Positive piezoresistive mortar based on carbon nanotube adhered aggregates: Damage at interfacial transition zone and mechanism

Traditional smart cement-based composites exhibit negative piezoresistivity (resistivity decreases with increasing compressive stress) due to enhanced contact between conductive phases. This behavior fails to directly monitor damage within the interfacial transition zone (ITZ), which is commonly associated with positive piezoresistive behavior (resistivity increases with increasing compressive stress). In this study, a highly conductive mortar was developed using aggregates prepared by bonding carbon nanotube to their surfaces. This approach directly incorporates the ITZ into the conductive network. The resulting mortars not only show distinct positive piezoresistive behavior but also achieve a 21.7% higher 28-day compressive strength than the control. The FCR under 30% of compressive strength and the stress sensitivity were as high as 40% and 14.67%/MPa, respectively, far surpassing those of other conductive aggregate mortars. This study opens a new avenue for redesigning the conductive network in cement, enabling not only enhanced conductivity but also controlled cracking of the network. In principle, this strategy removes the limit on FCR.