Assessment Of Carbonation Life Of Concrete Structures Based On Micro-environmental Effects

Currently,the majority of durability studies on concrete focus on various aspects such as the environment,materials,components,and structures.When analyzing the impact of environmental factors on the durability of building structures,spatial and temporal variations are considered.Spatial variations involve analyzing factors influenced by multiple environmental scales and proposing zoning criteria for environmental effects.Temporal variations involve considering the time-dependent cumulative effects on concrete structures and proposing durability design for the entire lifecycle of concrete.Some researchers have also discussed the patterns and mechanisms of environmental effects such as carbonation corrosion and chloride corrosion in concrete structures,taking into account the spatiotemporal variations,primarily focusing on temperature and relative humidity analysis.Most of the aforementioned studies are based on local or regional climates,often utilizing data from suburban weather stations as environmental parameters for predicting carbonation life.Temperature,relative humidity,CO₂ concentration,etc.,are assumed to be constant values such as daily averages or monthly averages.Additionally,these studies mainly focus on the component level rather than the structural level.Existing research has demonstrated significant differences between microclimates around buildings and local climates（weather station data）,such as the"urban heat island effect"and"school heat island effect."Furthermore,the microenvironment around buildings constantly changes over time and space.Therefore,the environmental factors considered in current research are not precise enough to capture the impact of carbonation on building structures.Consequently,assessing the carbonation life of concrete structures based on micro-environmental effects holds significant practical significance.To comprehensively analyze the distribution of microenvironments around buildings and their impact on carbonation in concrete structures,the author employed two research methods:field measurements and simulation modeling.Firstly,the distribution of microenvironments around buildings was obtained.Then,environmental influence factors f_i（C_d,T,RH）,and the relative carbonation rate K_i were proposed to evaluate the degree of environmental impact and carbonation conditions.The study analyzed the carbonation differences in different spatial configurations of buildings,different material reflectivity,and extreme weather conditions.Ultimately,based on the influence of micro-environments,recommendations were made for the design of concrete durability at the present stage.The main work and conclusions are as follows:1)A micro-environment measurement device based on unmanned aerial vehicles is integrated with temperature and humidity,CO₂ concentration sensors and a small drone,which can be quickly deployed for on-site measurements.It is suitable for measuring characteristic parameters of micro-environment in hard-to-reach or high-risk locations,such as exterior walls of high-rise buildings,periphery of bridges,and surfaces of dams.2)The applicability of micro-environment simulation software was validated by comparative analysis of field measurements.The invented micro-environment measurement device was used for on-site measurements,and the measured experimental data was compared with simulated micro-environment data.The results showed that the simulated values and distribution of temperature,relative humidity,and CO₂ concentration in ENVI-met can reflect the actual situation on site well,which verified the applicability of the micro-environment simulation software ENVI-met.3)The differences in carbonation of concrete structures under different spatial organizations of buildings were studied using three typical layouts:matrix,enclosure,and staggered.The results showed that the carbonation rate of all three decreased with height（0-18m）,and the maximum difference in carbonation rate was 1.0213 times.In matrix and staggered layouts,the carbonation rate in the western area was greater than that in the central area,which was greater than that in the eastern area,with an average carbonation rate difference of 1.0113 times.In the enclosure layout,the carbonation rate in the central area was greater than that in the western area,which was greater than that in the eastern area,and the maximum difference in carbonation rate was 1.0123 times.However,in all three layouts,the environmental impact within each zone weakened from west to east.In matrix and enclosure layouts,the outer perimeter of the west,south,and north directions had a faster carbonation rate than the inner area,while the outer perimeter of the east direction had a slower carbonation rate than the inner area.In the staggered layout,the outer perimeter of all four directions had a faster carbonation rate than the inner area.4)The micro-environment and carbonation differences were analyzed based on three factors:ground material,wall material,and roof material.The results showed that the decrease in reflectance of ground material had the greatest impact on the reduction of carbonation rate of building structures.To improve the durability of concrete structures,the preferred method is to increase the reflectance of ground materials.5)The differences in carbonation rate of concrete structures were studied under cold and hot meteorological environments by changing the input values of simulated meteorological parameters.The results showed that the carbonation rate in winter was up to 1.0784 times higher than that in summer.Based on a 50-year design life of buildings,the depth of carbonation in the northeastern region of China is about 11mm higher than that in the southern region.6)The lifespan of concrete structures was analyzed and evaluated based on micro-environmental factors.The carbonation lifespan of the lower level was shorter than that of the upper level in all three building organization forms.In matrix and staggered layouts,the carbonation lifespan in the western area was shorter than that in the central area,which was shorter than that in the eastern area.In the enclosure layout,the carbonation lifespan in the central area was shorter than that in the western area,which was shorter than that in the eastern area.The carbonation lifespan of the outer perimeter was shorter than that of the inner area in all three building organization forms,but in matrix and enclosure layouts,the carbonation lifespan of the outer perimeter in the eastern direction was longer than that of the inner area.7)Combining with an actual engineering case in a specific location,the differences in micro-environment and carbonation rate of the existing buildings before and after renovation are analyzed,and suggestions are made for the preliminary structural design of the new buildings in the project.