Research On Deterioration Mechanism And Life Prediction Of Green Concrete Under Multi-factor Environment

China has a vast territory,and the service environment for concrete is extremely complicated,and it often experiences the co-erosion of several environmental factors at the same time.In this paper,green concrete materials were taken as the research object.The method of combining theoretical analysis with laboratory tests,macro tests and micro analyses were used to evaluate the deterioration of concrete under the conditions of coupling freeze-thaw cycles with sulfate attack,and between wet and dry cycles with sulfate attack.The following research work has been performed:(1)The service conditions of concrete structures in the coupled environment of freeze-thaw cycles and sulfate erosion,and the combination of dry and wet cycles and sulfate erosion were simulated,the experimental and theoretical analysis methods were used to study the changes of physical and mechanical properties of green concrete,and the influence of different erosion age,erosion environment,different amount of fly ash and bentonite on the main mechanical properties of green concrete were quantitative analysed,the "degradation process time order" for the degradation of green concrete performance in a multi-factor coupling environment was established.The results show that the influence caused by the sodium sulfate solution to the freeze-thaw cycle was promoted first and then suppressed,while the magnesium sulfate solution always had an inhibitory effect on the damage of the concrete under the freeze-thaw cycle.In the magnesium sulfate solution and water,when the ratio of fly ash to cement was 1.2-1.8,it can effectively reduce the degree of deterioration of concrete,generally up to 30%.In magnesium sulfate solution,the degree of deterioration of concrete decrease d with the increase of the amount of bentonite,which can reach 15% under normal circumstances.(2)Under the environment of coupled freezing and thawing cycle and sulfate erosion,and coupled environment between dry and wet cycle and sulfate erosion,the green concrete prepared by adding appropriate amount of bentonite and fly ash to the fiber concrete was taken as the research object,environmental scanning electron microscope,X-ray Diffraction and nuclear magnetic resonance test methods were used to systematically study the influence of different amount of fly ash and bentonite,different erosion environments and erosion age to 324 green concrete specimens on the aspect of microstructure evolution and chemical composition changes,the distribution and development of internal pore were summarized,microcracks formation process and expansion trend were summarized,general types of erosion products and morphological evolution were summarized,and the deterioration mechanism of green concrete durability in a multi-factor coupling environment was revealed,and the corresponding durability design recommendations was given.The results show that the erosion products were mainly composed of calcium silicate hydrate,gypsum and ettringite,among which the amount of ettringite,gypsum crystals and micro-cracks increased with the age of erosion.At the same time,as the age of erosion increased,the tiny pores inside the green concrete gradually expanded due to the salt erosion,and eventually connect to form larger pores.In addition,adding a certain amount of fly ash and bentonite can reduce the number of tiny pores in the corresponding material,indicating that the durability was improved.According to the research results,it is suggested that the ratio of the amount of fly ash to the amount of cement is 1.2-1.8,and the amount of bentonite is about 3% of the amount of cement.(3)Under the environment of coupled freezing and thawing cycle and sulfate erosion,and coupled environment between dry and wet cycle and sulfate erosion,green concrete with different amounts of fly ash and bentonite was taken as the research object.The uniaxial compression stress-strain full curve of 972 specimens under different erosion environments and erosion age were measured,the corresponding stress-strain full curve equation were established considering the different content of fly ash and bentonite,and the effect of the content of fly ash and bentonite on the full curve in different environments were analyzed.The results show that after 150 freeze-thaw cycles,the peak stress of concrete decreased the most in magnesium sulfate solution and the least in water,while the peak strain increased the most in magnesium sulfate solution,the value increased by 83.2%.(4)Under the conditions of freeze-thaw cycle and dry-wet cycle,green concrete with different contents of fly ash and bentonite was taken as the research object,and the entire process of the generation and expansion of micro cracks in green concrete under uniaxial compression was dynamically monitored using an acoustic emission instrument.The corresponding parameters of the acoustic emission signal were analyzed,and the parameters of the acoustic emission signal from the initial loading to the destruction were analyzed.The damage evolution model of green concrete w ere established based on gray system theory and catastrophe theory,and the damage degree of green concrete during loading process was quantitatively evaluated,where the loading stress dropped to(80-90)% of the peak stress,the acoustic emission signal was significantly enhanced,indicating that the degree of material damage was increased.The analysis results showed that the model can reflect the damage state of the material in the whole process of uniaxial compression.(5)Based on the Weibull distribution theory,the two-parameter Weibull distribution was used to describe the failure probability distribution of green concrete.The degree of concrete damage was characterized by the relative dynamic elastic modulus,and the damage evolution model and the service life prediction model of green concrete under freeze-thaw cycle and dry-wet cycle were established..At the same time,according to the relevant regulations of the minimum limit of concrete durability in the current "Code for Durability Design of Concrete Structures" in China,the recommended service life of green concrete under the action of freeze-thaw cycle and dry-wet cycle were given,it can be used as a reference for further research and corresponding durability design.