Scalable carbon nanotube-coated aggregates for synergistic piezoresistive enhancement in self-sensing concrete

Self-sensing concrete offers a direct route for structural health monitoring by converting loading, strain development, and crack evolution into measurable electrical responses within the cementitious matrix. Conventional electrically conductive cementitious composites commonly rely on directly dispersed conductive fillers, but their performance is limited by filler agglomeration, unstable conductive paths, high dosage requirements, and the large volume fraction of non-conductive aggregates. To address these limitations, this study combines scalable fabrication of carbon nanotube (CNT)-coated conductive aggregates with a dual-phase conductive network formed by CNT-coated aggregates and chopped carbon fibers (CFs). Quartz sand was used as the aggregate substrate, polyacrylic resin as a thermoplastic bonding interlayer, and multi-walled CNTs as the conductive phase after comparison with graphene and activated carbon. The conductive aggregates were prepared using a two-stage coating method and characterized by packed-aggregate resistivity testing, scanning electron microscopy (SEM), and two-stage thermal decomposition analysis. The optimized aggregates were then incorporated into cementitious specimens with varying conductive aggregate replacement ratios and CF dosages to evaluate electrical percolation, interfacial morphology, and piezoresistive behavior under cyclic loading. CNTs formed a more effective surface conductive network than graphene or activated carbon, reducing the packed-aggregate resistivity to approximately 580 Ω·cm at a CNT dosage near 1.0 wt% of sand. Two-stage thermal decomposition analysis indicated that the final coating contained approximately 1.6 wt% polyacrylic resin and 0.427 wt% CNTs. In the cementitious composites, CNT-coated aggregates acted as conductive nodes and local CNT sources, whereas CFs bridged neighboring aggregates and microcracks. This hybrid network lowered the percolation requirement, stabilized resistivity in the semiconductor range. These results indicate that CNT-coated conductive aggregates provide a manufacturable functional aggregate and a practical platform for high-performance self-sensing concrete.