Influence Of Environmental Conditions On The Piezoreresistive Effect Of Carbon Nanofiber Concrete

Well-dispersed Carbon Nanofiber （CNF） could improve the mechanical andelectrical property of concrete, because of its Nanoscale size, large length to diameterratio, high strength, low resistivity. Overseas studied CNFC had a fine property ofpiezoresistive effect. So CNFC applied to structural health monitoring not onlyovercome the compatibility problem of embedded sensors and concrete, but alsodecreased the cost on the acquisition system. So CNFC could conform to the needs ofthe long-term structural health monitoring and had been the advanced item in this field.This issue had systematic researched the optimum adding amount of CNF, conductivemechanism of CNFC and all kinds of environmental conditions influencing thepiezoresistive effect. The main research contents are as follows:1. CNF dispersed condition is related to the concrete workability, modified by thepolycarboxylate high range water reducer solution. Research shows that it would usemore and more polycarboxylate high range water reducer solution so as to make theconcrete workability parameter T₅₀₀ and T_h within2s-5s and5s-20s respectively withthe increase of CNF adding amount, which confirm the CNF well dispersed in CNFC.2. CNFC conductive mechanisms were discussed based on the percolation theoryand the tunnel effect theory. The CNFC conductive paths consist of contact type andno-contact type, the former mainly include CNFs connection, connection between theCNFs and matrix interface and matrix conduction. The latter mainly includes electronsand positive holes transition.3. The electrical property and piezoresistive effect were researched under thecondition of different concrete age, aggregate types, CNFs adding amount, dry and wetstate of spicemen. The research results are following:（1）The resistivity of CNFCdecreased with the change of the CNFC age, while polarization time changed irregularly.（2） CNF cement, CNF mortar and CNFC had been molded to research their electricalproperties and piezoresistive effect. The results implied the resistivity values range isCNF cement＜CNF mortar＜CNFC and Polarization time presented irregularly. Thepiezoresistive effect curve shapes of the CNF mortar and CNFC kept similar and theirvariation rates is CNF mortar＜CNFC while the CNF cement was unsatisfactory.（3）Themore CNF added, the more resistivity of CNFC decreased. When the adding amountsvary from0.5%to1.8%, the piezoresistive curves show the linerly down with theloading increased. The change of polarization time still kept irregularly, piezoresistiveeffect curve kept same shape, the resistivity change rates was smaller.（4）Dried CNFCown shorter polarization time and lager resistivity than that with water content, whilepiezoresistive effect remained unchanged. 4. The electrical property and piezoresistive effect were discussed under theatmosphere of freezing and thawing cycle, NaCl corrosion, different temperature andhumidity. The research results are as follows:（1）After175freezing and thawing cycles,the electrical property of CNFC had not changed obviously, piezoresistive effect curvekept same shape, the resistivity change rates was smaller.（2）The Nacl adding amountschanged from0.5%to1.0%, the resistivity of CNFCs increased and polarization timealso changed irregularly. The shape of piezoresistive curves kept same, but theresistivity varing rates decreased gradually with the same level loadings. Thepiezoresistive effect curve kept same shape, the resistivity variration rates was smaller.（3）The resistivity of CNFC increased rapidly and polarization time curve present downwhen the temperature gradually rised.（4） The resistivity of CNFC decreased and thepolarization time increased with the humidity rised, the piezoresistive effect remainedunchanged.