Study On The Inhomogeneity Of Physical State In The Interfacial Zones Between The New And Old Concrete In Assembly Structure

Prefabricated concrete structure is widely used in the construction field.There are a lot of interfacial zones in the fabricated structure.The physical heterogeneity of the interfacial zone often causes the interfacial zone to become the weakest zone in the whole structure,which affects the mechanical properties and durability of the whole structure.In this paper,the interface roughness,coarse aggregate,pore structure and chloride diffusion coefficient of new and old concrete are studied by using three-dimensional laser scanner,scanning electron microscope and digital image processing.The 3D coordinates of different chiseled surfaces(2mm,4mm,6mm,8mm,10mm)were obtained by 3D laser scanning,and then the digital model of chiseled surfaces was reconstructed by MATLAB software,and the 3D fractal dimension of each reconstructed surface was calculated and analyzed by cube box covering method.On this basis,the relationship model of chiseling depth,sand filling depth and threedimensional fractal dimension is established.Through the digital image processing technology,the distribution of the area fraction of the coarse aggregate on both sides of the interface of the new and old concrete specimens with different chiseling depth(2mm,4mm,6mm,8mm,10mm)is studied.The results show that the minimum area fraction of the coarse aggregate in the interface zone of the new and old concrete increases with the increase of the chiseling depth,the increase decreases when the chiseling depth is greater than 6mm,and the minimum area fraction of the coarse aggregate is obtained.At present,the distance between the right side of the interface and the interface is 1 ? 2mm;at the same time,the theoretical model of the area fraction distribution of coarse aggregate on both sides of the interface is established considering different chiseling depths.The new and old concrete with different water cement ratio was observed by SEM.It is found that,regardless of the water cement ratio,the pore diameter in interfacial zone is much larger than the internal pore diameter,and the number of pores is more.Based on the image processing technology of MATLAB,the porosity of the new and old concrete interface at different positions is obtained from the SEM image.It is found that the porosity decreases with the increase of distance in the range of 4mm from the interface,and tends to be stable when the distance is greater than 4mm.Then the pore structure of the new concrete interface is observed when the chiseling depth is 6mm.It is found that the interface transition zone is formed at the interface of the new concrete due to the close contact between the new concrete and the coarse aggregate of the old concrete chiseling surface.Based on the above study,a porosity distribution model considering water cement ratio,gouging depth and distance from the interface is established.Through the 90 day experiment of chloride solution dry wet cyclic erosion on the new and old concrete specimens with different water cement ratio and different chiseling depth.It is found that the chloride concentration increases with the increase of water cement ratio at the interface or inside the concrete;when the chiseling depth is not more than 6mm,the chloride concentration in the interface zone decreases with the increase of the chiseling depth,and the effect index in the interface zone decreases.When the chiseling depth is more than 6mm,the chloride concentration and the effect index in the interface zone first increase,and then decrease;and The peak point of ion concentration shifted to the side of the old concrete with the increase of chiseling depth.Based on the study of the area fraction distribution and porosity distribution of the coarse aggregate on both sides of the interface of the new and old concrete,the chloride diffusion coefficient model of the interface zone considering the water cement ratio,the chiseling depth and the distance from the interface is established,which is proved to be applicable.There are 65 figures,16 tables and 142 references in this thesis.