Research On Freeze-Thaw Damage Mechanism Of Pore Structure Of Mold-bag Concrete Based On Fractal Theory

Due to the influence of factors such as natural environment and climate,the mold-bag concrete tank lining in the Hetao Irrigation District of Inner Mongolia has experienced seasonal freeze-thaw,and there are durability problems such as skin cracking and peeling.In this article,according to the mixing ratio of the existing formwork bag concrete,the concrete is prepared by double mixing solid wastes such as fly ash and silica fume instead of part of the cement.Through experimental research on the macro-mechanical properties and micro-pore structure evolution of mold-bag concrete under freeze-thaw environment,the freeze-thaw failure mechanism and fractal characteristics of mold-bag concrete are discussed,which lays the foundation for further research.The promotion and application of bag concrete in Hetao irrigation area and severe cold area in Inner Mongolia.The main findings are as follows:（1）The macro-mechanical test results under the effects of freeze-thaw show that an appropriate amount of silica fume and fly ash can improve the mechanical properties of bagged concrete.When the total amount of the double admixture is about 20%,the early mechanical properties of the mold-bag concrete and the mechanical properties after freeze-thaw are better than other mixing ratios.When the mixing ratio is F15S4,the compressive strength of bag concrete is the best,and F10S8 is the second.（2）SEM and XRF experiments show that as the number of freeze-thaw cycles increases,the damage to the pore structure of the mold-bag concrete increases,and the surface gradually changes from a uniform transition to fiber ettringite.The presence of SO₄^2-and Cl^-in the Yellow River water gradually increases the ettringite and C-S-H gel,squeezing the pore wall to expand the pore structure,and causes the damage of the bagged concrete to increase gradually.（3）Based on the SEM and NRM test results of the fractal theory,the fractal dimension of the bag concrete under the freeze-thaw environment is calculated.It is found that the fractal dimension of two-dimensional SEM and the fractal dimension of three-dimensional NMR are positively correlated with the damage degree of bag concrete.Different sizes of fractal dimensions can characterize the freeze-thaw damage of bagged concrete.（4）Through the regression estimation of the wavelet transform,the macroscopic and mesoscopic are connected,and the fractal constitutive model is established.The macroscopic model reflects the influence of many factors on the macroscopic mechanical properties and durability at the micro and micro level.Through the fractal dimension,the two-dimensional SEM image is extended to the three-dimensional space structure based on nuclear magnetic resonance,thus fully showing the space filling ability,the complexity of the hole and the roughness.Through the quantitative analysis of the fractal characteristics of bagged concrete caused by freeze-thaw damage,the exploration of the macroscopic mechanics and durability of concrete in fine,micro and three-dimensional spaces provides a new theoretical basis and research ideas.