Self-sensing concrete with recycled carbon fibres and EAF slag aggregate: Piezoresistive response under static and cyclic flexural loading

This study presents the development and evaluation of sustainable self-sensing concrete mixtures incorporating recycled carbon fibres (rCF), short and large metallic steel fibres (mFM and MFM), and electric-arc-furnace slag aggregate (EAF slag) for potential use in structural health monitoring applications. Six mixtures (three high-strength concretes, HSC, and three conventional concretes, CC) were designed to assess mechanical performance and piezoresistive response under static and cyclic loading. Electrical behaviour was investigated under direct current (DC) and alternating current (AC) in dry, ambient and saturated environments. Fibre-reinforced mixtures, particularly those incorporating rCF, exhibit strong piezoresistive behaviour, with measurable variations in electrical resistivity corresponding to crack initiation and propagation. The HSC mixture combining rCF with short metallic steel fibres (HS-B) achieved the best overall performance, showing high ductility, low impedance and a fractional change in resistivity (FCR) of up to 93% under fatigue loading. The CC-B mixture, which combines rCF with EAF slag and steel macrofibres, demonstrated improved electrical sensitivity and mechanical resilience compared to other CC formulations. The synchronised mechanical–electrical analysis identifies three operational regimes of the piezoresistive response and, from a single fatigue specimen per mixture, a biexponential kinetic law for the fatigue response that connects the static and cyclic mechanism. The slow time constant of the fatigue response shows a preliminary scaling with the baseline DC bulk resistance, suggesting a route to be validated for predicting the fatigue response from a non-destructive baseline measurement. These findings confirm the viability of using recycled and industrial waste materials to produce multifunctional concrete for real-time damage detection.