Engineering Multifunctional Lime-Based Mortars for Repair and Monitoring of Historic Buildings: Effects of Fibre Dosage, Length, and Aggregate Type

This study examines the properties of lime-based mortars enhanced with carbon microfibres (CMFs) to create multifunctional materials for structural health monitoring (SHM) and repair of heritage structures. An experimental campaign tested mortars with different CMF doses (0.0–0.2% by binder weight), fibre lengths (3–12 mm), and two types of aggregate. The mortars were examined for compressive and flexural strength, electrical resistivity, and self-sensing capabilities. Results reveal nonlinear effects of CMF dosage and fibre length on material behaviour. Mechanical strength initially decreased at low fibre contents but improved at higher doses, while electrical resistivity reached a minimum at about 0.1% CMF. Fibre length affected the formation of the conductive network: 6 mm fibres produced the lowest resistivity and highest compressive strength, whereas 12 mm fibres offered the greatest strain sensitivity. The type of aggregate had a significant impact: EN 196–1 standard sand increased stiffness, reduced resistivity, and raised the gauge factor compared with building sand. Taguchi analysis identified 0.1% CMF, 6–12 mm fibres, and standard sand as the optimal combination for achieving balanced mechanical performance and reliable self-sensing capabilities. The findings showed that lime-based mortars can be engineered for multifunctional behaviour, allowing for integrated repair and structural health monitoring of heritage masonry.