Study On Mechanical And Microscopic Properties Of Concrete In Corrosive Environment

Concrete material has gradually become one of the most widely used building materials in the world because of its advantages such as easy to obtain materials,rich resources and simple construction technology.However,concrete material is essentially a kind of in homogeneous porous material composed of sand,stone,cement and other mixtures,which will inevitably be chemically corroded by many external factors such as carbon dioxide,water,sulfate,chloride and other medium.As a result,the damage of concrete is accelerated and its durability is greatly reduced.In other words,the corrosion damage caused by sulfate and chloride corrosion is an important reason for the deterioration of the durability of concrete structures.Therefore,a systematic and comprehensive understanding of the changes of mechanical properties of concrete after corrosion is the key to ensure the durability and safety of engineering structures.Mastering the performance of concrete after corrosion and the change law of internal structure is the theoretical basis to ensure the safety of concrete structure operation,improve concrete quality and realize the long-term service of concrete in construction engineering.Taking the early-age concrete subjected to temperature as the research object,this paper explores the change law of mechanical properties of concrete corroded by sulfate and chloride salt,and combines nuclear magnetic resonance(NMR)and scanning electron microscope(SEM)techniques to prove the reasons for the change of macroscopic phenomena from a microscopic point of view.The main contents and results of this paper are as follows(1)After one day of temperature damage at 60℃,90℃ and 120℃,the compressive strength of concrete decreased by 4.16%,8.40%and 11.68%respectively compared with the base group,and this trend became more and more obvious with the increase of early age temperature.After 3 days of temperature treatment,the decreasing trend of strength was more significant,which decreased by 5.34%,10.07%and 14.50%,respectively,compared with the benchmark group.When the concrete was moved into the standard curing environment,when its age was 7 days,the difference between the strength of each specimen and the reference group decreased by 3.41%4.92%and 5.65%respectively after the early age temperature,and the development of concrete strength reached a turning point at this time.When the age is 28 days,the strength is 99.0%of the compressive strength of the base group,98.32%and 98.18%,respectively.At this time,the effect of early age temperature on the strength characteristics of concrete is no longer obvious.(2)After the concrete is corroded by single sulfate,chloride salt and sulfate-chloride compound salt,the apparent morphology begins to change,which is mainly manifested as the defect of the upper and lower end face and the exfoliation of one layer from the outside to the inside.The change of specimen quality before and after corrosion was tested.With the increase of corrosion time,the change rate of concrete quality under different corrosion schemes experienced a process of first increasing and then decreasing.Taking scheme 1 as an example,when the specimen was corroded for 30 days,the quality of the specimen was 3.5g higher than that before corrosion,and after 150 days,it was found that the quality of the specimen decreased by 2.77g,and the quality was lower than that before corrosion,and the concrete quality began to reverse growth.With the extension of corrosion time,the quality of the specimen decreased further and decreased by 0.44%compared with the original value(3)Through the uniaxial compression test of concrete specimens under the action of temperature at different early ages.The results show that the peak stress,peak strain and elastic modulus of concrete do not change obviously with the influence of early age temperature.The peak strength of the specimens corroded by 10%,15%and 20%sodium sulfate solution in 7 months decreased by 7.25%,9.82%and 15.67%compared with that in 5 months.The peak strength of the specimens corroded by 10%,15%and 20%sodium chloride solution in 7 months decreased by 6.51%,7.40%and 13.96%compared with that in 5 months.The peak stress of the specimens corroded by three concentrations of sodium sulfate and sodium chloride mixed solution for 7 months decreased by 3.01%,3.50%and 5.57%,respectively.When concrete is corroded,with the increase of solution concentration,the full stress-strain curve of the specimen moves to the right,the peak stress and elastic modulus decrease,the peak strain increases and the ductility increases.With the extension of corrosion time,the change trend of the above mechanical indexes is the same as that caused by concentration factors.(4)The peak stress of concrete under cyclic loading and unloading is less than that under the full stress-strain curve test.for example,after 5 months of sulfate corrosion with a concentration of 10%,15%and 20%,the peak stress of cyclic loading and unloading is 5.60%,7.68%and 17.59%lower than that under the full stress-strain curve test,respectively.The results show that the cyclic loading and unloading action weakens the bearing capacity of concrete again,and the more serious the corrosion damage is,the more obvious the cyclic loading and unloading effect is.(5)The results of nuclear magnetic resonance(NMR)and scanning electron microscope(SEM)can well characterize the changes of internal pore size,number and product number of concrete after corrosion.The change mechanism of macroscopic mechanical properties such as mass loss rate,strength and elastic modulus of concrete is revealed from the microscopic point of view.Figure 44 Table 22 Reference 74...