Effect Of Load On CO₂and Chloride Ion Transport In Cement-based Materials And Its Characterization By XCT

Carbonation and chloride ingress on protective layer of concrete is one of the main reasons for the internal steel bar corrosion and the deterioration of the whole performance of the structure.It is very important to study the transport characteristics and regularity of CO₂ and chloride ions in concrete for predicting the corrosion time and service life.Based on the domestic and abroad study on the current situation of the transport of CO₂ and chloride ions in concrete based on the domestic and abroad,This thesis has carried on the thorough research in two aspects: the changes of the transport performance in the concrete under external axial loads and characterization of carbonation process of hardened cement paste by X ray computed tomography?XCT?.With regard to the substance transport pattern in the concrete under loads,concrete is considered to be a heterogeneous material made of cement paste,the interfacial transition zone of the aggregate and the mortar with different mechanical and chemical properties.In this experiment,different levels of non-destructive axial tensile and compressive loads are applied to different phases for study of the transport characteristics of CO₂ and chloride ions.The results of this study provide the experimental basis for multiphase transport models of concrete and predicting the effective diffusion coefficient of chloride ion in concrete.The results show that the rate of transport of CO₂ and chloride ions in three phases of concrete,ITZ >mortar> paste;the compressive stress makes pore microstructure of the cement based material shrink which inhibit the transport process,and the tensile stress makes the pore microstructure expand that promotes the transport process;for mortar with different carbonation ages,in contrast to its influence on carbonation,it is more sensitive for influence of stress action on chloride.In the process of CO₂ transport,the effect of stress on the transport of CO₂ in cement paste is the highest.For the transport process of chloride ion,the effect of stress on the transport of chloride ion in the interface transition zone of aggregate is the most obvious.Therefore,the influence of the non-destructive load applying the material on CO₂ and chloride transport should be taken into account in the substance transport process for concrete.In the process of monitoring the carbonation process of cement based materials by XCT,the microstructure morphology changes of the cement based materials with different w/c ratios were tested.Due to the small change of equipment loss and test environment,the direct comparison of the results obtained in different time points is quite inaccurate.To solve this problem,the results are adjusted through the operation steps of the gray threshold definition,normalization of image magnification,micro adjustment of gray values in 2D and 3D level to make a direct comparison with the graphical results.By comparing the variation of the slice area and volume of the material with different gray values,the carbonation depth and the changes of the internal microstructure of the material caused by carbonation are obtained intuitively.