Electromechanical and piezoresistive behaviour of hydraulic lime mortar incorporating carbon microfibers for self-sensing applications

Self-sensing mortars, a type of smart material capable of monitoring structural behaviour under mechanical loading, have recently attracted significant attention. Although most research in this area has focused on cement-based systems, lime-based mortars remain relatively underexplored despite their importance in sustainable construction and heritage preservation. This study investigates the impact of carbon microfibers (CMFs) as a functional filler on the mechanical, electrical, and piezoresistive properties of hydraulic lime-based mortars. Mortar and paste mixes were prepared with different dosages of CMF (0 %, 0.05 %, 0.1 %, and 0.2 % by weight of the binder), both with and without silica fume as a pozzolanic additive. Mechanical tests, including flexural and compressive strength, as well as electrical resistivity measurements and electromechanical (piezoresistivity) evaluations, were performed. The results show that while silica fume enhances compressive strength, it tends to lower flexural strength, likely due to increased brittleness. The addition of CMF exhibited a nonlinear influence on the mechanical properties of the mixes, with strength decreasing up to 0.1 % CMF, followed by a slight increase at 0.2 % CMF. Similarly, the electrical properties (particularly piezoresistivity) also followed a nonlinear trend, showing enhanced sensitivity up to 0.1 % CMF and a reduction beyond that point. These trends suggest the presence of a percolation threshold around 0.1 % CMF The piezoresistive performance, assessed through gauge factor and linearity, also reached its peak at this dosage. The formulation of 0.1 % conductive material mixed with silica fume achieved a gauge factor of 311, which is 34 % higher than the mix containing 0.0 % CMF. This mix also displayed a coefficient of determination (R²) of 0.97. In contrast, the mix without silica fume attained a gauge factor of 339, representing a 79 % increase compared to the mix with 0.0 % CMF, with an R² of 0.94. These results confirm that the sensing response was stable and highly reliable. Overall, 0.1 % CMF was identified as the optimal amount for improving the self-sensing behaviour of lime-based mortars. These findings confirm that CMF can effectively enhance the electromechanical response of traditional lime matrices without compromising their structural performance or their suitability for heritage materials.