Mechanical properties, electrical resistivity and piezoresistivity of carbon fibre-based self-sensing cementitious composites

Carbon fibre (CF) is one of the most effective materials in improving the conductivity of the composites by developing a conductive network within the matrix, which also enhanced the piezoresistivity behaviour of the cementitious composites and has a potential application for structural health monitoring. A systematic study of the effect of sizing condition and fibre length on the piezoresistivity behaviour of cementitious composites by adopting unsized CF with the length of 3, 6 and 12 mm, and desized CF of 6 and 12 mm as functional fillers. Each type of CF was added at four different weight fractions of 0.1, 0.3, 0.5 and 0.7% to determine the optimal fibre content. Electrical resistivity and piezoresistivity tests were conducted for samples before and after drying treatment to evaluate the effect of water content on electrical properties. Besides, fresh properties of the fresh mixture, mechanical properties and microstructure of the composites were also investigated. Results showed that unsized CF is more effective in enhancing composites flexural strength and reducing the electrical resistivity, which also showed a stronger bonding with the cement matrix and also demonstrated a better dispersive ability. In terms of piezoresistivity behaviour, for a given fibre length, desized CF showed higher sensitivity and repeatability compared to unsized CF; however, the signals showed more noise. The best piezoresistivity behaviour was obtained for composites containing 3 mm CF at 0.7 wt%, which showed a fractional change in resistivity (FCR) value of approximately 70%. An equation was developed, which can successfully describe the relationship between the FCR of cementitious composites containing CF and the applied external stress.