Effects of salt freeze-thaw cycles and cyclic loading on the piezoresistive properties of carbon nanofibers mortar

This paper aims to investigate the evolution of electrical resistance and piezoresistivity of carbon nanofibers (CNFs) mortar with freeze-thaw cycles in solutions containing 0%, 1.5% and 3.0% NaCl. Two temperature ranges selected for the freeze-thaw cycles are −10 °C to 10 °C and −20 °C to 20 °C respectively. Four groups of cement mortars with water to cement ratios (w/c) of 0.5, 0.45, 0.4 and 0.35 were prepared by incorporating 2.25% CNFs by cement volume. Additionally, cyclic loading damage was applied to selected samples before exposure to freeze-thaw cycles. Experimental results showed that the strain-sensing sensitivity and linearity of CNFs mortar with low w/c ratios were decreased by the freeze-thaw cycles under temperature of −10 °C to 10 °C. However, the samples with high w/c ratios exhibited an opposite tendency with freeze-thaw cycles. The higher concentration of NaCl solution induced the worse degradation of sensitivity and linearity of low w/c ratio samples. The salt freeze-thaw cycles under temperature of −20 °C to 20 °C resulted in the obvious degradation of piezoresistivity for all the samples. Moreover, the cyclic loading history before freeze-thaw cycles debased the sensitivity and linearity of strain-sensing property of CNFs mortars. Therefore, negative effects from both mechanical loading and freeze-thaw environment should be fully considered for the field application of this type of self-sensing material.