Double percolation phenomenon of carbon nanotube/cement composites as piezoresistivity sensing elements with exposure to salt environment

It has been known that the carbon nanotube (CNT)/cement composites can be used to reflect the external loading information and detect premature damage; however, the agreement on whether the piezoresistivity as a function of external loading is positive or negative has yet to be achieved, and the innate electrically conductive mechanism remains unclear. In this study, to minimize the application cost, small-size CNT/cement composites were embedded into the surface of the concrete as sensing elements, and the piezoresistivity of the CNT/cement composites under external loading along with/without salt attacks was tested. The results indicate that the piezoresistivity significantly depends on the percolation backbone density and the critical exponent of the CNT networks near the percolation threshold. Without salt environments, an "M" shape was observed in the curve of the fractional electrical resistivity (FCR) as a function of cyclic loading applied on the concrete samples. However, when the concrete samples were exposed to a salt environment, the "M" shape in the FCR curve disappeared. In addition, based on the percolation theory, the electrical resistivity changes as a function of strain fits well with an exponential function. The microstructure analysis demonstrates that the pore structure of the CNT/cement composites can be divided into spherical pores and cracks with layered structures. This study not only provides a new insight into the electrical conduction mechanism in CNT/cement composites systems but also sheds light on how to accurately monitor and analyze concrete structures with external loads, especially along with salt environment.