| Super-fine stainless wire(SSW)has excellent tensile strength and electrical conductivity.It can be widely distributed in concrete at a low volume content,thereby significantly improving the strength and toughness as well as electrical conductivity of concrete.Carbon nanotubes and nano titanium dioxide(NT)have been proved to be representative one-dimensional and zero-dimensional nanomaterials that improve the microstructure,mechanical properties as well as smart properties of concrete from the nanoscale.In this dissertation,the enhancement or modification of the strength,toughness,electrical conductivity and pressure-sensitive property of reactive powder concrete(RPC)incorporating multi-walled carbon nanotubes(MWCNTs)/NT combined with SSW was studied.The effects of filler content and cement type on the properties of RPC were analyzed.The enhancement/modification mechanisms of hybrid nano-fillers and SSW on the properties of RPC were analyzed through theoretical calculation of composite materials.The main research contents and conclusions are as follows:(1)RPC with different cement types and filler contents of hybrid MWCNTs and SSW was prepared,and its flexural strength,compressive strength after flexure,flexural toughness,compressive toughness,and elastic modulus were tested.The results showed that the incorporation of 0.25%MWCNTs increased the flexural strength and toughness of RPC by up to 17.11%and 62.31%,respectively.The use of Portland cement and the addition of 0.5%MWCNTs were more favorable for the increase of compressive strength after flexure and compressive toughness of RPC.The strength and toughness of RPC enhanced with the increase of SSW content.In terms of flexural strength and flexural toughness,hybrid MWCNTs and SSW can play a synergistic strengthening/toughening effect of "1+1≥2" when the volume content of SSW is 1.2%.However,hybrid nano and micro fillers generally has no synergistic effect on the compressive strength after flexure of RPC.Hybrid 0.25%MWCNTs and 1.2%SSW had the best compounding effect,which increased the flexural strength,compressive strength after flexure,flexural toughness and compressive toughness of RPC by 150.66%,94.60%,313.56%and 206.09%,respectively.The elastic modulus of SSW reinforced RPC and RPC with hybrid MWCNTs and SSW was generally higher than that of the control RPC,with the maximum increase of 22.92%.When Portland cement is used,RPC composites obtain higher strength increase,toughness increase,maximum compressive strength after flexure and maximum compressive toughness.However,ordinary Portland cement helps RPC composites achieve higher flexural strength and flexural toughness and improve the elastic modulus of RPC blank group.(2)RPC with different cement types and filler contents of hybrid NT and SSW was prepared,and its strength,toughness and elastic modulus were tested.The results showed that the addition of 1.5%NT increased the flexural strength and flexural toughness of RPC by 37.87%and 48.44%,respectively.3%NT was the optimal content to increase the compressive strength after flexure and compressive toughness of NT reinforced RPC.The mechanical properties of RPC with hybrid NT and SSW increase significantly with the increase of SSW content,and the synergic strengthening/toughening law of hybrid NT and SSW was basically the same as that of hybrid MWCNTs and SSW.The addition of hybrid 1.5%NT and 1.2%SSW resulted in the greatest increase in flexural strength and flexural toughness of RPC,with the maximum growth rates being 171.59%and 466.46%,respectively.However,the compressive strength and compressive toughness of RPC with hybrid NT and SSW were not significantly different from those of RPC containing SSW only.Adding NT or hybrid NT and SSW had no significant effect on the elastic modulus of RPC.The influence of cement type on the strength and toughness of RPC with hybrid NT and SSW was the same as that of RPC with hybrid MWCNTs and SSW.(3)The enhancement mechanisms of MWCNTs/NT in combination with SSW on the mechanical properties of RPC were analyzed through thetheoretical calculation of composite materials.The theoretical calculation results of composite materials showed that the average spacing of SSW in RPC composites decreases with the increase of SSW content,indicating that a more extensive reinforcement network is formed.With the increase of MWCNTs content,the failure mode of MWCNTs changes from breaking to pulling out,while the failure mechanism of SSW in RPC composites is always breaking.Additionally,the bond strength between SSW and the matrix decreases with the increase of SSW content.The addition of an appropriate content of MWCNTs or NT makes the bonding strength between SSW and the matrix in RPC with hybrid nano fillers and SSW higher than that of SSW reinforced RPC,and ordinary Portland cement contributes to higher bonding strength of RPC composites against Portland cement.(4)The direct current(DC)and alternating current(AC)conductive properties of RPC with hybrid MWCNTs and SSW and their pressure-sensitive properties under cyclic and monotonic compressive loadings were tested.The results showed that as the content of MWCNTs increases,the resistivity of MWCNTs reinforced RPC gradually decreased,while its pressure-sensitive properties gradually improved.The use of ordinary Portland cement was beneficial to obtain better conductivity and pressure-sensitive properties.Adding 0.5%MWCNTs reduced the DC resistivity and AC resistivity of RPC by up to 52.16%and 62.2%,respectively.The strain sensitivity of composites under cyclic and monotonic compressive loadings was 51.53 and 36.88,respectively.The DC resistivity(AC resistivity)of SSW reinforced RPC reduced by 2(1),3(2),5(4)and 6(5)orders of magnitude by incorporating 0.15%,0.3%,0.6%,1.2%SSW,respectively.When the SSW content was low,the resistivity of RPC with hybrid MWCNTs and SSW might be lower than that of SSW reinforced RPC.However,a more extensive conductive network was formed in RPC containing 0.6%or 1.2%SSW through SSW overlapping,so further addition of MWCNTs resulted in an increase in resistivity and there is no significant decrease in resistivity under monotonic compressive loading.RPC with 0.3%SSW had outstanding pressure-sensitive properties,and its strain sensitivity under cyclic and monotonic compressive loadings was 13.96 and 135.05,respectively.The pressure-sensitive properties of RPC with hyhbrid 0.5%MWCNTs and 0.15%SSW under cyclic and mono tonic compressive loadings were superior to RPC with the corresponding single filler,while the pressure-sensitive performance of RPC with hyhbrid 0.5%MWCNTs and 0.3%SSW under cyclic and monotonic compressive loadings were close to that of SSW reinforced RPC.The cement type and mix proportion affect the conductivity and pressure-sensitivity properties of RPC.RPC made with ordinary Portland cement has higher resistivity.Increasing the water-binder ratio reduces the resistivity of RPC and increase its pressure-sensitive performance under monotonic compressive loading.(5)The DC and AC conductive properties of RPC with hybrid NT and SSW and their pressure-sensitive properties under cyclic and monotonic compressive loadings were tested.Adding 1.5%or 3%NT did not significantly reduce the resistivity of RPC.Increasing NT content improved the pressure-sensitive property of RPC.The resistivity of RPC with hybrid NT and SSW decreased with the increase of SSW content.Compared with SSW reinforced RPC,RPC with hybrid 1.5%NT and 0.6%SSW,and RPC with hybrid 1.5%NT and 0.15%SSW and with high water to binder ratio achieved lower resistivity.When Portland cement was used,the strain sensitivity of RPC with hybrid 3%NT and 0.15%SSW under cyclic and monotonic compressive loadings was 134.77%and 45.80%higher than that of SSW reinforced RPC,respectively.When ordinary Portland cement and high water to binder ratio were used,RPC with hybrid NT and SSW also had better pressure-sensitive properties than SSW reinforced RPC. |
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