Study On The Concrete Structural Durability Based On The Carbonation Degree In Atmospheric Environment

Due to global warming,the emission of greenhouse gases increases exponentially.The increased temperature and the CO₂ concentration lead to the carbonation life of reinforced concrete structures decreasing.Especially for hydraulic concrete structures,combined with the role of carbon sources in the reservoir area,durability is more worthy of attention.The prediction of the carbonation life of the existing concrete structure is mainly based on the carbonation depth;however,in terms of theory,the inducement of corrosion of reinforcement in concrete under an atmospheric environment depends on the carbonation degree.Secondly,the relative environmental humidity is the primary variable in the prediction model of concrete carbonation depth,and the influence of the internal moisture content of concrete is not considered.In addition,the carbonation depth of concrete is almost always defaulted to the depth of the non-colored area of phenolphthalein spraying.The test results show that when the moisture content of the specimen is high or low,the test results are not accurate and there are significant errors.Therefore,in order to evaluate the carbonation life of concrete structures in service more accurately,in this paper,the evaluation method of carbonation degree of cement-based materials is constructed from the aspects of the test procedure,experiment method,theoretical models and the stress characteristics of structures under service.Furthermore,based on the experiment and theoretical analysis,the carbonation degree of the surrounding concrete when the reinforcement begins to corrode is determined and the carbonation effects are identified.As a threshold,the critical carbonation degree is defined.Thus,similar to the normal serviceability limit state of the structure,the relationship between the carbonation effect and the critical carbonation degree is constructed,and the life prediction method for concrete structures under service based on the critical carbonation degree is proposed.The main work and conclusions of this paper are as follows:（1）The characteristics of frequently-used test methods for carbonation of cement-based materials such as the acid-base indicator method,the simulated pore solution method and the material composition analysis method are analyzed and summarized.Because of the shortcomings of the traditional phenolphthalein alcohol indicator method for measuring the carbonation depth,the python language has been used for digitizing the scanning images based on solid upper scanner invented by the research group to obtain the average carbonization depth more accurately.The test process of the simulated pore solution method is standardized,which can indirectly characterize the carbonation degree more precisely.Independent research and development,design and manufacture of precision miniature digital carbonation measuring device,which can reflect the carbonation degree of concrete directly by testing the content of Ca CO₃ in the specimen.The correlation between test results of the three methods is established on the basis of the comparative analysis of experimental data.（2）The influence of carbonized specimens pretreatment method on test results is researched by experiments.The results show that the pretreatment method used in the specification is not reasonable,and the water exchange exists in the carbonization process of the treated specimens affects the test results.After pretreatment of the specimens by the method proposed in this paper,the specimens are in a uniform wet distribution during the carbonation process.The measurement results are more accurate,which can reflect the carbonation performance of concrete components in practical projects.Under the wet steady state,when the relative environmental humidity is 53%,the carbonation depth of the phenolphthalein alcohol indicator method is the largest.When the relative environmental humidity is 70%,the carbonation degree in the complete carbonation zone was the largest.（3）The theoretical prediction model of cement-based material carbonation degree considering water transport is established based on the theoretical analysis.The applicability of the theoretical model is verified by comparing and analyzing the experiment results and theoretical model calculation results of specimens with different internal and external moisture content.Further researches show that the characteristics of cement-based material carbonation area are affected by the relative environmental humidity and the pore water saturation.When the pore water saturation is 0.7 and the relative environmental humidity is50%,the length of the complete carbonation zone is the largest.When the pore water saturation is 0.9 and the relative environmental humidity is 90%,the length of the complete carbonation zone is the smallest.When the environmental humidity is 50%and the pore water saturation is 0.1,the length of the partial carbonation zone is the longest.When the pore water saturation is 0.8 and the relative environmental humidity is 90%,the length of the partial carbonation zone is the shortest.（4）The carbonation characteristics of mortar specimens under axial tensile in different tensile stress levels are investigated experimentally.The test results show that there is no significant variance in the carbonation depth measured by the phenolphthalein indicator method under different tensile stress levels.With the increase in tensile stress level,the carbonation depth measured by the carbonate content method decreases first and then increases.The correlation between the carbonation volume shrinkage of cement-based materials and the tensile stress levels is poor,which is mainly affected by the composition of raw materials.The cumulative volume strain is the fundamental reason that affects the carbonation depth of the same cement-based materials under different tensile stress levels based on the test results detected with the carbonate content method.The greater the total volume strain contraction is,the smaller the carbonation depth is.The larger the total volume strain shrinkage,the smaller the carbonation depth.For conventional concrete,when the tensile stress level reaches 0.8f_t,the influence on carbonation degree cannot be ignored,and the influence coefficient of carbonation degree should be determined as 1.5.（5）Through an accelerated carbonation test,the critical carbonation degree of reinforcement in cement-based materials at the beginning of corrosion is studied by electrochemical measurement,macroscopic observation and microscopic detection.The results indicate that when the steel reinforcement begins to corrode,the concrete surrounding is partially carbonated,and the critical carbonation degree is about 55%.Taking this as the threshold,based on the established theoretical prediction model of carbonation degree,the structural life estimation method for concrete structures under service with critical carbonation degree as the standard was proposed.