Experimental Study On The Durability Of Multi Walled Carbon Nanotube Ultra-high Performance Concrete(MWCNTs-UHPC) Under Salt Erosion And Freeze-thaw Coupling Environment

Durability failure is one of the main causes of concrete structure failure.Especially in various civil engineering in cold areas such as northern China,concrete materials will inevitably be affected by different degrees of salt erosion and freeze-thaw.Using multi walled carbon nanotubes(MWCNTs)to prepare ultra-high performance concrete(UHPC)has important theoretical and practical significance for improving the durability of concrete structures in complex environment.In this paper,cement-based materials were used as the matrix to prepare UHPC by adding MWCNTs,and its mechanical properties and durability were studied.The main research contents and achievements of this paper are as follows:(1)The effects of different MWCNTs content(0%,0.02% 0.05%,0.10%,0.15%,0.20%,0.30%)and different water binder ratio(0.16,0.17,0.18,0.19,0.20)on the mechanical properties of UHPC are studied,and the optimal mix proportion was obtained based on the mechanical properties.The results show that the compressive strength and flexural strength of the specimens increase first and then decrease with the increase of the content of MWCNTs.Considering the cost factor and based on the mechanical property test results,the optimal mix proportion to prepare MWCNTs-UHPC was water binder ratio of 0.19 and MWCNTs content of 0.1%.At this time,the compressive strength of the specimen was 122.7 MPa and the flexural strength was 14.2 MPa,which was 34.1% and 13.6% higher than that of the control group with 0 content respectively.(2)The salt erosion resistance of MWCNTs-UHPC under different salt erosion(bicarbonate,chloride and sulfate)was studied.The results show that with the increase of erosion age,the compressive strength and corrosion resistance coefficient of concrete in clean water increase.Under different salt erosion,the compressive strength and corrosion resistance coefficient of concrete decrease first,then increase slowly,and then decrease gradually.When the erosion age is 180 days,the compressive strength of the five groups of specimens under different erosion environments were 3.43%,2.69%,1.22%,2.58%,-0.81% lower than that of the control group that have not been eroded.The corrosion resistance coefficients were 96.57%,97.31%,98.78%,97.42%and 100.81% respectively.MWCNTs-UHPC has good salt erosion resistance under the condition of long-term immersion erosion of salt solution.(3)According to the changes of apparent morphology,compressive strength,mass loss rate and relative dynamic elastic modulus(RDME),the damage law of MWCNTs-UHPC subjected to salt attack and freeze-thaw(FT)cycle under different salt attack is analyzed.The results show that with the increase of FT cycles,the micro pores on the surface of the specimen gradually increase and the mortar is gradually exposed.The compressive strength decreases gradually.After 1500 FT cycles,the loss rate of concrete compressive strength under different salt erosion were 47%,40%,13%,24% and 14%respectively.The mass loss rates were-1.09%,-0.85%,0.73%,1.73% and 3.18%respectively.RDME was 91.3%,94.3%,99.36%,99.03% and 99.55%respectively.The more complex the salt component of the erosion solution is and the greater the mass fraction is,the more serious the frost heave damage of concrete is.(4)Based on nuclear magnetic resonance technology(NMR),the microstructure changes of concrete under different environmental factors were revealed.The results show that the microstructure of concrete is still dense after1500 salt freezing cycles.In the single salt freezing environment,the number of small holes in concrete samples increases,while in the compound salt freezing environment(coupling of bicarbonate,chloride and sulfate erosion with FT),the number of small holes decreases.The proportion of medium and large pores decreases slightly,while the proportion of small pores increases more.The larger the mass fraction of erosion solution,the smaller the change of pore ratio.The porosity of the samples in five different salt freezing environments and the control group without salt freezing were 3.09%,3.35%,4.69%,4.69%,4.39% and 3.56% respectively.MWCNTs UHPC has good salt freezing resistance and durability in the complex salt freezing environment with a design service life of 150 years.