Investigations On Electrical Properties And Mechanical-Electrical Impedance Effects Of Ultra-High Performance Concrete

Ultra high performance concrete(UHPC)has excellent mechanical properties and is widely used in civil engineering structures.In this paper,by tapping the force-impedance effect of UHPC,the smart UHPC is developed to realize the self-sensing function of the structure,so as to provide a strain sensor with good durability and low cost for the health monitoring system of civil engineering structures.At present,DC resistance is the main electrical parameter used to characterize the strain self-sensing characteristics of smart concrete,while impedance(including real part and imaginary part)and capacitance based on AC test are seldom used to characterize its characteristics.In view of this,this paper focuses on the strain self-sensing performance of smart UHPC under AC test by experimental method.The main work of this paper is as follows:(1)Five kinds of UHPC with different steel fiber content were prepared to study the influence of steel fiber content,fiber diameter and testing method on the mechanical and electrical properties of UHPC,and establish an equivalent circuit model to analyze the mechanism of its conductivity and electrical polarization.The results show that the addition of steel fiber increases the compressive strength and elastic modulus of UHPC,decreases the resistivity and impedance of UHPC,and increases the capacitance of UHPC.The resistivity measured by AC method is obviously lower than that measured by DC method,and there is little difference between the impedance measured by four-electrode method and two-electrode method.With the increase of steel fiber content,the radius of high frequency arc of UHPC AC impedance spectrum decreases,that is,the ion transfer resistance of fiber-matrix interface decreases.Meanwhile,the non-Debye phenomenon is becoming more pronounced.When the diameter of steel fiber is reduced,the radius of high frequency arc of UHPC AC impedance spectrum increases,and the ion transfer resistance at fiber-matrix interface increases.(2)Four electrode DC test,four electrodes were used respectively to exchange and two electrodes AC testing methods such as research UHPC when unidirectional compression and uniaxial tensile force-impedance effect,compare the UHPC under different test methods for strain sensing sensitivity,signal-to-noise ratio and linear features,and explore the mechanism of the strain response such as resistors,capacitors.The results show that the AC resistance and capacitance obtained by the two-electrode AC test method are more sensitive to characterize the strain self-sensing performance of UHPC.When the volume content of steel fiber is 1.5%,the strain sensitivity and linearity of UHPC are better.When UHPC is pressed,AC resistance decreases and capacitance increases with the increase of strain.The signal-to-noise ratio of AC resistance to strain is better than that of capacitance,and the sensitivity represented by AC resistance is slightly higher than that of capacitance,so measuring AC resistance of UHPC is more conducive to strain self-sensing than measuring capacitance.(3)At 100 Hz,1 kHz,10 kHz,100 kHz and 300 kHz frequencies,the changes of the real and imaginary parts of the impedance of UHPC during cyclic compression were studied,and the influence of AC test frequency on the sensitivity and repeatability of the force impedance effect of UHPC was discussed.The results show that the real part of the impedance of UHPC has the highest sensitivity to strain perception at 100 Hz,and the imaginary part of the impedance has the highest sensitivity to strain perception at 100 Hz or 1 kHz.With the increase of frequency,the sensitivity of strain perception decreases,but the repeatability becomes better.