Electro-mechanical behaviour of mortars reinforced with alternative electrically conductive inclusions

The incorporation of electrically conductive inclusions in structural materials can impart self-sensing functionalities, making them ideal for structural health monitoring applications. However, the use of more sustainable alternatives and their effect on key engineering properties remain largely unexplored, while the adoption of different testing protocols for the characterisation of electrical/self-sensing properties can lead to different results, thus questioning their reliability, even for existing smart composites. This paper investigates systematically the effect of recycled carbon fibres and graphite powder on the mechanical, electrical, transport properties and piezoresistive performance of cementitious mortars. Virgin carbon fibres, at dosages equivalent to those of recycled fibres, were also examined to establish a performance benchmark. Fibre content ranged from 0.05% to 1% vol., while graphite powder was added as sand replacement at contents varying from 0.3% to 3% vol. The effect of existing testing protocols and electrode layout on the piezoresistive performance was also examined, and the associated limitations and challenges are discussed in detail. The results demonstrate the potential of recycled carbon fibres as a cost-effective alternative in smart applications, without compromising electrical and piezoresistive performance. The use of 0.25%vol. of recycled or virgin carbon fibres was found to provide the desirable synergy between structural performance, cost and self-sensing properties, yielding a 50-60% increase in flexural strength, and good piezoresistivity with a gauge factor of 90-110. In contrast, the use of graphite powder resulted in composites with poor self-sensing ability even at the highest content examined (3%vol.), also accompanied by a reduction in compressive strength up to 33%.