Research On The Internal Damage Of Nano-Material Modified Concrete Under The Coupling Of Multiple Factors

It is a complex physical and chemical reaction process to destroy the durability of concrete under the coupling of sulfate attack,carbonation and freeze-thaw cycles.At present,some research results have been made at home and abroad,but due to the inconsistency of the experimental system and evaluation criteria,the conclusions of the study are different.Moreover,there are many salt lakes and saline soil areas in most of the northwest areas of China.Due to the influence of the environment,the temperature difference between day and night is large.With the effect of various environmental factors,the damage degree of concrete buildings is aggravated,and the service life is difficult to meet the design requirements.Therefore,it is of practical significance to study the internal damage of nano modified concrete under sulfate attack,carbonation and freeze-thaw cycles.At present,nano-materials are widely used in many fields such as mechanics,pharmacology,material science and mechanics.They also have broadened application prospects in the fields of chemical engineering,medicine,aerospace and aviation,which are hailed as the most promising materials in the 21st century.In this thesis,the internal deterioration of nano-modified concrete with different contents（0.2%,0.4%,0.6%,0.8%）,different sizes and types（15nm SiO₂,30nm SiO₂ and30nm TiO₂）is mainly studied under the combined effect of various environmental factors.Nano-modified concrete here is subjected to sulfate attack（3 days）,carbonization（3 days）and freeze-thaw cycles（0 times,25 times,50 times,75 times）,and the mass loss as well as dynamic elastic modulus in the freeze-thaw cycle are measured.Industrial CT is used for non-destructive testing under each environmental factor,and damage evaluation indicators,such as mass loss rate,relative dynamic elastic modulus,porosity and pore distribution of concrete,are discussed and analyzed.The results are as following:（1）In freeze-thaw cycles,the mass loss rate shows an increasing trend,and the relative dynamic elastic modulus shows a decreasing trend.（2）The mass loss rate and relative dynamic elastic modulus of 15nm SiO₂,30nm SiO₂and 30nm TiO₂modified concrete with 0.6%content are smaller,which show the trend:15nm SiO₂<30nm SiO₂<30nm TiO₂.It indicates that 15nm SiO₂ with the content of 0.6%can better improve the internal structure of concrete and reduce the mass loss rate and relative dynamic elastic modulus loss during the freeze-thaw cycles.（3）The porosity change trend of nano-modified concrete under different content is consistent with the change trend of mass loss rate and dynamic elastic modulus,which all show a decreasing trend during the freeze-thaw cycles.When the content is 0.6%,the porosity reduction range of nano-modified concrete is relatively small.The porosity reduction of 15nm SiO₂,30nm SiO₂ and 30nm TiO₂modified concrete is about 21.27%,23.14%,and 30.84%,respectively.（4）In carbonation environment,the porosity of nano-modified concrete is decreasing due to the formation of reaction product Ca CO₃,which fills the pores.In freeze-thaw environment,as freeze-thaw cycles increase,the internal pore structure of concrete occurs with the change,and the expansion speed of the internal pores increases,which result in a significant change in internal porosity of concrete,showing an increasing trend.（5）In carbonation environment,the reaction product Ca CO₃ is generated,which may fill the pores of 0.01-0.1mm³and reduce some pores of 0.01-0.1mm³ to≤0.01mm³.Therefore,the pore volume of≤0.01mm³ shows an increasing trend.In the freezing and thawing environment,water enters the pores and freezes into ice with volume expansion,squeezing the pore walls and then increasing the pore volume.SiO₂ may react with the hydration product and produce hydrated silicate gel,which will enhance the density of the transition area and inhibit the initiation of micropores during the freezing and thawing process,and thus the pore volume of≤0.01mm³ tends to decrease,while the pore volume of>10 mm³ is increasing on the whole.（6）During the freeze-thaw cycles,there exists frost heaving force,as well as effects of nano-materials on inhibiting micro-pores initiation and improving the internal structure of concrete,which result in the different tendency of small pores expanding into large pores.Therefore,the pore volume ratio of 0.1-1 mm³ and 1-10 mm³ fluctuates.In this thesis,the internal damage of nano-modified concrete under the coupling of multiple factors is studied,which will provide a certain idea and basis for the mix proportion design and durability evaluation of nano-material modified concrete.