analysis and numerical analysis is used to study the meso pore structure characteristics,the numerical analysis of the seepage process and the numerical analysis of the mechanical model of the antifreeze permeable concrete.With the help of high-precision CT scanning technology,researches have been carried out on the three-dimensional reconstruction of antifreeze permeable concrete,the analysis of real meso pore structures,the characteristics of pore water seepage,and the force characteristics under uniaxial and multiaxial compression and tension conditions.The main research work and results of this paper are as follows:1.Three-dimensional reconstruction models of the real mesostructure of the antifreeze permeable concrete has been established based on CT scan images and digital image processing technology(1)The antifreeze permeable concrete is co nsider as a multi-phase composite material composed of aggregate,mortar,interface transition zone and pores.The Trainable Weka Segmentation(TWS)method is used to segment the CT images and the mesostructures of aggregates,mortar,interface transition zone and pores have been extracted based on digital image processing technology.(2)Three-dimensional reconstruction of the mesostructure of aggregates,mortar,interface transition zone and pores has carried out in the three-dimensional visualization software.Then the models have been combined to three sets of 3D reconstruction models of the real mesostructure of antifreeze permeable concrete specimens with different aggregate particle sizes and pore structures.2.Pore structure models have been extracted based on the 3D reconstruction models of the real mesostructure of the antifreeze permeable concrete specimen.The accuracy of the pore models have been verified by calculating the porosity and the 2D and 3D pore structure characteristics have been analyzed.(1)The results of porosity analysis show that the average value of total porosity calculated based on the continuous 2D images and the 3D connected porosity of the 3D model are close to experimental results,which indicates that the pore structure model extraction method based on CT scanning and digital image processing technology can characterize the real pore structure.(2)The analysis results of the pore diameter show that both 2D and 3D pore diameters are normally distributed and mainly concentrated in 0-10 mm and 0-15 mm respectively,in which range the number of pores account for more than 70%.With the increase of the aggregate particle size,the proportion of large pores in the specimen gradually increases.(3)The plane porosity in different areas of the specimen shows similar change trend with the slice position and the values of pore fractal dimension in different areas are relatively close,indicating that the pore structure has certain fractal characteristics.The fractal dimension shows a highly linear relationship with the plane porosity,which can be used as a method for testing the porosity of antifreeze permeable concrete.3.Numerical seepage models have been generated based on the 3D reconstruction models of the real mesostructure of the antifreeze permeable concrete specimen.The reliability of the seepage models has been verified by calculating the numerical permeability coefficients and seepage simulations under different pressure gradients have been carried out.(1)The simulation results of the permeability coefficient are close to the experimental results and the errors are within a reasonable range,indicating that the pore structure model and numerical analysis method in this study have a certain degree of reliability.(2)The results of seepage simulation show that in the pore structure model with the same aggregate size,the distribution of seepage pressure and seepage velocity under different pressure gradients is basically the same.The seepage pressure gradually decreases along the seepage direction,and the largest seepage pressure appears in the pores near the pressure inlet.The seepage pressure will obviously increase when the pore diameter decreases or the pore curvature changes significantly.(3)The seepage velocity gradually increases with the increase of the pressure gradient.As the seepage reaches a steady state,the water tends to flow through the pores that penetrate the upper and lower surfaces of the specimen,which are the dominant channel with the least resistance during the seepage process.The seepage velocity increases sharply when water flows through narrow pores.When the aggregate particle size is small,the path line distribution is significantly denser.There are more dominant channels for water flow in smal er pores.(4)Through the results of cross-sectional flow and seepage velocity obtained by t he numerical seepage analysis,the seepage curves of three models have been obtained.The flow of pore water complies with Darcy-Fochheimer's law,and the critical pressure gradient and critical Reynolds number of three sets of antifreeze permeable concrete specimens have been obtained according to the seepage curves.4.Numerical models of mechanical structure have been established based on the 3D reconstruction model of the real mesostructure of the antifreeze permeable concrete specimen.The reliability of the mechanical models have been verified by the crack development of the numerical models under uniaxial compression process and the numerical analysis of uniaxial and multiaxial compression and tension processes have been performed.(1)The numerical analysis results of uniaxial compression show that the cracks are vertical cracks parallel to the loading direction,which is in good agreement with the crack characteristics of the physical test specimens.This indicates that the mechanical model construction method can reflect the force characteristics of the specimen(2)During the uniaxial compression and tension of the antifreeze permeable concrete specimen,the stresses of the internal components continuously redistribute with the loading process.The inhomogeneity of the mechanical properties of each component leads to extremely uneven stress distribution.During the loading process,stress concentrat ion will occur at the edges of the pores and the sharp corners of the aggregates,which will lead to crack and destruction of the specimen.(3)The inhomogeneity of the spatial distribution of the components inside the specimen causes uneven deformation of the specimen during the loading process.The edges of the specimen and the internal open large pores are the weak positions during the stress process.Large local deformations will occur at these positions during the compression and tensile process,causing tensile and shear failures on the surface of the specimen.(4)The edges of the pore structure and interface transition zone are the weak links that affect the mechanical properties of antifreeze permeable concrete specimens.The cracks successively occur at the pore edge,mortar,interfacial transition zone and aggregate.The microcracks gradually extend to the main fracture zone and finally intersect and penetrate to form macrofractures.During the compression process,the cracks on the surface of the specimen are mostly tension cracks and diagonal shear cracks and oblique shear failure is the main failure characteristic.During the tensile process,the cracks on the surface of the specimen are mainly tensile cracks,and the damage of the specimen is ma inly local tensile failure.In this paper,real mesostructure models of antifreeze permeable concrete have been established based on CT scan images and digital processing technology.Based on the numerical model,quantitative analysis of pore characteristics,numerical analysis of pore water seepage under different water pressure gradients and numerical analysis of the mechanical model under uniaxial and multiaxial loads have been carried out,which has certain theoretical significance for the enrichment and development of the theoretical research on antifreeze permeable concrete. |
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