Cross-scale Model Study On Piezoresistive Effect Mechanism Of Carbon Nanotube Concrete Member

Carbon nanotube concrete,as a composite material with multiple phases,has different structural characteristics and mechanical properties at different scales,and the nano,micro and micro structural characteristics are the root of reflecting the macro properties.In order to better reveal the mechanism of piezoresistive effect of carbon nanotube concrete,it is necessary to establish a cross-scale model.The model can directly reflect the relationship between the four scales and piezoresistive law,which is of great significance for the application of carbon nanotube concrete in structural health monitoring.Therefore,it is necessary to study the cross-scale model of carbon nanotube concrete components.In this paper,carbon nanotubes cement-based components at macro and meso scales are studied by combining theoretical analysis and laboratory tests.A cross-scale model study on piezoresistive effect of carbon nanotube concrete member was carried out.At the same time,the four-point bending test of carbon nanotube steel spacer trabeculae was also carried out.The main research contents and conclusions of this paper are as follows:（1）The piezoresistive effects of carbon nanotube concrete and carbon nanotube mortar were analyzed.Firstly,four scales of carbon nanotube concrete are defined.Secondly,the piezoresistive tests of macroscopic and meso-scale components of carbon nanotube concrete were carried out.Finally,the piezoresistive mechanism of carbon nanotube concrete at four scales is analyzed.The results show that both macroscopic and microscopic components have piezoresistive repeatability,and their resistivity is about150Ω·m.And there is some difference between the two laws,the resistivity change value of the microspecimen is large,and the difference between them is about 45%.The piezoresistive mechanism and characteristics of carbon nanotube concrete at four scales are revealed,and a two-step homogenization model is established.（2）A cross-scale model of piezoresistive effect mechanism of carbon nanotube concrete member is established.Firstly,the piezoresistive coefficient and other physical parameters of carbon nanotubes were measured and calculated by piezoresistive test.Secondly,four scale piezoresistive models of carbon nanotube concrete are established respectively.Finally,by integrating the piezoresistive effect models of four scales,the final cross-scale model is established.The results show that according to the test and calculation formula of piezoresistive coefficient of carbon nanotube concrete,the value of piezoresistive coefficient of carbon nanotube concrete is 3.08×10^-8 m²/N.The piezoresistive effect models at the four scales are uniform and can be related by the changes of parameters、β.The cross-scale model of piezoresistive effects of carbon nanotube concrete members can be further improved by two correction coefficients.（3）The piezoresistive effect of trabeculars of carbon nanotube steel spacer under four-point bending loading was analyzed.Firstly,the best model of carbon nanotube reinforcement spacer was selected by contrast test.Secondly,the four-point bending test of carbon nanotube steel spacer trabecula was carried out.Finally,the reason of piezoresistive law is analyzed.The results show that through the comparison of electrode making and piezoresistive effect,the square steel bar spacer is selected as the model of intelligent steel bar spacer,and its resistivity is about 150Ω·m.Under four-point flexural loading test,the internal bar spacer also has piezoresistive repeatability,and its resistance change rate is about 2.5%.Its resistivity has a certain volatility,but the final change of the general trend is predictable.