In recent years, biochar has been successfully investigated to improve the mechanical properties, internal curing, and CO₂ capture of cementitious materials [1]. Thus, the aim of this work was to study the sensing ability of commercial biochar (Nera biochar) made from certified wood wastes as a strain sensor in mortar samples.

The as-received material was first characterized by laser granulometry, X-ray diffraction, x-ray fluorescence and scanning electron microscopy. Later, the biochar particles were manually ground and sieved at 125 microns with a steel mesh. The passing fraction was first dispersed in distilled water using a bath sonicator and then, added to a commercial resin for restoration (Primal B60 from Rohm & Haas, a copolymer of ethyl acrylate/methyl methacrylate) under magnetic stirring. Multiwalled carbon nanotubes (CNTs, Nanocyl 7000) were also used to increase the sensitivity of the sensors because of their higher aspect ratio. Different compositions were investigated: pure biochar, 85% biochar/15% CaCO₃, 80% biochar/20% CaCO₃, 65% biochar/15% CaCO₃/20% CNTs and 60% biochar/20% CaCO₃/20% CNTs. Two stainless steel mesh (1 × 2 cm²) were manually inserted as electrodes into mortar prisms (4 × 4 × 16 cm³) prepared with standard sand immediately after casting and vibrating (1:2 water to cement ratio and 3:1 sand to cement ratio). The samples were demolded after 24 hours and cured under water for 28 days. After drying, the sensing materials were finally painted by brush on the cementitious samples (strip of 1 cm in width between the two stainless steel electrodes at a distance of about 10 cm). 

The prisms were tested in three-point bending, and the sensing films were alimented with a 1 V AC current at 1 kHz frequency using an impedancemeter (Hioki 3301). The change in the impedance value was monitored during mechanical testing. The measurements were first done on samples with a certain humidity content and then on fully dried ones. The best results were obtained with the sensing film containing 60% biochar/20% CaCO₃/20% CNTs, which showed a 23% change in the impedance value under a force of 2000 N.