Feasibility of using biochar as conductive filler in cement-based strain sensors

Securing sustainable and cost-effective conductive materials is vital for the advancement of cement-based smart composites. This study explores the partial replacement of high-cost multi-walled carbon nanotubes (MWCNT) with low-cost biochar to develop self-sensing cementitious composites. Six types of specimens were prepared: a control (OPC), MWCNT only (MWC), untreated biochar (BC), Fe modified biochar (FC), and two hybrid systems combining MWCNT with either untreated (BMC) or modified biochar (FMC). Mechanical, electrochemical, and piezoresistive properties were systematically evaluated. FC showed a 43.2% increase in compressive strength compared to OPC, while BMC and FMC also demonstrated enhanced mechanical performance and deformation capacity. All specimens incorporating conductive fillers exhibited electrical resistivity values approximately 5–10 times lower than OPC across varying moisture conditions. Notably, the synergistic interaction between the porous biochar structure and the percolated MWCNT network in the BMC and FMC gave rise to hierarchically separated bulk conduction, interfacial polarization, and diffusion-controlled transport mechanisms, resulting in the appearance of three distinct semicircular arcs in the electrochemical impedance spectroscopy (EIS) results. Under repeated compressive loading, specimens without conductive fillers exhibited noisy piezoresistive signals due to polarization effects. In contrast, both BMC and FMC maintained high repeatability and stable responses under cyclic loading, with minimal signal distortion within the elastic range. Although MWC demonstrated superior electrical conductivity, it also resulted in the highest cost and environmental impact. By comparison, BMC and FMC achieved comparable sensing performance while reducing both cost and environmental burden by approximately 48%, highlighting their potential as sustainable alternatives for smart infrastructure applications. • This study confirms the potential effectiveness of low-cost biochar-based sensing materials for detecting the structural condition of cementitious composites. • The results suggest that the electrical conductivity and sensing capability of biochar can be enhanced through metal modification. • The findings demonstrate the feasibility of using biochar-based composites as an eco-friendly and cost-effective alternative to expensive carbon-based materials