Influence Of Environmental Condition On Piezoresistivity Of Carbon Nanofibers Cement-based Materials

Structural health monitoring is an important approach for ensuring the health and safety of civil infrastructures and also becomes a technique for research of the damage accumulation or even disaster evolving characteristics of civil concrete infrastructures.Recently,self-sensing concrete is a hot topic for health monitoring research of the concrete structures.Self-sensing concrete is fabricated through incorporating some conductive fillers or fibers into cement-based materials to increase its ability to sense the stress,strain and displacement by itself.Comparing with other smart sensors,the self-sensing concrete has the advantages of long-term durability,reliability and good compatibility with concrete structures.Carbon nanofibers(CNFs)possess excellent conduction and mechanical properties,while they are easily dispersed in cement matrix.Therefore,more attention has been paid to the preparation of smart cement-based materials by using carbon nanofibers.This thesis aims to study the influence of rheological parameters of fresh cement-based paste on the dispersion of carbon nanofibers(CNFs).Additionally,the electrical resistance and the piezoresistive performances of CNFs cement-based materials under different temperatures,moisture contents or freeze-thaw environments were studied in this paper.The experimental study and mechanism analysis of these researches provided the basis for the preparation of CNFs cement-based materials and their self-sensing application in different environments.The main research contents are as follows:The percolation zone of CNFs cement-based materials is obtained by experiments.A reasonable content of CNFs was selected in the percolation zone in order to research the influence of rheological parameters(the slump flow,the plastic viscosity and the yield stress)on fibers dispersion.Cement paste with different rheological properties were obtained by changing the water cement ratios(w/c),adding different mineral admixtures and water reducing agent.The variability coefficient(Cv)of electrical resistivity for each group of samples were determined to evaluate the dispersing efficiency and the influencing mechanism.Results showed that for each group of cement paste the slump flow from 205 mm to 215 mm were most favorable for enhancing the dispersion of CNFs,an excessive high or low fluidity or viscosity led to an adverse effect.CNFs cement paste with the best fibers dispersion led to the optimal linearity and sensitivity of the piezoresistivity.The influence of electrical testing conditions(including different AC frequencies,drying state,immersed in Na Cl solution for long time)on the electrical parameters of CNFs cement paste(CNFP)with embedded electrodes and pasted electrodes were studied.Results indicated that the electrical resistivity,the variability coefficient(Cv)and the capacitive reactance were reduced by the increase of AC frequency effectively when the CNFs was in low dosage.At the same time the immersion of Na Cl solution reduced the resistivity and improved the capacitive reactance of CNFP obviously.The CNFP with pasted electrodes presented higher resistivity and capacitive reactance than the specimen with embedded electrodes when the CNFs content was smaller than a certain critical value.The CNFP showed favorable piezoresistivity as the electrical resistivity ranges from 1.0k?·cm~20k?·cm.A multi-axis strain piezoresistive model of smart cement-based materials was selected to calculate the piezoresistive results of CNFs cement paste.The experimental piezoresistive results satisfied the theoretical piezoresistive results.The influence of temperatures and moisture contents on electrical resistance and piezoresistivity of carbon nanofibers cement mortar were investigated.The mechanisms were analysed by the carrier transport conduction,the electric phenomena,the electric polarization and the macroscopic tunneling effect.The CNFs mortar with appropriate CNFs amount performed favorable piezoresistivity when tested in the drying state and normal temperature.The sensitivity and linearity of strain-sensing property for CNFs mortar declined as the temperature decreased from 20? to-25? with the same moisture contents.As the temperature is higher than 20?,the sensitivity and linearity of strain-sensing property decreased as well.However,the sensitivity first decreased and then increased with the increasing moisture contents at the same temperature.The damage characteristics and piezoresistive evolution of CNFs mortar exposed to freeze-thaw(F-T)environment in the medium of water,1.5% NaCl solution and 3.0% NaCl solution at temperatures of-10?~10? and-20?~20? respectively were determined in this paper.Results showed that no obvious mass loss happened in the CNFs mortar after 300 F-T cycles at temperature-10?~10?,while the relative dynamic modulus of elasticity decreased and the fractional change in electrical resistivity increased obviously.The strain sensing sensitivity and linearity of the CNFs mortar with low w/c performed the highest in drying state while it decreased obviously after F-T cycles.However,the CNFs mortar performed the worst sensitivity and linearity in drying state with high w/c.Nevertheless,the sensitivity and linearity of the CNFs mortar increased after F-T cycles.Mass loss and piezoresistivity deterioration were accelerated by salt F-T environment at temperature-20?~20?.The F-T damages and the piezoresistivity deterioration were increased by cyclic load.The damages,the piezoresistivity and the flexural self-monitoring performances with three point bending experiment of CNFs concrete subjected to F-T cycles were researched in this thesis.Results indicated that the sensitivity and linearity of piezoresistivity and flexural sensitivity deteriorated after F-T cycles(The deterioration of salt F-T cycles was more obvious).The theoretical flexural sensitivitive equations by flexural self-sensing models were approximate linear equations.Under the same conditions,flexural self-sensing property by the surface resistance monitoring was more sensitive than the volume resistance monitoring,while the linearity was worse.