Coupled Ionic–Electronic Finite Element Modeling of Piezoresistivity in Carbon Nanotube-Reinforced Cement Composites

Carbon nanotube (CNT)-reinforced cement composites are promising self-sensing materials for monitoring buildings and civil infrastructure, but their piezoresistive response arises from coupled, spatially heterogeneous conduction mechanisms that empirical models do not resolve. This study develops a three-dimensional electromechanical finite element framework in which the capillary porosity and CNT content vary spatially, the conductivity combines ionic conduction through the pore solution with electronic conduction through CNT contact and tunneling, and the connectivity of both networks evolves with a signed principal-strain measure that distinguishes gap closure from gap opening. The framework was calibrated against cyclic uniaxial compression tests on mortar containing 0.5% multi-walled CNTs by mass of cement in three moisture states. The simulations reproduced the negative piezoresistive response, and the agreement with the measurements improved as drying reduced polarization. Parametric analyses showed that the response is governed by the connectivity of the pore and CNT networks and by its strain sensitivity. Reducing the matrix porosity from 0.30 to 0.22 decreased the minimum normalized voltage from 0.949 to 0.841, and triaxial compression produced the largest response (0.82 versus 0.94 under uniaxial compression) because confinement suppresses local tensile zones. The framework supports the interpretation and design of CNT–cement sensors embedded in building structures.