Development of self-sensing ultra-high-performance concrete using hybrid carbon black and carbon nanofibers

The self-sensing ultra-high-performance concrete (UHPC) was developed in this study by incorporating nano carbon black (CB) and carbon nanofiber (CNF) as additives into the UHPC matrix. Single CB and hybrid CB/CNF filled UHPCs were compared in terms of strength, microstructures, percolation threshold, conductivity, and piezoresistive sensing performance. The results indicate that hybrid CB/CNF filled UHPC consistently exhibits superior compressive strength compared to the counterpart with single CB. The percolation threshold begins at approximately 0.5 % CB content, regardless of the inclusion of CNF. The CNF links the surrounding conductive passages contributed by CB nanoparticles, demonstrating the positive effect of hybrid nanofillers with multiple dimensions. AC impedance spectroscopy (ACIS) and equivalent circuit modelling were performed to understand the synergetic effect of CB/CNF on the electrical microstructure in UHPC matrix and to compare the conductive characteristics between single CB and hybrid CB/CNF filled self-sensing UHPC. The essential information can help comprehend the conductive behaviours and working mechanisms at the microstructural level, providing new insight into the material design strategy to enhance the electrical and sensing performances of UHPC-based cementitious sensors. In terms of the piezoresistive performance, the stability of sensing performance in response to dynamic cyclic load improves with an increasing content of conductive fillers; the hybrid fillers of CB/CNF enhance the stability of piezoresistive sensing performance of self-sensing UHPC with less signal noise under monotonic compressive loading. This study integrates the piezoresistive self-sensing capacity with UHPC to promote the application of cement-based sensors in civil infrastructure.